1
Numerical Modeling and Simulation of the Stability of Earth Slopes
A Thesis Submitted to the Department of Nuclear Engineering
DEPARTMENT OF NUCLEAR ENGINEERING
SCHOOL OF NUCLEAR AND ALLIED SCIENCES
COLLEGE OF BASIC AND APPLIED SCIENCES
UNIVERSITY OF GHANA
BY
BRENDAN DAGEMANYIMA ATARIGIYA 10507155
BSc (KNUST Kumasi) 2012
In Partial Fulfilment of the Requirements for the Degree of
MASTER OF PHILOSOPY
IN
COMPUTATIONAL NUCLEAR SCIENCE AND ENGINEERING
July 2016
ii
DECLARATION
I hereby declare that with the exception of references to other peoplersquos work which
have duly been acknowledged this Thesis is the result of my own research work and
no part of it has been presented for another degree in this University and elsewhere
helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip helliphelliphelliphelliphelliphelliphelliphelliphelliphellip
Brendan Dagemanyima Atarigiya Date
(Candidate)
I hereby declare that the preparation of this project was supervised in accordance with
the guidelines of the supervision of Thesis work laid down by the University of Ghana
helliphelliphelliphelliphelliphelliphelliphelliphelliphellip helliphelliphelliphelliphelliphelliphelliphelliphelliphellip
Dr Nii Kwashie Allotey Nana (Prof) A Ayensu Gyeabour I
(Principal Supervisor) (Co-Supervisor)
helliphelliphelliphelliphelliphelliphelliphelliphelliphellip helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip
Date Date
iii
DEDICATION
I dedicate this work to God Almighty my family and friends
iv
ACKNOWLEDGMENT
Firstly I want to thank and praise God for the good health and strength given me during
all this period of schooling
My sincere and utmost gratitude goes to my Principal Supervisor Dr Nii Kwashie
Allotey of the Ghana Atomic Energy Commission (GAEC) and the University of Ghana
for his expertise in the field of research and exemplary guidance towards the progress
of this research I am also grateful to Prof (Nana) A Ayensu Gyeabour my Co-
Supervisor for his creative suggestions and motivation through this research project
My thanks also goes to Ms Rita Awura Abena Appiah of GAEC thank you for pushing
me to make this possible You helped me improve my programming skills a lot
To my big family the Atarigiya Aguyire and Allotey families thank you for the
support I could not have done it without you
To my amazing course mate Linda Sarpong thank you for your words of
encouragement during out period in school you are a strong woman Furthermore to
the nuclear engineering department (Samiru Efia Matilda Henryhellip) thanks for the
wonderful time we had together
Last and not the least to the woman who stood firmly behind me from day one from
when this journey began Mercy Selina Somhayin Namateng I LOVE YOU
GOD RICHLY BLESS YOU ALL
v
ABSTRACT
Ghana as most other countries has a considerable variation in its topography In an
attempt to build cheaper but yet the safe structures (ie roads apartments etc) we
are most often times faced with building on hill-sides and in valleys This then calls
for the need to correctly assess the stability of any adjacent slopes
In recent times due to the extensive need for stability analysis in engineering practice
slope stability analysis programs have been developed It is noted that these
commercial slope stability programs are used extensively in the industry but are very
expensive and require purchasing yearly licenses As a result of this slope stability
analysis is not routinely conducted in local geotechnical engineering practice The need
for cheaper more accessible options is thus considered needful
This research initiative uses MATLAB a commercially available user-friendly and
easy to access computing platform to develop a slope stability analysis program The
method used is the General Limit Equilibrium Method (GLE) with the adoption of the
Morgenstern-Price (M-P) factor of safety (FoS) approach to develop a cheap efficient
and yet effective model for slope stability analysis and design The results of the
program are validated by comparing with the results of SLOPEW a commercial slope
stability program
The results show four model outputs from the developed program and SLOPEW for a
homogeneous material Two different failure mechanisms are shown (ie toe and base
failures) It is noted that the percentage error in the M-P FoS is less than 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
vi
Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
LIST OF FIGURES ix
LIST OF TABLES xi
CHAPTER ONE INTRODUCTION 1
11 Background 1
12 Problem Statement 3
13 Relevance and Justification of Study 4
14 Research Goal 5
15 Research Objectives 5
16 Scope 5
17 Format of the Thesis 6
CHAPTER TWO LITERATURE REVIEW 7
21 Factors Causing Instability 7
22 Types of Slip Surfaces 8
23 Definition of FoS 10
24 Slope Stability Analysis Methods 12
241 Limit Analysis Method 12
242 Variational Calculus Method 13
243 Strength Reduction Method 14
244 General Discussion on Limit Equilibrium Method 15
25 General Limit Equilibrium Method of Slices (GLE method) 19
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
ii
DECLARATION
I hereby declare that with the exception of references to other peoplersquos work which
have duly been acknowledged this Thesis is the result of my own research work and
no part of it has been presented for another degree in this University and elsewhere
helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip helliphelliphelliphelliphelliphelliphelliphelliphelliphellip
Brendan Dagemanyima Atarigiya Date
(Candidate)
I hereby declare that the preparation of this project was supervised in accordance with
the guidelines of the supervision of Thesis work laid down by the University of Ghana
helliphelliphelliphelliphelliphelliphelliphelliphelliphellip helliphelliphelliphelliphelliphelliphelliphelliphelliphellip
Dr Nii Kwashie Allotey Nana (Prof) A Ayensu Gyeabour I
(Principal Supervisor) (Co-Supervisor)
helliphelliphelliphelliphelliphelliphelliphelliphelliphellip helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip
Date Date
iii
DEDICATION
I dedicate this work to God Almighty my family and friends
iv
ACKNOWLEDGMENT
Firstly I want to thank and praise God for the good health and strength given me during
all this period of schooling
My sincere and utmost gratitude goes to my Principal Supervisor Dr Nii Kwashie
Allotey of the Ghana Atomic Energy Commission (GAEC) and the University of Ghana
for his expertise in the field of research and exemplary guidance towards the progress
of this research I am also grateful to Prof (Nana) A Ayensu Gyeabour my Co-
Supervisor for his creative suggestions and motivation through this research project
My thanks also goes to Ms Rita Awura Abena Appiah of GAEC thank you for pushing
me to make this possible You helped me improve my programming skills a lot
To my big family the Atarigiya Aguyire and Allotey families thank you for the
support I could not have done it without you
To my amazing course mate Linda Sarpong thank you for your words of
encouragement during out period in school you are a strong woman Furthermore to
the nuclear engineering department (Samiru Efia Matilda Henryhellip) thanks for the
wonderful time we had together
Last and not the least to the woman who stood firmly behind me from day one from
when this journey began Mercy Selina Somhayin Namateng I LOVE YOU
GOD RICHLY BLESS YOU ALL
v
ABSTRACT
Ghana as most other countries has a considerable variation in its topography In an
attempt to build cheaper but yet the safe structures (ie roads apartments etc) we
are most often times faced with building on hill-sides and in valleys This then calls
for the need to correctly assess the stability of any adjacent slopes
In recent times due to the extensive need for stability analysis in engineering practice
slope stability analysis programs have been developed It is noted that these
commercial slope stability programs are used extensively in the industry but are very
expensive and require purchasing yearly licenses As a result of this slope stability
analysis is not routinely conducted in local geotechnical engineering practice The need
for cheaper more accessible options is thus considered needful
This research initiative uses MATLAB a commercially available user-friendly and
easy to access computing platform to develop a slope stability analysis program The
method used is the General Limit Equilibrium Method (GLE) with the adoption of the
Morgenstern-Price (M-P) factor of safety (FoS) approach to develop a cheap efficient
and yet effective model for slope stability analysis and design The results of the
program are validated by comparing with the results of SLOPEW a commercial slope
stability program
The results show four model outputs from the developed program and SLOPEW for a
homogeneous material Two different failure mechanisms are shown (ie toe and base
failures) It is noted that the percentage error in the M-P FoS is less than 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
vi
Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
LIST OF FIGURES ix
LIST OF TABLES xi
CHAPTER ONE INTRODUCTION 1
11 Background 1
12 Problem Statement 3
13 Relevance and Justification of Study 4
14 Research Goal 5
15 Research Objectives 5
16 Scope 5
17 Format of the Thesis 6
CHAPTER TWO LITERATURE REVIEW 7
21 Factors Causing Instability 7
22 Types of Slip Surfaces 8
23 Definition of FoS 10
24 Slope Stability Analysis Methods 12
241 Limit Analysis Method 12
242 Variational Calculus Method 13
243 Strength Reduction Method 14
244 General Discussion on Limit Equilibrium Method 15
25 General Limit Equilibrium Method of Slices (GLE method) 19
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
iii
DEDICATION
I dedicate this work to God Almighty my family and friends
iv
ACKNOWLEDGMENT
Firstly I want to thank and praise God for the good health and strength given me during
all this period of schooling
My sincere and utmost gratitude goes to my Principal Supervisor Dr Nii Kwashie
Allotey of the Ghana Atomic Energy Commission (GAEC) and the University of Ghana
for his expertise in the field of research and exemplary guidance towards the progress
of this research I am also grateful to Prof (Nana) A Ayensu Gyeabour my Co-
Supervisor for his creative suggestions and motivation through this research project
My thanks also goes to Ms Rita Awura Abena Appiah of GAEC thank you for pushing
me to make this possible You helped me improve my programming skills a lot
To my big family the Atarigiya Aguyire and Allotey families thank you for the
support I could not have done it without you
To my amazing course mate Linda Sarpong thank you for your words of
encouragement during out period in school you are a strong woman Furthermore to
the nuclear engineering department (Samiru Efia Matilda Henryhellip) thanks for the
wonderful time we had together
Last and not the least to the woman who stood firmly behind me from day one from
when this journey began Mercy Selina Somhayin Namateng I LOVE YOU
GOD RICHLY BLESS YOU ALL
v
ABSTRACT
Ghana as most other countries has a considerable variation in its topography In an
attempt to build cheaper but yet the safe structures (ie roads apartments etc) we
are most often times faced with building on hill-sides and in valleys This then calls
for the need to correctly assess the stability of any adjacent slopes
In recent times due to the extensive need for stability analysis in engineering practice
slope stability analysis programs have been developed It is noted that these
commercial slope stability programs are used extensively in the industry but are very
expensive and require purchasing yearly licenses As a result of this slope stability
analysis is not routinely conducted in local geotechnical engineering practice The need
for cheaper more accessible options is thus considered needful
This research initiative uses MATLAB a commercially available user-friendly and
easy to access computing platform to develop a slope stability analysis program The
method used is the General Limit Equilibrium Method (GLE) with the adoption of the
Morgenstern-Price (M-P) factor of safety (FoS) approach to develop a cheap efficient
and yet effective model for slope stability analysis and design The results of the
program are validated by comparing with the results of SLOPEW a commercial slope
stability program
The results show four model outputs from the developed program and SLOPEW for a
homogeneous material Two different failure mechanisms are shown (ie toe and base
failures) It is noted that the percentage error in the M-P FoS is less than 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
vi
Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
LIST OF FIGURES ix
LIST OF TABLES xi
CHAPTER ONE INTRODUCTION 1
11 Background 1
12 Problem Statement 3
13 Relevance and Justification of Study 4
14 Research Goal 5
15 Research Objectives 5
16 Scope 5
17 Format of the Thesis 6
CHAPTER TWO LITERATURE REVIEW 7
21 Factors Causing Instability 7
22 Types of Slip Surfaces 8
23 Definition of FoS 10
24 Slope Stability Analysis Methods 12
241 Limit Analysis Method 12
242 Variational Calculus Method 13
243 Strength Reduction Method 14
244 General Discussion on Limit Equilibrium Method 15
25 General Limit Equilibrium Method of Slices (GLE method) 19
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
iv
ACKNOWLEDGMENT
Firstly I want to thank and praise God for the good health and strength given me during
all this period of schooling
My sincere and utmost gratitude goes to my Principal Supervisor Dr Nii Kwashie
Allotey of the Ghana Atomic Energy Commission (GAEC) and the University of Ghana
for his expertise in the field of research and exemplary guidance towards the progress
of this research I am also grateful to Prof (Nana) A Ayensu Gyeabour my Co-
Supervisor for his creative suggestions and motivation through this research project
My thanks also goes to Ms Rita Awura Abena Appiah of GAEC thank you for pushing
me to make this possible You helped me improve my programming skills a lot
To my big family the Atarigiya Aguyire and Allotey families thank you for the
support I could not have done it without you
To my amazing course mate Linda Sarpong thank you for your words of
encouragement during out period in school you are a strong woman Furthermore to
the nuclear engineering department (Samiru Efia Matilda Henryhellip) thanks for the
wonderful time we had together
Last and not the least to the woman who stood firmly behind me from day one from
when this journey began Mercy Selina Somhayin Namateng I LOVE YOU
GOD RICHLY BLESS YOU ALL
v
ABSTRACT
Ghana as most other countries has a considerable variation in its topography In an
attempt to build cheaper but yet the safe structures (ie roads apartments etc) we
are most often times faced with building on hill-sides and in valleys This then calls
for the need to correctly assess the stability of any adjacent slopes
In recent times due to the extensive need for stability analysis in engineering practice
slope stability analysis programs have been developed It is noted that these
commercial slope stability programs are used extensively in the industry but are very
expensive and require purchasing yearly licenses As a result of this slope stability
analysis is not routinely conducted in local geotechnical engineering practice The need
for cheaper more accessible options is thus considered needful
This research initiative uses MATLAB a commercially available user-friendly and
easy to access computing platform to develop a slope stability analysis program The
method used is the General Limit Equilibrium Method (GLE) with the adoption of the
Morgenstern-Price (M-P) factor of safety (FoS) approach to develop a cheap efficient
and yet effective model for slope stability analysis and design The results of the
program are validated by comparing with the results of SLOPEW a commercial slope
stability program
The results show four model outputs from the developed program and SLOPEW for a
homogeneous material Two different failure mechanisms are shown (ie toe and base
failures) It is noted that the percentage error in the M-P FoS is less than 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
vi
Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
LIST OF FIGURES ix
LIST OF TABLES xi
CHAPTER ONE INTRODUCTION 1
11 Background 1
12 Problem Statement 3
13 Relevance and Justification of Study 4
14 Research Goal 5
15 Research Objectives 5
16 Scope 5
17 Format of the Thesis 6
CHAPTER TWO LITERATURE REVIEW 7
21 Factors Causing Instability 7
22 Types of Slip Surfaces 8
23 Definition of FoS 10
24 Slope Stability Analysis Methods 12
241 Limit Analysis Method 12
242 Variational Calculus Method 13
243 Strength Reduction Method 14
244 General Discussion on Limit Equilibrium Method 15
25 General Limit Equilibrium Method of Slices (GLE method) 19
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
v
ABSTRACT
Ghana as most other countries has a considerable variation in its topography In an
attempt to build cheaper but yet the safe structures (ie roads apartments etc) we
are most often times faced with building on hill-sides and in valleys This then calls
for the need to correctly assess the stability of any adjacent slopes
In recent times due to the extensive need for stability analysis in engineering practice
slope stability analysis programs have been developed It is noted that these
commercial slope stability programs are used extensively in the industry but are very
expensive and require purchasing yearly licenses As a result of this slope stability
analysis is not routinely conducted in local geotechnical engineering practice The need
for cheaper more accessible options is thus considered needful
This research initiative uses MATLAB a commercially available user-friendly and
easy to access computing platform to develop a slope stability analysis program The
method used is the General Limit Equilibrium Method (GLE) with the adoption of the
Morgenstern-Price (M-P) factor of safety (FoS) approach to develop a cheap efficient
and yet effective model for slope stability analysis and design The results of the
program are validated by comparing with the results of SLOPEW a commercial slope
stability program
The results show four model outputs from the developed program and SLOPEW for a
homogeneous material Two different failure mechanisms are shown (ie toe and base
failures) It is noted that the percentage error in the M-P FoS is less than 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
vi
Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
LIST OF FIGURES ix
LIST OF TABLES xi
CHAPTER ONE INTRODUCTION 1
11 Background 1
12 Problem Statement 3
13 Relevance and Justification of Study 4
14 Research Goal 5
15 Research Objectives 5
16 Scope 5
17 Format of the Thesis 6
CHAPTER TWO LITERATURE REVIEW 7
21 Factors Causing Instability 7
22 Types of Slip Surfaces 8
23 Definition of FoS 10
24 Slope Stability Analysis Methods 12
241 Limit Analysis Method 12
242 Variational Calculus Method 13
243 Strength Reduction Method 14
244 General Discussion on Limit Equilibrium Method 15
25 General Limit Equilibrium Method of Slices (GLE method) 19
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
vi
Contents
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
LIST OF FIGURES ix
LIST OF TABLES xi
CHAPTER ONE INTRODUCTION 1
11 Background 1
12 Problem Statement 3
13 Relevance and Justification of Study 4
14 Research Goal 5
15 Research Objectives 5
16 Scope 5
17 Format of the Thesis 6
CHAPTER TWO LITERATURE REVIEW 7
21 Factors Causing Instability 7
22 Types of Slip Surfaces 8
23 Definition of FoS 10
24 Slope Stability Analysis Methods 12
241 Limit Analysis Method 12
242 Variational Calculus Method 13
243 Strength Reduction Method 14
244 General Discussion on Limit Equilibrium Method 15
25 General Limit Equilibrium Method of Slices (GLE method) 19
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
vii
26 The Ordinary or Fellenius Method (OMS) 20
27 Simplified Bishoprsquos Method 21
28 Janbursquos Simplified Method 22
29 Morgenstern-Price (M-P) Method 23
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE USED 27
31 Selection of Factor of safety method 27
32 Morgenstern-Price Method 28
33 Assumptions 29
34 Numerical Method Development 29
35 Derivation of Equations 30
36 Structured Program 32
37 Numerical Algorithm 33
38 Software and Programs Used 34
381 SLOPEW 34
382 MATLAB 36
CHAPTER FOUR RESULTS AND DISCUSSION 37
41 Introduction 37
42 Programme Test Examples 37
421 Toe Failure 37
422 Base Failure 42
43 Comparison with Cases from Literature 46
431 Case 1 46
432 Case 2 50
CHAPTER FIVE CONCLUSIONS 53
51 Conclusions 53
52 Recommendations 54
REFERENCES 55
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
viii
APPENDICES 60
Appendix A Structured Programme 60
Appendix B MATLAB Code for Solving FoS 63
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
ix
LIST OF FIGURES
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road 3
Figure 21 Types of circular slip failure surface 9
Figure 22 Typical non-circular slip surfaces 10
Figure 23 Various definitions for FoS 11
Figure 24 Swedish Slip Circle Method 16
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass 17
Figure 26 Forces considered in the Ordinary Method of Slices 21
Figure 26 Forces considered in the Ordinary Method of Slices 22
Figure 28 Forces considered in the M-P method 24
Figure 29 Inter-slice force function types 25
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method 26
Figure 31 Sketch of a Slope Section 30
Figure 32 Forces acting on a Single Slice from a Mass Slope 31
Figure 33 Algorithm flowchart for solving for the FoS 33
Figure 34 SLOPEW KeyIn Analyses Page 35
Figure 35 SLOPEW KeyIn Material Page 35
Figure 36 SLOPEW KeyIn Entry and Exit Range Page 36
Figure 41 SLOPEW Output of Toe Failure Case 1 38
Figure 42 MATLAB Output of Toe Failure Case 1 39
Figure 43 SLOPEW Output for Toe Failure Case 2 40
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
x
Figure 44 MATLAB Output for Toe Failure Case 2 41
Figure 45 SLOPEW Output for Base Failure Case 42
Figure 46 MATLAB Output for Base Failure Case 1 43
Figure 47 SLOPEW Output for Base Failure Case 2 44
Figure 48 MATLAB Output for Base Failure Case 2 45
Figure 49 Homogeneous Slope without Foundation 46
Figure 410 Analysis using SLOPEW - FoS = 1385 47
Figure411 FoS based on M-P approach for Toe failure - FoS = 1451 48
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375 49
Figure 413 Slope Model Geometry from Slide 3 50
Figure 414 FoS based on M-P Approach for ACAD Problem 51
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
xi
LIST OF TABLES
Table 21 The Main Limit Equilibrium Methods 18
Table 31 Brief Comparison of Limit Equilibrium Methods 27
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1 38
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2 40
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1 42
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2 44
Table 45 Slope dimensions and material properties 46
Table 46 Slope Dimensions and Material Properties for ACAD Problem 50
Table 51 Summary of FoS Outputs for all Case Studies 53
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
xii
LIST OF SYMBOLS AND ABBREVIATIONS
A cross-sectional area of slice
dx width of slice
l length of the bottom of the slice
c cohesion of soil
G total unit weight of soil
τf shear strength
τ shear stress
γw unit weight of water
angle of internal friction of soil
α inclination from horizontal of the bottom of the slice (degrees)
cal cos(α)
sal sin(α)
tph tan()
H Height of slope
N total normal force on the bottom of the slice
S shear force on the bottom of the slice
W weight of the slice
havg = average height of slice
u pore water pressure
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
xiii
∆X shear component of the inter-slice force
∆E inter-slice force on the downslope of the slice
ns number of slice faces
FoS factor of safety
F assumed factor of safety
Ff force factor of safety
Fm moment factor of safety
Xc x coordinate of centre of slip circle
Yc y coordinate of centre of slip circle
R Radius of slip circle
ytop Ground surface
ybot Slope surface
FSom Factor of Safety for Ordinary Method of Slices
FFm Moment Factor of Safety for Morgentern-Price Method of Slices
FFf Force Factor of Safety for Morgentern-Price Method of Slices
F(x) Interslice force function
λ Scale factor of the assumed f(x)
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
1
CHAPTER ONE INTRODUCTION
11 Background
Slope stability is the potential for ground slopes to resist movement [1] Slope
instability has been the subject of continued concern because of the tremendous loss of
life property and infrastructure caused annually in many places in the world [2] In the
field of construction slope instability can occur due to rainfall increasing the water
table and the change in stress conditions Similarly tracks of land that have been stable
for years may suddenly fail due to changes in the geometry external forces and loss of
shear strength [3]
Slope failures also called slides or landslides whether sudden or gradual are due to
the increased stress of slope materials or foundations compared with their mobilized
strength [3]
The majority of the slope stability analyses performed in practice still use traditional
limit equilibrium approaches involving the method of slices and has remained virtually
unchanged for decades
Analysis of the slope stability is carried out to assess the safety of artificial or natural
slopes (eg dams road cuts mining open pit excavations and landfills) For human
made slopes analysis of slope stability is used to evaluate various design options which
then provides a basis for a form of engineering design with associated costing
comparisons The efficient engineering of natural and artificial slopes has therefore
become a common challenge faced by both researchers and practitioners
Slope stability assessment mainly involves the use of the factor of safety (FoS) method
to determine how close a given slope is to the onset of instability or to what extent the
state of the slope is from failure
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
2
When this ratio is well above 1 the resistance to shear failure is generally higher than
the driving shear stress and the slope is considered stable When this ratio is near to 1
the shear strength is almost equal to the shear stress and the slope is close to failure If
the FoS is less than 1 the slope is considered to have failed or considered to be trigger-
point ready [4]
Ghana is not noted to be a frequent serious victim of mass movement (slope failures)
It is however noteworthy that Ghana has not been without slope failures In Ref [5]
reference is made of a slope failure which occurred in 1968 near Jamasi in the Sekyere
District involving about 1500 cubic meters of rock soil and vegetation The failure
blocked the main Kumasi-Mampong truck road for a total of ten days
Reference is also made in Refs [6 amp 7] on potential slope failures on the stretch of the
Accra-Aburi road when rocks began to fall unhindered onto the road in 2014
Furthermore Ref [8] notes that in 2013 after a heavy downpour of rain loose parts of
the Akoaso Mountain fell and blocked the Kasoa-Weija portions of the road resulting
in a significant traffic jam for hours
These recent records of slope instability in the country have served as a wakeup call for
researchers and practicing engineers to take a critical look at this issue
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
3
Figure 11 Over 40 m high slope on the Ayi-Mensah-Aburi road
12 Problem Statement
In an attempt to build cheap and yet the safe structures (ie roads living apartments
etc) for man-kind we are most times faced with building in valleys and on mountains
Either way we are faced with the problem of slope instability
In the past decades computer software for slope stability analysis and design have been
developed and marketed extensively These commercial software which have been
developed over many years are able to perform rigorous stability calculations and give
fast and accurate answers to complex slope stability problems These software have
become widely accepted in industry and are now part of most large design engineering
offices These software are however expensive and normally require the annual
renewal of licenses Notwithstanding their wide acceptance in industry most
Ghanaian geotechnical engineers still resort to the use of the rule of thumb slope
stability analysis methods and old charts for their daily slope stability analysis This is
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
4
due to the relatively high cost of these commercial software and the limited financial
capacity of local engineering firms
This has created a gap between local and international engineers and has resulted in
major geotechnical engineering projects involving complex slope instability problems
being awarded to international firms rather than local engineering firms
13 Relevance and Justification of Study
Landslides rock falls and mass movement of any kind are undoubtedly one of the
oldest natural disasters that have resulted in huge damages loss of lives and a great
deal of pain to mankind Like other mountainous countries Ghana has large variations
in its topography The impending threat of landslides in the case of [5 or 6] or rock
falls in the case of [7] is now accepted as life threating and the need for these slopes to
be properly engineered is critical
It has been already noted that the available commercial slope stability programs that
are extensive slope stability analysis are very expensive and require purchasing yearly
licenses This has necessitated the need to develop a simple yet efficient slope stability
program that can be easily accessed by local engineers to appraise local slope stability
problems using the most rigorous and accurate methods available
This research initiative uses MATLAB (a commercially available and easily
accessible computing platform with great user-friendly interface) using General Limit
Equilibrium method (GLE) and adopting the Morgenstern-Price FoS to develop a cheap
and efficient yet effective model for slope stability analysis and design
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
5
14 Research Goal
The goal of this thesis is to develop and implement a slope stability numerical model
that will aid local geotechnical engineers to readily appraise the stability of slopes in
local practice
15 Research Objectives
The objectives of the research are to
Develop a physical model to represent the problem
Develop mathematical equations to solve the problem
Develop a numerical algorithm and write a code to solve for the FoS of earth
slopes
Verify and validate the code using Geoslope Internationalrsquos SLOPEW
commercial slope stability programme
16 Scope
For the purpose of this study this thesis is limited to the development of a slope stability
programme for homogenous soil and rock media In this regard the goal of the study
is to develop the generic algorithm for slope stability analysis Furthermore similar to
the existing commercial programmes the study is limited to two-dimensional slope
stability problems
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
6
17 Format of the Thesis
Following the introduction to slope stability problems in Chapter 1 a detailed literature
review of the methods of slope stability analysis is provided in Chapter 2 Chapter 3
then presents the proposed solution method for calculating the factor of safety in which
the GLE method is explained and the solution algorithm developed
Chapter 4 presents sample results from the developed MATLAB code It presents
comparisons between the results of the developed code and results from the commercial
slope stability programme SLOPEW Chapter 5 finally presents the conclusions of
the study and also provides recommendations for further studies
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
7
CHAPTER TWO LITERATURE REVIEW
Analysis of slope stability problem is an important subject area in geotechnical
engineering The phenomenon of landslides and related slope instability is a problem
in many parts of the world Slope failure mechanisms and the geological history of a
slope can be very complicated problems and required complex forms of analyses
21 Factors Causing Instability
The failure of the slopes occurs when the downward movements of soil or rock material
because of gravity and other factors creates shear stresses that exceed the inherent shear
strength of the material Therefore factors that tend to increase the shear stress or
decrease the shear strength of a material increases the risk of the failure of a slope
Various processes can lead to a reduction of the shear strength of a soilrock mass
These include increased pore pressure cracking swelling decomposition of
argillaceous rock fills creep under sustained loads leaching softening weather and
cyclic loading among others
On the other hand shear stress within a rocksoil mass may increase due to additional
loads on top of the slope and increase in water pressure due to cracks at the top of the
slope an increase in the weight of soil due to increasing water content the excavation
of the base of the slope and seismic effects Furthermore additional factors that
contribute to the failure of a slope include the rocksoil mass properties slope geometry
state of stress temperature erosion etc
The presence of water is the most critical factor that affects the stability of slopes This
is because it increases both the driving shear stress and also decreases the soilrock
massrsquo shear strength The speed of sliding movement in a slope failure can vary from
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
8
a few millimeters per hour to very fast slides where great changes have occurred within
seconds Slow slides occur in soils with a plastic stress-strain characteristics where
there is no loss of strength with increased strain Fast slides occurs in situations where
there is a sudden loss of strength as in the liquefaction of sensitive clay or fine sand
Increase in shear stresses across the soil mass result in movement only when the shear
strength mobilized on given possible failure surface in the ground is less than the
driving shear stresses along that surface
22 Types of Slip Surfaces
To calculate the FoS of a slope it is always assumed that the slope is failing in some
shape normally in a circular or non-circular shape For computational simplicity the
slide surface is often seen as circular or composed of several straight lines [9] Different
sliding surfaces are normally assumed with the computation of a corresponding FoS
The sliding surface with the minimum FoS is then selected as the FoS of the slope in
question
A circular sliding surface like that shown in Figure 21 is often used because it is
suitable to sum the moments about a centre The use of a circle also simplifies the
calculations Wedge-like surfaces have their failure mechanisms defined by three or
more straight line segments defining an active area central block and the passive area
as shown in Figure 22 This type of sliding surface can be used for analysis of slopes
where the critical potential sliding surface comprises a relatively long linear sector
through low material bounded by a stronger material
As noted above the critical slip surface is the surface with the lowest factor of safety
The critical slip surface for a given problem analysed by a given method is found by a
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
9
systematic procedure to generate sliding test surfaces until the one with the minimum
safety factor is found [9]
Figure 21 Types of circular slip failure surface [3]
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
10
Figure 22 Typical non-circular slip surfaces [9]
23 Definition of FoS
FoS is usually defined as the ratio of the ultimate shear strength to the shear stress
mobilized at imminent failure There are several ways to formulate the FoS The most
common formulation assumes the safety factor to be constant along the sliding surface
and it is defined in relation to limit equilibrium force and moment equilibrium [4]
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
11
These definitions are given in Figure 23 below As will be developed further in the
limit equilibrium method the first definition is based on the shear strength which can
be obtained in two ways a total stress approach (su‐analysis) and an effective stress
approach (crsquo- φrsquo minusanalysis) The type of strength consideration depends on the soil
type the loading conditions and the time elapsed after excavation The total stress
strength method is used for shortndashterm conditions in cohesive soils whereas the
effective stress method is used in long- term conditions in all soil types or in short-term
conditions in cohesive soils where the pore pressure is known [3]
Figure 23 Various definitions for FoS [3]
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
12
24 Slope Stability Analysis Methods
Slope stability problems deal with the condition of ultimate failure of a soil or rock
mass Analyses of slope stability bearing capacity and earth pressure problems all fall
into this area The stability of a slope can be analysed by a number of methods among
others of which are the
Limit analysis method
Variational calculus method
Strength reduction method and
Limit equilibrium method
241 Limit Analysis Method
The limit analysis method theory is based on a rigid-perfectly plastic model material
Drucker and Prager [10] first formulated and introduced the upper and lower bound
plasticity theorems for soilrock masses
The general analysis process includes construction of a statically admissible stress field
for the lower-bound analysis or a kinematically admissible velocity field for the upper-
bound analysis
For both upper- and lower-bound analysis one of the following two conditions has to
be satisfied
Geometrical compatibility between internal and external displacements or
strains This is usually concerned with kinetic conditions ndash velocities must be
compatible to ensure no gain or loss of material at any point
Stress equilibrium ie the internal stress fields must balance the externally
applied stresses (forces)
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
13
The basis of limit analysis rests upon two theorems which can be proved
mathematically In simple terms these theorems are
Lower Bound any stress system in which the applied forces are just sufficient
to cause yielding
Upper Bound Any velocity field that can operate is associated with an upper
bound solution
The lower- bound approach has been used in 2D slope stability analysis by Zhang [11]
and Kim et al [12] The upper-bound approach was first used in 2D slope stability
analysis by Drucker and Prager [10] to determine the critical height of a slope
Subsequently Refs [13 14 amp 15] also applied and extended the upper-bound
approaches in 2D slope analysis Michalowski [16] proposed an upper-bound approach
based on a translational failure mechanism The vertical slice techniques which are
often used in traditional limit equilibrium approaches were employed to satisfy the
force equilibrium condition for all individual slices Two extreme kinematical solutions
neglecting the inter-slice strength or fully utilizing the inter-slice strength of the soil
were then obtained The traditional limit equilibrium solutions for slices with a proper
implicit assumption of failure mechanism can fall into the range of these two extremes
Donald and Chen [17] also presented an upper-bound method on the basis of a multi-
wedge failure mechanism in which the sliding body was divided into a small number
of discrete blocks
242 Variational Calculus Method
The variational calculus approach does not require assumptions on the inter-slice
forces It was first used for 2D stability analysis by Baker and Garber [18] This
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
14
approach was subsequently employed by Jong [19] for vertical-cut analysis in cohesive
directionless soils Cheng et al [20] also used it in their research where they developed
a numerical algorithm based on the extremum principle by Pan [21] The formulation
which relies on the use of a modern try and error optimization method and can be
viewed as an equivalent form of the variational method in a discretized form but is
applicable for a complicated real life problem
243 Strength Reduction Method
In recent decades there have been great developments in the area of the strength
reduction method (SRM) for slope stability analysis The general procedure of the
SRM analysis is the reduction of the strength parameters by the FoS while the body
forces (due to the weight of soil and other external loads) are applied until the system
cannot maintain a stable state This procedure can determine the FoS within a single
framework for both two and three-dimensional slopes
The main advantages of the SRM are as follows
The critical failure surface is automatically determined from the
application of gravity loads andor the reduction of shear strength
It requires no assumption about the distribution of the inter-slice shear
forces
It is applicable to many complex conditions and
It can give information such as stresses movements (deformations) and
pore pressures
One of the main disadvantages of the SRM is the long time required to develop the
computer model and to perform the analysis to arrive at a solution With the
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
15
development of computer hardware and software 2D -SRM can now be done in a
reasonable amount of time suitable for routine analysis and design This technique is
also adopted in several well-known commercially available geotechnical finite element
or finite difference programs In strength reduction analysis the convergence criterion
is the most critical factor in the assessment of the FoS
Investigation results show that the FoS obtained and the corresponding slip surface
determined by the SRM demonstrate good agreement with the results of the Limit
Equilibrium Method (LEM)
244 General Discussion on Limit Equilibrium Method
2441 Swedish Slip Circle Method of Analysis
The Swedish Slip Circle method assumes that the friction angle of the soil or rock is
zero In other words when angle of friction is considered to be zero the effective stress
term tends to zero which is therefore equivalent to the shear cohesion parameter of the
given soil The Swedish slip circle method assumes a circular failure interface and
analyses stress and strength parameters using circular geometry and statics as shown in
Figure 24 The moment caused by the internal driving forces of a slope is compared
with the moment caused by resisting forces in the sliding mass If forces resisting
movement are greater than the forces tending to cause movement then the slope is
assumed to be stable
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
16
Figure 24 Swedish Slip Circle Method [3]
2442 Method of Slices
Despite all the above methods limit equilibrium methods are by far the most used form
of analysis for slope stability studies They are the oldest best-known numerical
technique in geotechnical engineering These methods involve cutting the slope into
fine slices so that their base can be comparable with a straight line The governing
equilibrium equations equilibrium of the forces andor moments Figure 25 are then
developed According to the assumptions made on the efforts between the slices and
the equilibrium equations considered many alternatives have been proposed in Table
21 They give in most cases quite similar results The differences between the values
of the FoS obtained with the various methods are generally below 6 [22]
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
17
Figure 25 Slice Discretization and Slice Forces in a Sliding Mass [2324]
The idea of dividing a potential sliding mass into slices dates back to the early 1900rsquos
The first documented use of the method of slices is the analysis of the 1916 failure at
the Stigberg Quay in Gothenburg Sweden [25]
This Limit Equilibrium method is well known to be a statically indeterminate problem
and assumptions about the inter-slice shear forces are needed to make the problem
statically determinate On the basis of the assumptions on the internal forces and the
force andor moment equilibrium there are more than ten methods developed for
analysis of slope stability problems [26] Famous methods include those by Fellenius
[27] Bishop [28] Janbu [29 30] Spencer [31] Morgenstern-Price [32] and the General
Limit Equilibrium (GLE) [25]
Table 21 shows the differences between the various methods of stability analysis on
the basis of forces and moments equilibrium
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
18
Table 21 The Main Limit Equilibrium Methods [5]
No Methods Moment
Equilibrium
Force
Equilibrium
Inter
Slice
Normal
Forces
Inter
Slice
Shear
Forces
Moment
Factor of
Safety
Force
Factor of
Safety
Inter Slice
Force Function
1 Fellenius Swedish Circle or
Ordinary Method (1936) Yes No No No Yes No No
2 Bishop Simplified (1955) Yes No Yes No Yes No No
3 Janbu Simplified (1954) No Yes Yes No No Yes No
4 Spencer Method (1967) Yes Yes Yes Yes Yes Yes Constant
5 Morgenstern-Price Method
(1965) Yes Yes Yes Yes Yes Yes
Constant Half-
Sine
Clipped-Sine
Trapezoidal
Specified
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
19
All limit equilibrium methods utilise the Mohr‐Coulomb criterion to determine the
shear strength (τf) along the sliding surface The mobilised shear stress at which a soil
fails in shear is defined as the shear strength of the soil According to Janbu [29] a state
of limit equilibrium exists when the mobilised shear stress (τ) is expressed as a fraction
of the shear strength Nash [33] states that at the moment of failure the shear strength
is fully mobilised along the failure surface when the critical state conditions are
reached The shear strength is usually expressed by the Mohr‐ Coulomb linear
relationship as
119891 = 119888 + tan() (21)
and
= 120591119891119865119900119878 (22)
where
c is the soil cohesion and is the soil frictional angle
The strength of the soil available depends partially on the type of soil and the normal
stresses acting on it The mobilized shear strength on the other hand depends on the
external forces acting on the soil mass This defines the FoS as a ratio of the τf to τ in a
limit equilibrium analysis [30] as defined in Equation 22
25 General Limit Equilibrium Method of Slices (GLE method)
The GLE formulation was developed by Fredlund at the University of Saskatechewan
[34 35] This method encompasses the key elements of all the other methods of slices
(the Ordinary Bishoprsquos Janbu and M-P methods) The GLE formulation is based on
two factors of safety equations One equation gives the factor of safety with respect to
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
20
moment equilibrium Fm (Equation 23) while the other equation gives the factor of
safety with respect to horizontal force equilibrium Ff (Equation 24)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (23)
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (24)
Where
l is the length of the bottom of the slice R is the radius of the slip circle W is the weight
of slice Ni is the total normal force on the bottom of the slice c is the soil cohesion
microis the pore water pressure α is the inclination of the slip surface at the middle of the
slice
26 The Ordinary or Fellenius Method (OMS)
The ordinary method is considered the simplest of the methods of slices since it is the
only procedure that results in a linear factor of safety equation It is generally stated that
the inter-slice forces can be neglected because they are parallel to the base of each slice
[26] This notwithstanding the Newtons principle of action equals reaction is not
satisfied between slices The change in direction of the resultant inter-slice forces from
one slice to the next results in factor of safety errors that may be as much as 60 [37]
The normal force on the base of each slice is derived either from summation of forces
perpendicular to the base or from the summation of forces in the vertical and horizontal
directions The forces considered to act in this method are represented in Figure 26
below The FoS is based on moment equilibrium and computed as
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
21
Figure 26 Forces considered in the Ordinary Method of Slices
119865119898 =119888lowast119897+119873lowasttan()
119882lowastsin(120572) (25)
N = W lowast cos(α)ndash microl (26)
where
W is the weight of each slice
microis the pore water pressure
l is the base length of the slice
α is the inclination of the slip surface at the middle of the slice
27 Simplified Bishoprsquos Method
The simplified Bishop method neglects the inter-slice shear forces and thus assumes
that a normal or horizontal force adequately defines the inter-slice forces as shown in
Figure 27 below The normal force on the base of each slice is derived by summing
forces in a vertical direction The factor of safety is derived from the summation of
moments about a common point as in OMS since the inter-slice forces cancel out
Therefore the factor of safety equation is the same as for the ordinary method [28]
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
22
However the definition of the normal force is different The FoS is based on moment
equilibrium and computed as
Figure 27 Forces considered in the Simplified Bishoprsquos Method
119865119898 =119888lowast119897+119873lowasttan()
119882lowastx (27)
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119898]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119898
(28)
where lsquoxrsquo is the horizontal distance from the mid-base of the slice to the centre of
rotation
As can be seen in the equation above the expression is nonlinear due to the appearance
of Fm in both sides of the equation and will require an iterative procedure to reach a
solution
28 Janbursquos Simplified Method
In Janbus simplified method the normal force in each slice is derived from the
summation of vertical forces with the inter-slice shear forces ignored The horizontal
force equilibrium equation is used to derive the factor of safety The sum of the inter-
slice forces must cancel and FoS equation becomes
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
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[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
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[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
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Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
23
119873119894 =119882minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865119891]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865119891
(29)
119865119891 =sum119888lowast119897lowastcos(120572)+(119873119894minusμlowast119897)lowastcos(120572)lowasttan()
119873119894lowastsin(α) (210)
29 Morgenstern-Price (M-P) Method
This has become the most widely used method developed for analysing generalized
failure surfaces The method was initially described by Morgenstern and Price [36]
The Morgenstern‐Price method also satisfies both force and moment equilibriums and
the overall problem is made determinate by assuming a functional relationship between
the inter-slice shear force and the inter-slice normal force According to Morgentern
and price [32] the inter-slice force inclination can vary with an arbitrary function f(x)
as
119883119894 = 119864119894 lowast λ lowast f(x) (211)
where
f(x) = inter-slice force function that varies continuously along the slip surface and
λ = scale factor of the assumed function
The method suggests assuming any type of force function f(x) for example half‐sine
trapezoidal or user defined as shown in Figure 29 The relationships for the base normal
force (N) and inter-slice forces (E X) are the same as given in Janbursquos generalised
method The forces considered to act in this method are represented in figure 28 below
For a given force function the inter-slice forces and the Newton-Raphson numerical
technique can be used to solve the moment and force equations for the FoS and until
Ff is equals to Fm in equations (212) and (213) [33] below
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
24
Figure 28 Forces considered in the M-P method [24]
119865119891 = sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (212)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (213)
where
119873119894 =119882minus∆119883119894minus[
119888lowast119897lowastsin[120572119894]minus119906lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
Cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(214)
F is Fm or Ff depending on which equilibrium equation is being solved
and
∆119883119894 =∆119864119894 lowast λ lowast f(x) (215)
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
25
Figure 29 Inter-slice force function types
An alternative derivation for the Morgenstern-Price method was proposed by Fredlund
and Krahn in [34] It presents a complete description of the variation of the factor of
safety with respect to λ On the first iteration the vertical shear forces are set to zero
On subsequent iterations the horizontal inter-slice forces are first computed and then
the vertical shear forces are computed using an assumed λ value and side force function
Fm and Ff are solved for a range of λ values and a specified side force function These
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
26
FoS are plotted on a graph as shown in Figure 210 The FoS vs λ are fit by a second
order polynomial regression and the point of intersection satisfies both force and
moment equilibrium
Figure 210 Variation of FoS with respect to Fm and Ff vs λ for the M-P method [34]
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
27
CHAPTER THREE RESEARCH METHODOLOGY AND SOFTWARE
USED
31 Selection of Factor of safety method
There are different limit equilibrium methods having varying superiorities over each
other as discussed in Chapter 2 Methods are based on different assumptions on
equilibrium conditions to be satisfied Duncan and Wright [24] summarized some of
limit equilibrium methods with respect to their limitations assumptions and
equilibrium conditions to be satisfied as shown in Table 31
Table 31 Brief Comparison of Limit Equilibrium Methods [24]
Procedure Use
Swedish circle method Applicable to slopes where = 0
Ordinary method of slices
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle Very convenient for hand calculations In
accurate for effective stress analysis with high pore water pressure
Simplified Bishop method
Applicable to non-homogeneous slopes soils where slip surface can be
approximated by a circle More accurate than ordinary method of slices
especially for analysis with high pore water pressures Calculations
feasible by hand or spreadsheet
Spencers method
An accurate procedure applicable to virtually all slope geometries and soil
profiles The simplest complete equilibrium procedure for computing
FoS
Morgenstern and Prices
method
An accurate procedure applicable to virtually all slope geometries and soil
profiles Rigorous well-established complete equilibrium procedure
Requires solution of nonlinear equations with an iterative procedure
The analysis and design of failing slopes requires an in-depth understanding of the
failure mechanism in order to choose the right slope stability analysis method
In this research as stated above the slice approach for GLE procedure is used and the
FoS according to the Morgenstern-Pricersquos procedure which satisfies all the
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
28
requirements for static equilibrium is adopted Regardless of whether equilibrium is
considered for a single free body or a series of individual vertical slices there are more
unknowns (forces locations of forces factor of safety etc) than the number of
equilibrium equations the problem of computing a FoS is thus statically indeterminate
Therefore assumptions must be made to achieve a balance of equations and unknowns
This method allows for analysis of any failure shape (circular non-circular or
compound) The solution for the (FoS) is derived from the summation of forces
tangential and normal to the base of a slice and the summation of moments about the
centre of the base of base slice The steps required to provide the input data for
performing the slope stability analysis include [37]
A survey of the elevation of the ground surface on a section perpendicular to
the slope
Estimation of ground stratigraphy from borehole logs and soilrock properties
from engineering soilrock tests
The determination of ground water level from piezometer readings to estimate
ground water pore-water pressures
32 Morgenstern-Price Method
Morgenstern and Price [32] developed the method for FoS similar to the Spencer
method [31] The method considers both normal inter slice force and shear forces
Therefore it satisfies both moment and force equilibrium
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
29
33 Assumptions
To begin with the generalized formulation the following assumptions are made for a
body of mass slope
1 The failure surface is assumed to be circular
2 The soilrock is a homogeneous
3 The soilrock is an isotropic material
4 The failure mass is a rigid body
5 The base normal force acts at the middle of each slice
6 The Mohr-Coulomb failure criterion is used
34 Numerical Method Development
In this work the geotechnical generalized method of slices approach which is a form
of the Finite Strip Method would be used for the computation of the Factor of Safety
(FoS) which is a measure of the degree of safety of the slope The Finite Strip Method
is a variant form of the known differential equation numerical discretization
approaches It discretizes the equilibrium differential equation (based on slices) and
imposes equilibrium boundary conditions at the ends to solve for the required
unknowns
The method of slices method is a numerical approach used to solve the equilibrium
differential equation at the limiting condition ie at the point of slope failure The
method consists of dividing the slope into a number of fine slices so that their bases can
be comparable with a straight line The equilibrium differential equations are then
developed for each slice and then the global force and moment equilibrium problem
solved numerically to obtain the FOS [24]
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
30
This study will make available a locally developed slope stability program that would
simulate the behaviour of a simple slope undergoing circular failure The results from
this work which will include both factor of safety for moment and force equilibrium
will be visualized in the command window of MATLAB
35 Derivation of Equations
Figure 31 is a sketch of a slope section and Figure 32 is a sketch of a slice of mass
from Figure 31 with the forces acting on it at the point of failure By taking moment
about the midpoint of the base of the slice in Figure 31
119864prime [(119910 minus 119910119905prime) minus (minus
dy
2)] minus (119864prime + dEprime) [119910 + dy minus 119910119905
prime minus dy119905prime + (
dy
2)] minus 119883 (
dx
2) minus (119883 +
dX) (dx
2) = 0 (31)
After simplification and proceeding to the limit as dxrarr0 it can be shown readily that
119883 =119889(119864prime119910119905
prime)
dxminus 119910(
dEprime
dx) (32)
Figure 31 Sketch of a Slope Section [23]
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
31
Figure 32 Forces acting on a Single Slice from a Mass Slope [24]
For equilibrium in the N direction we have from Figure 32
dNprime = dWcos[120572] minus dXcos[120572] minus dEprimesin[120572] (33)
For equilibrium in the S direction we have from Figure 32
dS = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (34)
The Mohr-Coulomb failure criterion in terms of effective stresses may be expressed as
dS =1
119865119888primedxsec[120572] + (dNprime)tan[120601prime] (35)
Equation (35) defines the factor of safety in terms of shear strength
Eliminating dS from Equations (34) and (35)
1
119865[119888primedxsec[120572] + (dNprime)sin[120601prime]] = dEprimecos[120572] minus dXsin[120572] + dWsin[120572] (36)
By eliminating dNprime from Equations (33) and (36) and dividing by dxcos[120572] we have
119888prime
119865sec2[120572] +
tan[120601prime]
119865[dW
dxminus
dX
dxminus
dEprime
dxtan[120572]] =
dEprime
dxminus
dX
dx+
dW
dxtan[120572] (37)
In the specified co-ordinate system tan[120572]=-dy
dx and equation (37) becomes
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
32
119888prime
119865[1 + (
dy
dx)2
] +tan[120601prime]
119865[dW
dxminus
dX
dx+
dEprime
dx(dy
dx)] =
dEprime
dxminus
dX
dxminus
dW
dx(dy
dx) (38)
Therefore the governing differential equation becomes using equations (32)
dEprime
dx[1 minus
tan[120601prime]
119891(dy
dx)] +
dX
dx[tan[120601prime]
119865+
dy
dx] =
119888prime
119865[1 + (
dy
dx)2] +
dW
dx[tan[120601prime]
119865+
dy
dx](39)
Ersquo and Ersquo+dErsquo are the horizontal inter-slice forces
X and X+dX are the vertical inter-slice forces
120601prime is the effective friction angle
W is the weight of ith slice
dN and dS are resultants of the normal and tangential forces on the slice base of length
li
α is the inclination of the slice with respect to the horizontal
F is the factor of safety
The governing differential equation is developed for each slice and summed up to
develop the equilibrium equation for entire mass Based on the assumption on the inter-
slice forces as provided in Chapter 2 the various expressions for Fm Ff N X amp E for
the different slices are computed
36 Structured Program
Below (Appendix A) is an algorithm developed to solve for the force and moment factor
of safetyrsquos (Ff and Fm) using the M-P method
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
33
37 Numerical Algorithm
Figure 33 shows the numerical algorithm used to solve the problem Equation numbers
referred to here are that from appendix A
Figure 33 Algorithm flowchart for solving for the FoS
No
Yes
Input H
Xc Yc R
Generate ytop
and ybot eq
1amp2
Input c and phi
Compute lb W and alpha-eq
10 7amp 6
Compute FSom-eq 15 amp16
Set lambda and
initial FoS for the
first iteration
Is FSm-FoSlttol
Is FSf-FoS lttol else
NFSm=FSm NFSf=FSf
Increase
Compute N FSm FSf and
∆E-eq 17 15 16 amp11
Output
FSm(i)
FSf(i)
Output
FSm FSf
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
34
38 Software and Programs Used
381 SLOPEW
SLOPEW developed by GeoStudio International Canada is part of a professional
geotechnical software suite of programmes SLOPEW is the programme used for slope
stability analysis This software is based on the theories and principles of the LE
methods discussed in the previous sections To check the accuracy of the program
written in MATLAB and its output SLOPEW is employed For this study a full
licensed version of Geostudio 2007 has been used
For each model using the drawing tools the geometry of the slope is entered Then by
using the slip surface dialogue box the slip surface is specified by using a range
Then by using the ldquoMaterialsrdquo dialogue box soil parameters will be entered and the
selected soil will be assigned to the drawing in the software
After entering all of the input data into the software by hitting the ldquoStartrdquo button under
the ldquoSolve Managerrdquo the program starts to analyze the slope and find the minimum
factor of safety and its related failure surface Figures 34 to 36 show some of the stages
of developing the model using SLOPEW
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
35
Figure 34 SLOPEW KeyIn Analyses Page
Figure 35 SLOPEW KeyIn Material Page
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
36
Figure 36 SLOPEW KeyIn Entry and Exit Range Page
382 MATLAB
MATLAB (Matrix Laboratory) is a multi-paradigm numerical computing environment
and fourth-generation programming language A proprietary programming language
developed by MathWorks MATLAB allows matrix manipulations plotting of
functions and data implementation of algorithms creation of user interfaces and
interfacing with programs written in other languages including C C++ Java Fortran
and Python [38] The programme is also easily accessible and is available in most
design engineering offices These features of MATLAB make it suitable to be used for
this research work
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
37
CHAPTER FOUR RESULTS AND DISCUSSION
41 Introduction
This chapter presents outputs from the MATLAB code and SLOPEW programmes and
compares the results of the two First the analysis is run for assumed models (the same
geometry soil properties) of homogeneous soil mass and modelled for two failure
scenarios ie for toe and base failure conditions
The second set of comparison is done for existing case studies Case one is the study
from Griffiths and Lane [39] and Rocscience [40] and case two is from the study by
ACADS [41]
The MATLAB script is presented in appendix B
42 Programme Test Examples
The program was tested with two failure mechanisms toe and base failures In slope
stability design and analysis and for most design projects the worst case scenario is
always adopted The worst case when it comes to FoS in slope stability analysis is the
least FoS This is adopted in choosing the FoS in this MATLAB program in situations
when more than one FoS is outputted
421 Toe Failure
Two models were presented for the toe failure mechanism The slope angles for both
models are 45deg or 1H 1V (H horizontal distance V vertical distance) The rest of the
soil parameters and geometry are presented in Tables 41 and 42
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
38
Case 1
Table 41 Slope Dimensions and Material Properties for Toe Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 15 38 18 5
Figure 41 SLOPEW Output of Toe Failure Case 1
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
39
Figure 42 MATLAB Output of Toe Failure Case 1
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
40
Case 2
Table 42 Slope Dimensions and Material Properties for Toe Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 5 28 16 5
Figure 43 SLOPEW Output for Toe Failure Case 2
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
41
Figure 44 MATLAB Output for Toe Failure Case 2
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
42
It is noted from the outputs that the MATLAB code give reasonable outputs of FoS
compared to SLOPEW In both cases the percentage error is found to be 4 The
small error margin is an indication that the developed MATLAB code is working to an
acceptable degree
422 Base Failure
Two models were presented for the base failure mechanism The slope angles for both
models are 1H 1V The rest of the soil parameters and geometry are presented in Tables
43 and 44
Case 1
Table 43 Slope Dimensions and Material Properties for Base Failure Case 1
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 1 40 20 20 5
Figure 45 SLOPEW Output for Base Failure Case 1
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
43
Figure 46 MATLAB Output for Base Failure Case 1
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
44
Case 2
Table 44 Slope Dimensions and Material Properties for Base Failure Case 2
cacute rsquo γ H
(kPa) () (kNm3) (m)
Case 2 0 15 18 5
Figure 47 SLOPEW Output for Base Failure Case 2
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
45
Figure 48 MATLAB Output for Base Failure Case 2
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
46
43 Comparison with Cases from Literature
431 Case 1
The geometry of a homogeneous slope without foundation is shown in Figure 49 This
case follows the analyses performed by [39] and [40] which were used as benchmark
cases to study the applicability of 3-D FE analyses to slope stability The slope angle is
2625deg or 2H 1V for this case According to Griffiths and Lane the adopted parameters
are based on cʹγH = 005 The height of the slope is assumed to be 40 m thus the
corresponding parameters are summarized in Table 45
Figure 49 Homogeneous Slope without Foundation
Table 45 Slope dimensions and material properties
cacute rsquo γ H
(kPa) () (kNm3) (m)
40 20 20 40
The slope stability analyses using the computer program SLOPEW is shown in Figure
410 The FoS based on the GLE and the adopted M-P approach is conducted for both
toe and slope failures The FoS for toe and slope failures are found to be 1385 and
1373 respectively
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
47
The same analysis was conducted using the MATLAB code the FoS for toe and slope
failures are found to be 1431and 1375 respectively and shown in Figure 410-411
Figure 410 Analysis using SLOPEW - FoS = 1385
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
48
Figure411 FoS based on M-P approach for Toe failure - FoS = 1431
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
49
Figure 412 FoS based on M-P approach for Base Failure FoS = 1375
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
50
432 Case 2
In 1988 a set of 5 basic slope stability problems together with 5 variants was
distributed both in the Australian Geomechanics profession and overseas as part of a
survey sponsored by ACADS [41]
This problem is taken from the verification manual of Slide 30 (Verification 1) It was
distributed to the Australian Geomechanics profession and overseas in 1988 with some
other models to verify the efficiency and accuracy of Slide 30 The slope model
geometry is presented in Figure 413 The slope material properties are shown in Table
46
Figure 413 Slope Model Geometry from Slide 30 [41]
Table 46 Slope Dimensions and Material Properties for ACAD Problem
cacute γ H
(kPa) () (kNm3) (m)
3 196 202 10
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
51
Figure 414 FoS based on M-P Approach for ACAD Problem
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
52
The FoS from this case study when modelled with Slide 30 and the GLE (M-P)
approach adopted was found to be 0986 Upon modelling the same problem with the
MATLAB code as shown in Figure 414 above the FoS is found to be 1035 Again
this can be said to be a reasonably accurate estimate of the FoS by the MATLAB code
The percentage error is 47
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
53
CHAPTER FIVE CONCLUSIONS
51 Conclusions
The primary focus of this research was to
Develop a MATLAB code for solving the FoS of homogeneous earth slopes
Verify and validate the code with SLOPEW models and also with cases from
literature
Seven cases were studied in this research as summarized below in Table 51
Table 51 Summary of FoS Outputs for all Case Studies
Case 1 Case 2
Percentage
Error ()
SLOPEW MATLAB SLOPEW MATLAB Case 1 Case 2
Toe Failure 5649 5406 3659 3514 4 4
Base
Failure 3152 3168 0611 0614 05 05
Comparison with Literature
Griffiths
and Lane MATLAB
Slide
(Rocscience) MATLAB Case 1 Case 2
Toe Failure 1385 1431 32
Base
Failure 1373 1375 0986 1035 01 47
From the Table 51 above it is seen that the MATLAB program gives a good estimate
of the FoS when compared with SLOPEW and some other problem evaluated with
different programs for a homogeneous soil material Two different failure mechanisms
are shown (ie toe and base failures) It is noted that the maximum percentage error
in the M-P FoS is 5
It is anticipated that with the availability of this computer code Ghanaian Engineers
can more readily assess the safety of slopes in routine design works
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
54
52 Recommendations
The current work could be extended in the future to include the following
The provision for a more user friendly interface and development of the
programme into a stand-alone interactive program
The direct extension of the model to cater for heterogeneous soil and rock
material
The direct extension of the model to include earthquake effect surcharge
loading and increase in pore water pressures
It is hoped that at the end of these improvements this work that has started during this
research study would be found in most geotechnical design offices in Ghana and other
less-developed countries who cannot afford the existing expensive commercial
software available
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
55
REFERENCES
[1] Wikepedia httpsenwikipediaorgwikiSlope_stability accessed June 2016
[2] Shioi Y and Sutoh S Collapse of high embankment in the 1994 far-off Sanriku
Earthquake Slope Stability Engineering Eds Yagi Yamagami amp Jiang Balkema
Rotterdam pp559-564 1999
[3] Abramson L W Lee T S Sharma S and Boyce G M Slope Stability and
Stabilisation Method 2nd edition John Wiley amp Sons New York 2002
[4] Rouaiguia A Dahim M A Numerical modeling of slope stability analysis
IJESIT Vol 2 No 3 May 2013
[5] Ayetey J K The extent and effects of mass wasting in Ghana Bulletin of the
International Association of Engineering Geology No 43 Paris 1991
[6] CitifmOnline httpcitifmonlinecom20140609death-trap-rocks-hang-
dangerously-along-ayi-mensah-peduase-road accessed May 2016
[7] GraphicOnline httpgraphiccomghnewsgeneral-news22493-danger-
looms-on-aburi-accra-road-as-rocks-fall-from-cliffhtml accessed May 2016
[8] GraphicOnline httpgraphiccomghnewsgeneral-news8384-landslide-can-
occur-around-akoasa-mountainhtml accessed May 2016
[9] DeNatale J S Rapid identification of critical slip surfaces structure Journal
of Geotechnical Engineering ASCE Vol 117 No 10 pp 1568-1589 1991
[10] Drucker D C and Prager W Soil mechanics and plastic analysis or limit
design Quarterly Journal of Applied Mathematics Vol 10 pp 157-165 1952
[11] Zhang X Slope stability analysis based on the rigid finite element method
Geotechnique Vol 49 No 5 pp 585-593 1999
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
56
[12] Kim J Salgado R and Lee J Stability analysis of complex soil slopes using
limit analysis Journal of Geotechnical and Geoenvironmental Engineering ASCE
Vol 128 No 7 pp 546-557 2002
[13] Chen W F Limit Analysis and Soil Plasticity Elsevier Scientific Publishing
Co New York 1975
[14] Karal K Energy method for soil stability analyses Journal of the Geotechnical
Engineering Division Vol 103 No 5 pp 431-445 1977
[15] Izbichki R J Limit plasticity approach to slope stability problems Journal of
the Geotechnical Engineering Division Vol 107 No 2 pp 228-233 1981
[16] Michalowski R L Slope stability analysis a kinematical approach
Geotechnique Vol 45 No 2 pp 283-293 1995
[17] Donald I and Chen Z Y Slope stability analysis by the upper bound
approach fundamentals and methods Canadian Geotechnical Journal 34 853-862
1997
[18] Baker G and Garber M Theoretical analysis of the stability of slopes
Geotechnique Vol 28 No 4 pp 395-411 1978
[19] Jong G D Application of the calculus of variations to the vertical cut off in
cohesive frictionless soil Geotechnique Vol 30 No 1 pp 1-16 1980
[20] Cheng Y M Zhao Z H and Wang J A Realization of Pan Jiazhengrsquos
extremum principle with optimization methods Chinese Journal of Rock Mechanics
and Engineering 27(4) pp 782-788 (in Chinese) 2008
[21] Pan J L Goh A T C Wong K S and Selby A R 2002 Three-
dimensional analysis of single pile response to lateral soil movements International
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
57
Journal for Numerical and Analytical Methods in Geomechanics 26(8) pp 747-758
2002
[22] Duncan JM State of art limit equilibrium and finite-element analysis of
slopes Journal of Geotechnical Engineering Vol 122 No 7 pp 557-596 1996
[23] Duncan JM and Wright SG Soil Strength and Slope Stability John Wiley
amp Sons Inc 2005
[24] Zhao Y Tong Z Luuml Q Slope Stability analysis using slice-wise factor of
safety Mathematical Problems in Engineering Volume 2014 Paper No 712146 6
pages 2014
[25] Petterson KE The early history of circular sliding surfaces Geotechnique
Vol 5pp 275-296 1955
[26] Krahn J Stability Modeling with SLOPEW An Engineering Methodology
July 2012 Edition Geo-Slope International 2012
[27] Fellenius W Calculation of stability of earth dams Proceedings of the 2nd
Congress Large Dams pp 445-463 Washington D C 1936
[28] Bishop A W The use of the slip circle in the stability analysis of slopes
Geotechnique Vol 5 No 1 pp 7-17 1955
[29] Janbu N Slope Stability Computations Embankment-Dam Engineering
Casagrande Volume Ed R C Hirschfield and S J Poulos John Wiley and Sons New
York pp 47-86 1973
[30] Janbu N Application of composite slip surface for stability analysis European
Conference on Stability of Earth Slopes Vol 3 pp39-43 Stockholm 1954
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
58
[31] Spencer E A method of analysis of embankments assuming parallel inter-slice
forces-Geotechnique Vol 17(1) pp 11-26 1967
[32] Morgenstern N R and Price V E The analysis of stability of general slip
surfaces Geotechnique Vol 15 No 1 pp 77-93 1965
[33] Nash D A comparative review of limit equilibrium methods of stability
analysis Slope Stability Ed M G Anderson and K S Richards pp 11 -75 1987
[34] Fredlund DG and Krahn J Comparison of slope stability methods of
analysis Canadian Geotechnical Journal Vol 14 No 3 pp 429-439 1977
[35] Fredlund DG Krahn J and Pufahl DE The relationship between limit
equilibrium slope stability methods Proceedings of the International Conference on
Soil Mechanics and Foundation Engineering Vol 3 pp409-416 1981
[36] Whitman R M and Bailey W A Use of computers for slope stability
analysis Journal of the Soil Mechanics and Foundation Division ASCE Vol 93 No
SM4 pp 475-498 1967
[37] Baba K Bahi L Ouadif L and Akhssas A Slope stability evaluations by
limit equilibrium and finite element methods applied to a railway in the Moroccan Rif
Open Journal of Civil Engineering Vol 2 pp 27-32 2012
[38] Matlab httpsenwikipediaorgwikiMATLAB accessed April 3 2016
[39] Griffiths DV and Lane PA Slope stability analysis by finite elements
Geotechnique Vol 49 No 3 pp 387-403 1999
[40] Rocscience Inc Application of the Finite Element Method to Slope Stability
Rocscience 2004
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
59
[41] ACADS 2D elasto-plastic finite element program for slope and excavation
stability analyses - Slope Stability Verification Manual Rocscience 2011
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
60
APPENDICES
Appendix A Structured Programme
Below is an algorithm developed to solve for the force and moment factor of safetyrsquos
(Ff and Fm) using the M-P method
1 By using the equation of a straight line define the slope surface
119910top = 119898 lowast 119909 + 119888 (1)
2 By using the equation of a circle define the slip surface
119910bot = 119910119888 minus radic(abs(1198772 minus (119909 minus 119909119888)2 (2)
3 By using the equation of a straight line define the piezometric line
119910w = 119898 lowast 119909 + 119888 (3)
4 Divide the slope into n number of slices by vertical lines
5 For each slice the width dx bottom inclination α and average height havg are
determined
ℎavg =ℎ119894+ℎ119894+1
2 (4)
dx =119883
119899119904minus1 (5)
120572 = tanminus1((ℎ119894+1minusℎ119894)
dx) (6)
6 The area of the slice A is computed by multiplying the width of the slice (dx)
by the average height havg
7 The weight W of the slice is computed by multiplying the area of the slice by
the total unit weight of soil
8 W = γA (7)
9 If piezometric height is above slip surface continue with step 9 else skip to
step 17
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
61
10 The piezometric height is determined at the downslope boundary centre and
upslope boundary of each slice The piezometric height at the downslope and
upslope boundaries of the slice hb and ht respectively are used to compute the
forces from water pressures on the sides of the slice Here a triangular
hydrostatic distribution of pressures is assumed on the sides of the slice The
piezometric height at the centre of the slice hp represents the pressure head for
pore water pressures at the base of the slice
11 Hydrostatic forces from water pressures on the sides of the slice are computed
from
micro119887 =ℽ119908lowastℎ119887
2
2 (8)
micro119905 =ℽ119908lowastℎ119905
2
2 (9)
12 The pore water pressure is computed by multiplying the piezometric head at the
centre of the base of the slice by the unit weight of water micro = Ɣw hp
13 The length of the bottom of the slice l is determined the length can be
computed from the width dx and base inclination
119897 =119889119909
cosα (10)
14 A trial FoS (F) is assumed and a λ ranging from 0 to 1 set
15 Beginning with the first slice at the toe
∆119864119894 =119882[sin[120572119894]minus
tan[120601]cos[120572119894]
119865]minus
cl
119865
cos[120572119894]+tan[120601]sin[120572119894]
119865
(11)
16 The inter-slice shear force X for each slice is calculated from equationhellip
∆119883119894 = ∆119864119894λf(119909) (12)
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
62
Where λ is the percentage (in decimal form) of the function used and f(x) is
inter-slice force function representing the relative direction of the resultant
inter-slice force given by
119891(119909) = sin[(119883119894+1minus119883119894
119883119899minus119883119894) lowast 120587] (13)
17 The normal force N at the base of each slice is calculated from
119873119894 =119882minus(∆X)minus[
119888lowast119897lowastsin[120572119894]minus120583lowast119897lowastsin[120572119894]lowasttan[120601]
119865]
cos[120572119894]+sin[120572119894]lowasttan[120601]
119865
(14)
18 The new factor of safetyrsquos Ff and Fm are calculated from
Ff= sum119888lowast119897lowastcos[120572119894]+(119873119894minusμ119897)lowasttan[120601]lowastcos[120572119894]
119873119894lowastsin[120572119894] (15)
119865119898 = sum119888lowast119897lowast119877lowast119882lowastcos[120572119894]minusμl)lowast119877lowasttan[120601]
119882lowast119909 (16)
19 Iterate for a number of slip surfaces and determine the slip surface with the least
FoS
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
63
Appendix B MATLAB Code for Solving FoS
this code calculates the factor of safety of a homogeneous soil mass
using the method of slices and the M-P interslice force function
Clear all close all clearvars clc
Slope and soil parameters
H = 5 inpu(Enter the height of the slope) height of
slope
Xc =1 inpu(Enter the x coordinate of center of slip circle Xc should be
gt0)
x coordinate of center of circle
Yc = 7 input (Enter the y coordinate of center of slip circle Yc should
be greater than H) y coordinate of center of circle
R = 8 5 Yc+10 radius of center of slip circle
fr = 45 slope angle in degrees
ns = 31 number of slice faces
G = 20 unit weigh of soil
c = 40 cohesion of soil
phi = 20 frictional angle of soil (degrees)
tph = tand(phi) tangent of the frictional angle of soil
tfr = tand(fr) slope angle in gradient
Slip surface generation
xmin = Xc - sqrt(abs(R^2 - Yc^2)) Exit point of slip circle
xmax = Xc + sqrt(abs(R^2 - (Yc-H)^2)) Entry point of slip circle
x = linspace (xminxmaxns) Positions where the forces
will be analyzed
ybot = Yc - sqrt(abs(R^2-(x-Xc)^2)) Equation for generating slip
circle
Slope surface generation
ytop = (xgt=0)(xlt(Htfr))(xtfr) + (xgt=(Htfr))H Slope surface
equation of a line is used
figure(1)
hold on
plot (xybot-rxytop-r)
xlabel (Distance (m))
ylabel (Elevation (m))
plot(reshape([xxx]1[])reshape([ytopybotytopNaN]1[]))
hold off
Other parameters
hs = ytop-ybot Height at each node
havg = (hs(1end-1)+hs(2end))2 5) Average height of each slice
xavg = abs(((x(1end-1)+x(2end))2)-Xc) midpoint distance of each
slice
dx = (xmax-xmin)(ns-1) Width of each slice
yb = (diff(ybot))
alpha = rad2deg(atan2(ybdx)) Slice angle With atan2 the sign
is clear
sal = sind(alpha)
cal = cosd(alpha)
A = dxhavg 6) Area of each slice
W = GA 7) Weight of each slice
s= Rsal offset f for non-circular slip surfaces
l = dx cal 12) Length of the bottom of the slice assuming
straight border
u = 0
factor of safety computation
upO = sum((cl) + (Wcaltph) - (ultph))
downO = sum(Wsal)
FSom = upO downO
OFSm = FSom
NFSm = 12FSom initial guessed F
OFSf = FSom
NFSf = 12FSom
Tol =0001
t =-1020
y=t
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points
64
i=1
k=1
n=1 ns-1
for lbd=t
E (1) =0
d=1 ns-1
f=sind (((xavg (d)-xmin) (xmax-xmin))180)
X=Elbdf
while abs(OFSm-NFSm)gttol
OFSm = NFSm
Nbu = ((W-X) - ((clsal) - (ultphsal)) OFSm)
Nbd = cal + (tphsal) OFSm
Nob = Nbu Nbd
upb = sum((clR) + (NobtphR) - (ultphR))
downb = sum(Wxavg)
FSm = upb downb
FSmm = ((FSm)) (ns-1))
NFSm = FSmm
FSM (i)=FSmm
end
while abs(OFSf-NFSf)gttol
OFSf = NFSf
Nju = ((W-X) - ((clsal) - (ultphsal)) OFSf)
Njd = cal + (tphsal) OFSf
Noj = Nju Njd
upj = sum( (clcal) + (tphcalNoj) - (ultphcal) )
downJ = sum(Nojsal)
FSf = (upj downJ)
FSf = (FSf) (ns-1))
NFSf = FSf
FSF (i)= FSf
end
E = (((cl-ultph)cal) OFSm) + (salNob)-((tphcal)NobOFSm)
E = (((cl-ultph)cal) OFSm) + (salNoj)-((tphcal)NojOFSm)
OFSm = FSom
OFSf = FSom
i=i+1
end
fSM=FSM
fSF=FSF
figure(2)
plot(yfSM-ryfSF-b)
finding intersection point
[xi yi] = polyxpoly(y fSM y fSF)
xlabel(lambda)
ylabel(FoS)
legend(FSm FSf)
mapshow(xiyiDisplayTypepointMarkero MarkerEdgeColork)
axis normal
strValues = strtrim(cellstr(num2str([xi yi](03f03f))))
text (xiyistrValuesVerticalAlignmentbottom)
[xi yi] Display intersection points