lecture notes foundation engineering

Technical University of Mombasa

Department of Building and Civil Engineering

ECE 2414 Foundation Engineering II

Teaching notes

By Sixtus Kinyua Mwea

2016

Technical University of Mombasa - ECE 2414 -i-

Syllabus

ECE 2414 Foundation Engineering II

Foundation footings, strip, rafts, piles, piers and caissons. Foundation types: selection of suitable

types of foundations for given sittings such as footings, mats, strip, rafts, piles, piers. Site

investigations and exploration: planning, geological information, sub-surface exploration.

Retaining walls: design and failure modes, abutments, sheet piling and cofferdams. Site

investigations: boring and sampling, penetration tests, remote sensing, geophysical exploration.

Overlaps in yellow are topics which are spread in ECE 2311, 2406, 2414 and 2508

ECE 2311 Site investigations: reconnaissance, sampling, field test

ECE 2406 Site exploration: Planning, geological information, sub-surface exploration.

ECE 2414: Site investigations and exploration: planning, geological information, sub-surface

exploration.

Site investigations: boring and sampling, penetration tests,

ECE 2508: Advanced Geotechnical site investigations

Overlaps in green are topics which are spread in 2406, and 2414

ECE 2406: footings, mats and piles.

ECE 2414 Foundation types: selection of suitable types of foundations for given sittings

such as footings, mats, strip, rafts, piles, piers

ECE 2508: Load tests on piles.

Proposed ECE 2414 Foundation Engineering II

1. Foundation footings, strip, rafts Foundation types: selection of suitable types of

foundations for given sittings such as footings, mats, strip, rafts,, Foundation

characteristics of tropical and residual soils; properties, depth, and strength

2. Piles, piers and caissons piles Load tests on piles, piers

3. Retaining walls: design and failure modes, abutments, sheet piling and cofferdams,

4. Site investigations: reconnaissance, planning, geological information, sub-surface

exploration boring and sampling penetration tests field tests and. Advanced Geotechnical

site investigations; remote sensing and geophysical exploration

5. Excavation and bracing. Ground water; dewatering techniques. Laboratory work and

tutorials.

Technical University of Mombasa – ECE 2414 ii

Table of contents

Syllabus ....................................................................................................................................... i Proposed ECE 2414 Foundation Engineering II .................................................................... i Chapter one: - Shallow foundations ............................................................................................1

1.1 Types of foundations .................................................................................................1

1.2 Introduction to shallow foundations .......................................................................1

1.3 Proportioning of shallow foundations .....................................................................2

1.3.1 Contact pressure distribution ............................................................................... 2 1.3.1 Proportioning the foundations .............................................................................. 3 1.3.2 General consideration in the selection of the foundation depth ......................... 12

1.3.3 Foundations for common buildings ................................................................... 13

1.4 Foundations on difficult soils .................................................................................14

1.4.1 Foundations on expansive clays......................................................................... 14

1.4.2 Foundations on loose sands ............................................................................... 19 Chapter two: Deep Foundations................................................................................................22

2.1 Pile foundations .......................................................................................................22

2.1.1 Introduction ........................................................................................................ 22

2.1.2 Classification of Piles by materials and construction ........................................ 23 2.1.3 Driven piles ........................................................................................................ 24

2.1.4 Bored piles ......................................................................................................... 28 2.1.5 Determination of pile load carrying capacity ..................................................... 30 2.1.6 Determination of load carrying capacity dynamic methods .............................. 35

2.1.6 Determination of load carrying capacity pile testing ......................................... 37 2.1.7 Negative skin friction ......................................................................................... 39 2.1.8 Pile groups ......................................................................................................... 40

2.2 Drilled piers and Caisson Foundations .................................................................42

2.2.1 Drilled piers ....................................................................................................... 42

2.2.2 Caisson Foundations .......................................................................................... 43

2.4 Examples of Piling Schemes ...................................................................................47

2.5 Tutorial examples on chapter two .........................................................................47

Chapter Three: Retaining Walls ................................................................................................49

3.1 Introduction .............................................................................................................49

3.2 Types of retaining walls ..........................................................................................49

3.2.1 Gravity walls ...................................................................................................... 49 3.2.2 Cantilevered retaining walls .............................................................................. 50

3.2.3 Sheet pile wall .................................................................................................... 51 3.2.4 Bored pile ........................................................................................................... 51 3.2.4 Anchored ............................................................................................................ 52

3.3 Design of retaining walls ........................................................................................52

3.2.5 Examples on retaining walls ......................................................................................56 Chapter Four : Site Investigation .............................................................................................57

4.1 Introduction .............................................................................................................57

4.1.2 Planning a site investigation .............................................................................. 58

4.2 Preliminary and detailed stage site investigations ...............................................60

4.2.1 Preliminary stage site investigations .................................................................. 60 4.2.2 Detailed stage site investigations ....................................................................... 61 4.2.3 Sampling ............................................................................................................ 66

Technical University of Mombasa – ECE 2414 iii

4.2.4 Scope of Site Investigation ................................................................................ 69

4.2.5 Site Investigation Reports .................................................................................. 70 Chapter Six : Shoring and underpinning ...........................................................................72

6.1 Shoring .....................................................................................................................72

6.2 Underpinning...........................................................................................................74

Chapter Seven : Excavation , bracing, ground water, dewatering techniques. ....................76

7.1 Excavation and bracing ..........................................................................................76

7.2 Ground water and dewatering techniques ...........................................................77

Technical University of Mombasa - ECE 2414 -1-

Chapter one: - Shallow foundations

1.1 Types of foundations

Foundations that are encountered in practice may be classified into two broad categories

namely shallow and deep foundations. Under shallow foundations the following categories

are usually encountered:-

a) Strip foundations for wall and closely spaced columns

b) Spread or isolated footings for individual columns. In this category it is usual

to consider combined foundations for two or three closely spaced columns as

spread or isolated footings

c) Raft foundations covering large sections of the foundation area

The design and construction of shallow foundations is dealt with in this chapter.

Under deep foundations the following two types of foundations are encountered :-

a) Piles

b) Caissons

The design and construction of deep foundations is dealt with in the next chapter.

In the selection of the foundations to adopt for a structure it is usually necessary to consider

the function of the structure, its loads, the subsurface conditions and the cost of the

foundation being adopted in comparison to other possible types of foundations.

1.2 Introduction to shallow foundations

The foundation is the part of the structure that transmits the loads directly to the underlying

soil. If the soil is sufficiently strong it is possible to use shallow foundation. On the other

hand if the soil is not strong enough the foundation is taken deeper into the ground and is

referred to as a deep foundation. A definition which sometimes conflicts with the definition

of the shallow foundation defines a shallow foundation as one whose depth is less or equal to

its least width. The foundation must satisfy two fundamental requirements:-

1. The factor of safety against shear failure must be adequate. A value of 3 to 5 is

usually specified.

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- 2- Shallow foundations

Technical University of Mombasa – ECE 2414

2. The settlement of the foundation should be tolerable and in particular differential

settlement should not cause any unacceptable damage o interfere with the function of

the structure.

3. The allowable bearing capacity is defined as the pressure which may be applied to the

soil to enable the two fundamental conditions to be satisfied

The damage being mitigated in the design of the structures can be classified as architectural,

functional or structural. In the case of framed structures settlement damage is usually

confined to the cladding and finishes (architectural damage). It is usual to expect a certain

amount of damage. What is critical is to ensure that the damage to the services is limited.

Angular distortion limits were proposed by Craig (1987) and are shown on Table 1.1. In

general the limiting angular distortion to prevent damage is 1/300. For individual footings

this translates to a maximum settlement of about 50mm in sand and 75mm in clay. An

accurate damage criterion is to limit the tensile strain at which the cracking occurs. The

concept of tensile strain should be used in analysis using an idealization of the structure and

the foundation in elastic strain analysis when the fundamental properties of the foundations

are known.

Table 1.1 Angular distortion limits

1/150 Structural damage of general buildings may be expected

1/250 Tilting of high rigid buildings may be visible

1/300 Cracks in panel walls expected

Difficulties with overhead cranes

1/500 Limit for buildings in which cracking is not permissible

1/600 Overstressing of structural frames without diagonals

1/750 Difficulties with machinery sensitive to settlement

The design of the foundations is usually a two process exercise. The first is to determine the

allowable bearing of the soil while the second is to size the foundation on the design strata

based on the allowable bearing capacity. The first part was covered in ECE 2406. The second

part is now presented

1.3 Proportioning of shallow foundations

1.3.1 Contact pressure distribution

This is the distribution of the pressure below the base of the foundation and the ground. The

pattern of the distribution varies according to the stiffness of the foundation. The stiffness

may be described as yielding (elastic), rigid or flexible

Yielding foundation

The stiffness of such foundation is zero. Here the contact pressure distribution has the same

variation as that of the load. Because of its zero stiffness there will be no moments induced

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in the footing. Such a condition exists in fresh concrete before it sets. It has no practical

significance.

Rigid foundations

Contrary to the yielding foundation the rigid foundation has infinity rigidity. They are so

rigid that they do not deflect. Most of the foundations considered in practice are rigid

foundations. The analysis is simple and leads to economical design of the footings.

Flexible foundations

The stiffness of such foundations lies between rigid and the yielding foundations. The

foundations in this category deflect to a certain degree depending on the magnitude of their

stiffness. The analysis of such foundations is complicated but leads to an economical design.

However this is not usually done in practice and is not considered in these notes.

1.3.1 Proportioning the foundations

The proportioning of the foundations is usually the final step in the design of a structure. The

type of foundation, sizes and the level of the foundation depend on the result of the site

investigation. Usually partial factors would have been used in the design of the columns.

However unfactored loads would be used in the proportioning of the foundations. The

factored loads are however required in the determination of the foundation depths and design

of the foundation in accordance with BS 8110 (1997). The general procedure for the design

of the foundations follows the following steps

a) Evaluate the allowable bearing pressure in a site investigation exercise

b) Examine the existing and future levels around the structure and take into account the

ground bearing strata and the ground water level to determine the final depth of the

foundation

c) Calculate the loads and the moments if any on the individual footings with partial

safety factors on the structural loads.

d) Recalculate the loads and the moments on the individual columns and the walls

without partial factors of safety. In many cases it is sufficiently accurate to divide the

factored loads and moments with 1.45.

e) Calculate the plan area of the foundation using unfactored loads

The plan area of the foundations is determined assuming that all the forces are

transmitted to the soils without exceeding the allowable bearing pressure. The distribution of

the pressure is assumed to be planar. In no case should the extreme pressure be less than

zero. All parts of the foundation in contact with the soil should be included in the assessment

of the contact pressure. Subsequently the designer carries out the structural design of the

foundations. Typical foundations are now discussed

Strip and rectangular footings

A strip footing is significantly greater in length than in width. This type of foundation is used

to support walls and closely spaced columns. When and individual column is supported by a



footing then this foundation is referred to as a pad footing. When two or more columns are

supported by one footing, this is referred to as a combined footing.

Axially loaded strip and rectangular foundations

The contact pressure of these foundations is considered as uniform when loaded axially. The

pressure under the foundations should not exceed the allowable bearing pressure of the

supporting soil. Figure 1.17 shows the pressure distribution of such foundations.

Figure 1.1 Pressure distribution below individual and strip foundations under axial load

Eccentrically loaded rectangular foundations

When foundations are subjected to axial and moments at their foundations the soil pressure

resultant does not coincide with the centroid of the footing. The resulting pressure is a

combination of the compression and the moment stresses. While the columns can in almost

all cases resist the moments it is doubtful that the spread footing can sustain an applied

column moment. The base usually will rotate and induce more moment at the far end of the

column.

In conventional analysis the contact pressure distribution under eccentrically loaded

rectangular foundations (Figure 1.) are derived from the common flexural formula. The

general formula for the estimation of the pressure when there is eccentricity in the y and x

axis is given in Equation 1.34.

yIMxIMAP xxyyyx **),( 1. 1

Where

σ(x,y) = contact pressure at any given point (x, y)

P = the vertical load

x,y = coordinate of the point at which the contact pressure is calculated

My and Mx = the moment about y and x axis respectfully

Ix and Iy = moment of inertia of the footing area about the x and y axis respectively

=L*B3/12 and BL

3/12 respectively.

a) Pad foundation b) Strip foundation

d) Pressure distribution c) Combined foundation



Figure 1. 2 Rectangular foundation eccentrically loaded in two axis

When Equation 1.34 results in negative values in some areas, this means that the foundation

soil is taking tension. It is then necessary to change the dimensions to have only compression

pressure at the base. This is difficult and requires trial and error approach for solution of

maximum and minimum pressures. It is prudent to place the foundation such that that there

is only eccentricity in one axis direction as explained below.

Eccentrically loaded rectangular foundations in one axis

In design it is common to determine the magnitude of the contact pressure at the edges.

Equation 1.34 reduces to equation 1.35 shown below and Figure 1.19 shows the pressure

distribution.

xIA

Pq

y

M y *

12

3BL

yI

2

Lx

ePM *

)6

1(L

e

BL

Pq

1. 2

y

ey

My

Mx

L

B

P

My and Mx ey

ey x

My

M

x

L

B

ey

ex



Figure 1.3 Soil pressures below footing



When the eccentricity inside mid-third of the base (Figure 1.19a,e<l/6) the computed

minimum pressure is positive soil pressure and the computed maximum pressure should not

exceed the allowable bearing pressure. At e=l/6 Figure 1.19b the minimum soil pressure q=0

and the footing is fully effective in bearing. This limit of eccentricity means that as long as

the eccentricity is less than l/6 also described as falling within the mid-third of the foundation

the entire footing is effective. When the eccentricity is large (Figure 1.19c) and e>l/6 the

computed minimum pressure is negative soil pressure. This is an indication of a tensile stress

between the soil and footing. This in not feasible and the soil pressure has to be evaluated

neglecting any soil tension. The eccentricity is said to be outside mid-third.

For eccentricity outside middle third with respect one axis the maximum soil pressure

redistributes itself since the base cannot take negative pressure. The distribution of pressure

is triangular and is shown on Figure 1.20. The equations applicable in this case can be

derived as follows:-

Figure 1. 4 Eccentrically loaded rectangular out of middle third

eLL

23

' and )'(

2BL

qP

Solving the two equations to obtain the maximum soil pressure q, Equation 1. is obtained

)2/(3

*2

elB

Pq

1.3

Rectangular combined footings

It may not be possible to place columns at the centre of spread footings if they are near the

property line, near mechanical equipment or irregularly spaced columns. Columns located

off center will result in a non uniform soil pressure. In order to avoid the non uniform soil

pressure, an alternative is to enlarge the footing and place one or more of the columns in the

same footing to enable the center of gravity of the columns loads to coincide with the center

P

L

L’

L’/3

B

e=M/P

M

P



of the footing (Figure 1. . The assumption here is that the footing is rigid. The column loads

are taken as point loads and distributed into the footing. The footings are statically

determinate for any number of columns. The column loads are known and the resulting

pressure is shown in equation 1.37

APq / 1. 4

Figure 1.5 Combined rectangular footing

Trapezoidal shaped footings

A trapezoidal shaped footing is required when a combined rectangular footing will not result

in uniform pressure. This is usually so when the space between the combined footings is

constricted as shown on Figure 1.22.

Figure 1. 6 Trapezoidal footing

From Figure 1.22 the position of the centre of area of the footing is x’. The centre of the area

is to coincide with the center of gravity of the loads from the two or more columns being

supported by the trapezoidal footing. The position of the base cannot be extended beyond the

length dimension L. L is therefore a known dimension. The value of the area of the

foundation is obtained from the allowable bearing pressure and the total column loads (

aqPA / ). . The area of the base is shown in Equation 1.38 and the position of the centre

of the area is shown in Equation 1.39. The solution to the two equations leads to unique

values of a and b representing the dimensions of the trapezoidal footing.

Lba

A2

1. 5

a b X’

L

Variable S

P2 P1

X1



2**

3**)(

2

1*

2

1 LLa

LLabxL

baA

Therefore

ba

baLx

2*

3

1 1. 6

From Equation 1.39 and Figure 1.22 it can be seen that the solution for a=0 is a triangular

footing and for a=b it is a rectangle. The solution for a trapezoid footing exists only for

23

1 Lx

L

Strap or cantilever footings

A strap footing is designed to connect an eccentrically loaded column to an interior column

as shown on Figure 1.23. The strap is used to transmit the moment caused by eccentricity to

the interior column footing so that a uniform soil pressure applied to both footings. The strap

serves the same purpose as the interior portion of combined footing and is used in lieu of

combined rectangular or trapezoidal footing. Equations 1.40 through 1.43 are used to

proportion the footing dimensions. The value of eccentricity e is chosen arbitrary by the

designer. Unique solution of the strap footing is not always possible

SPSR 1

1

1 *

111S

SPR 1. 7

1212 RPPR 1. 8

xeL 2/1 1. 9

aqLBR ** 111 and aqLBR ** 222 1. 10

Figure 1. 7 Typical strap footing

Three basic considerations for strap footing design are:-

R2

x e

R1

P1 P2 S

L1/2 L2

S1



a) The strap must be rigid (Istrap/Ifooting>2. This rigidity is necessary to avoid rotation of

the exterior footing.

b) The footing should be proportioned to approximately the same soil pressures and

avoidance of large differential settlements

c) The strap should be out of contact with the soil so that there are no soil reactions and

is weightless

A strap footing is to be considered only as a last option when other options would not work.

The extra labor involved in the forming of the deep beam and accompanying costs make it

only an attractive alternative when other options have been exhausted.

Raft foundations

A raft foundation is a large concrete slab used as a foundation of a several columns in several

lines. It may encompass the entire foundation area or only a portion. Raft foundations are

generally used to support storage tanks, several pieces of industrial equipment or high rise

buildings. Figure 1.24 shows some typical raft foundations

A raft foundation is used where the supporting soil has a low bearing capacity.

Traditionally the raft is adopted when pad and structural wall foundations cover over half the

area enclosed by the columns and the structural walls. However this should be evaluated on a

case by case basis since the raft foundations end up with negative moments and top and

bottom reinforcement. This arrangement could end up being more expensive than closely

spaced pads which require only bottom reinforcement.

(a) Flat slab; (b) Thickened under columns or beam slab (c) Basement walls as part of the raft or

cellular construction

Figure 1. 8 Common types of raft foundations

(a) (b) (c)



The advantages of the raft foundations over the other foundations include:-

a) The effect of combining the column bases is increase in the bearing capacity of the

foundation. This is because the bearing capacity increases with the breadth of the

base.

b) The raft foundations bridge over the weak spots

c) They reduce settlement and are particularly suitable for structures sensitive to

settlement.

Raft foundations are usually designed as infinitely rigid in comparison to the supporting soil.

This assumption simplifies the pressure under the raft to a linearly distributed contact

pressure. The centroid of the contact pressure coincides with the line of action of the

resultant force of all the loads acting on the raft. Figure 1.25 shows the pressure distribution

and the resultant of the vertical loads.

Figure 1. 9 Linear pressure distribution below a rigid raft

A raft foundation is considered as rigid if the column spacing is less than 1.75/λ. λ is given by

Equation 1.44

4/1

**4

*

IE

bK

c

s 1. 11

Where Ks = coefficient of sub-grade reaction

B = width of strip of the raft between centers of adjacent bays

Ec = modulus of elasticity of concrete

I = the moment of inertia of the strip of concrete

λ. = characteristic coefficient

Bowles (1982) suggests that the coefficient of subgrade reaction be estimated from Equation

1.45.

as qFK **40 1. 12

σmax σmin

Resultant of column and wall loads

Resultant of soil pressure



Where F = the factor of safety applied to the ultimate bearing capacity

qa = the allowable bearing capacity

Equation 1.44 is applicable when the column loads do not vary in magnitude by more than

20%. The column loads should also be uniformly spaced. The design of the raft follows the

following basic steps

a) Compute the maximum column and wall loads

b) Determine the line of action of the resultant of all the loads

c) Determine the contact pressure distribution using Equation 1.46. Figure 1.26 shows

the arrangement of the columns and the eccentricities with respect to x and y axis.

y

x

x

y

yxI

xeP

I

yeP

A

P ****),(

1. 13

Where ∑P=total loads on the raft

A = Total area of the raft

x, y =Coordinates of any point on the x and y axis passing through the centroid of

the raft

Ix and I y = moment of inertia of the area of the raft with respect to the x and y axis

respectively

ex and ey = the eccentricities of the resultant force in the x and y direction

It is conventional to obtain the pressures at the four corners and then interpolate in between to

enable the determination of moments and shears for the structural design of the raft

Figure 1. 10 Raft foundation plan showing column loads

1.3.2 General consideration in the selection of the foundation depth

Once the geometry of the foundation of the foundation has been found, it is necessary to

determine an appropriate depth of the foundation. The following are general considerations

which the designers should take into consideration.

L

B

x

y

ex

ey

P1

P7 P9

P2

P4

P8

P6

P3

P5

∑P

P2

ex

ey Mx

My



a) Usually the foundation should be placed below the depth with minimum moisture

variation over the years. This eliminates the shrinkage and collapse effects of the

foundation soil. In this country a depth of between 1.0 and 1.5 metres is usually

sufficient.

b) The foundation should be placed below top soil and below depths with roots of tress.

The roots are potential water paths which weaken the foundations.

c) The foundations should be sited with due consideration to existing nearby structures.

The exaction of the foundation in the vicinity of the existing structures could lead to

loss of lateral support of the neighboring structures.

d) Special attention should be taken to foundations supported on expansive soils and

those on loose sandy silts which are likely to be saturated during the lifetime of the

structure.

e) For water structures viz: - river bridges it is necessary to take extra care to ensure that

scouring of the foundation vicinity does not impair the safety of the foundation. It is

usual to use gabions in areas where scouring is likely to erode the foundations such as

downstream of box culverts and around abutments and pier foundations

f) It is preferable to place foundations at one level throughout. None the less if it is not

practical to have the foundations at one level, the change of level should be at one

plane. Sloping foundation levels should be completely avoided even if they are on

rock. There is a risk of the foundation sliding.

1.3.3 Foundations for common buildings

This section deals with foundations for ordinary common buildings. These are single and

double storied buildings with structural walls as the main form of support. The spans should

generally not be bigger than six metres. The buildings are generally on good bearing soils.

The bearing soils include red coffee soils, gravelly soils and firm sandy, gravelly clays. The

footing for these common buildings is shown on Figure 1.27. The 600 mm width is a

practical width which allows masons to maneuver into the trench.

Figure 1. 11 Typical strip footing for an ordinary building

100mm slab with BRC no 65 at the top face

Damp proof membrane

100-200 mm thick hardcore

600mm wide x 200mm deep

mass concrete foundation

200-150 mm thick

masonry wall

200-150 DPC

A minimum of 1000 mm

depth of foundations

150 mm minimum drop dropasountonsd

100mm slab with BRC no 65 at the top face

Damp proof membrane

100-200 mm thick hardcore

600mm wide x 200mm deep

mass concrete foundation

200-150 mm thick

masonry wall

200-150 DPC

A minimum of 1000 mm

depth of foundations

150 mm minimum drop dropasountonsd



The following are the general considerations in the usage of the standard footing.

a) No reinforcement is needed for strips where the load can be distributed through 45o.

b) The foundations should be excavated and the last 150mm excavation be finalized

when the concreting can be done without further delay. This minimizes the softening

of the foundation

c) The mass concrete is in mass concrete usually by volume batching to achieve grade

15 concrete. A ratio of 1:3:6 for cement sand and ballast respectively is generally

sufficient.

d) Reinforced concrete foundations are done for areas with concentrated loads. These are

usually column supports. Grade 25 concrete is the lowest class of concrete allowed in

the new BS 8110, but grade 20 of concrete can be considered.

1.4 Foundations on difficult soils

1.4.1 Foundations on expansive clays

Introduction

The problems associated with expansive soils arise as a result of alternate heaving and

shrinkage of the clays. These soils are typically black or grey and are referred to as black

cotton soils in this country. The cycle of expansion and shrinkage is a result of ability of the

clays to take in water and retain it in its clay structure. The water absorption leads to

expansion of the clay and causes strains in the foundation and the structures supported

thereupon. The strains eventually cause the cracks to appear on the walls. The result is

structural safety and aesthetics of the buildings are compromised

The clay minerals include montmorillonite, illite and kaolinite as discussed in FCE

311. The montmorillonite clay mineral is particularly prone to heaving and shrinkage. Soil

having more than 20% of montmorillonite are particularly prone to swelling problems

In addition to visual identification the expansive soils can be identified by assessing

the swell potential of the soils. This is done by conducting an odometer test which measures

the free swell and the swell pressure attained in an odometer when a sample held in an

odometer ring is kept at the same volume as swelling is induced by allowing the sample to

take in water. Some of the Nairobi black cotton soils have been found to have a swell

pressure of up to 350 kN/m2.

Chen ( ) has related swell potential to plasticity index as shown

on

Table 1.2. The following methods can be applied to mitigate damage control

a) Moisture control

b) Soil stabilization

c) Structural measures



Table 1.2 Relationship of swelling potential and plasticity

Swelling Potential Plasticity index (PI)

Low 0-15

Medium 10-35

High 20-55

Very High Over 55

Source (Chen, )

Moisture control

The main course of heave and shrinkage is the fluctuations of moisture under and around the

structures in question. Depending on the topographical, geological and weather conditions

the natural ground water fluctuates during the year. This seasonal fluctuation decreases with

depth. In some areas the depth to the fluation zone is as low as 1.5 meters. In other areas it

will be deeper going down to over three meters. In addition to the ground water fluctuation

the surface water from rains or bust pipes seeps into the foundations and course moisture

migration.

A satisfactory solution to the problem would to devise an economical way of

stabilizing the soil moisture under and around the structure. It does not matter whether the

moisture is maintained high or low in so far as it can be maintained throughout the year. An

effective procedure of achieving this is to provide a water tight apron of approximately one

metre round the building. A subsurface drain one metre round the building is provided with

augur holes provided at every 2 meters. The holes are filled with sand and interconnected at

the top. In effect the augur drain is and the impervious apron ensures that the moisture at the

foundation area remains the same. Figure 1. 12 shows such an arrangement of the drains for

ensures that the moisture content of the foundations remain the same

The subsurface drain is used to intercept the gravity flow, or; perched water of free

water to lower ground. It also arrests capillary moisture water movement. The subsurface

drain should be lend to a positive outlet. In general the ground surface around the building

should be graded so that surface water will flow away from the building foundations all h the

time.

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a) Location of sand drain around a building

b) Sand drain and apron detail

Figure 1. 12 Typical sand drain treatment of a building

Soil stabilization

Soil stabilization consists of one of the following operations

(a) Pre-wetting or flooding the in-situ soil to achieve swelling prior to construction.

(b) Compaction control

(c) Soil replacement

(d) Chemical stabilization

Pre-wetting or flooding the in-situ soil to achieve swelling prior to construction involves the

flooding of the site under consideration prior to construction. The soil would heave and the

Compacted granular material at

high water content

Positive drain to outfall

away from the building

Building

Masonry

walling

Ground floor with double

mesh A142

2 meter wide water

tight apron

Original ground

level

Coarse sand drains

at 2 metre intervals

Expansive soil

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potential danger of cracking is eliminated. Pre-wetting has been used with success when the

active zones are not large. It is very difficult to saturate high plasticity clays. There is danger

that expansion of the clays could continue after the construction has taken place. This

procedure should be considered for stabilizing pavement or canal linings. In only rare cases

should the method be considered for use below ground floor slabs. Its application below

building foundations is risky and questionable.

Compaction control has been used in pavement construction. Expansive clays expand very

little when compacted at low densities and high moisture contents. But will expand

considerably when compacted to high densities at low moisture contents. The approach is to

compact swelling clays at moisture contents slightly above their natural moisture content for

good result. In this method it is not necessary to introduce large amounts of water into the

soil. Dry compaction of expansive soils was done along the Lodwar-Kakuma road.

Soil replacement is the simplest an easiest solution for slabs and footings founded on

expansive soils. The expansive foundation soils are replaced with non-heaving materials.

The method requires the selection of the replacement material and the depth to replacement.

In Nairobi the depth of the expansive black cotton soils is in the region of 1.0 to 1.5 metres.

In this case it has been found desirable to remove the entire expansive soil below buildings

and replace with suitable granular material. When the expansive soil is deeper building slabs

can be constructed above the compacted soil covering the expansive soil but the foundation

of main structure needs further consideration.

This method is particularly useful for the construction of highway pavement in a site

completely overlaid with expansive soils where the alternative to reroute the road is not

viable. In this case it the lower expansive soils are overlaid with the compacted replaced

material to a depth of 1.5 metres.

Chemical stabilization is the process of mixing additives like cement and lime to expansive

soil to alter its chemical structure and in the process retard its potential expansiveness. Lime

reduces the plasticity of the soil and hence its swelling potential. The amounts used range

from two to eight percent by weight. Cement on the other hand reduces the liquid limit,

plasticity and potential volume change. Stabilization has been used mainly in highway and

airport construction.

Structural measures include several methods have been reported in literature such methods

include

(a) Floating foundation

(b) Reinforcement of brick walls

(c) Foundation on piles

Floating foundation concept is a providing a stiffened foundation. This is essentially a slab

on ground foundation with the main supporting beams resting on non-cohesive non heaving

material. The slabs are designed fixed on the beams that assuming a heave pressure of 20

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kN/m2. This magnitude is small considering that the swell pressure of the expansive soils

commonly found in Kenya has been estimated at between 300 and 500 kN/m2. Results of

such an approach have been mixed where they have been tried. This method needs further

research.

Reinforcement of brick walls have been tried in South Africa. In this method reinforcement is

placed in brick walls. The reinforcement is placed where cracking usually takes place. This

is typically above and below openings. The structure is made also semi flexible by providing

joints in the brickwork so that when heave takes place the building will conform to the new

ground shape and consequently reduce the bending moment induced in the walls. The joints

are typically 1.5cm.

Foundation on piles is a very successful procedure which ignores the heave by placing the

footing to a sufficient depth (Figure ). The depth of the pile should leave an expansion zone

between the ground and the building to allow the soil to swell without causing detrimental

effect to the building. One way of installing the piles is to provide a pile with bell at the

bottom. The bell or under reamed section should be well below the active zone. The bell is

installed with special equipment and anchors the pile into the ground. The pile can be

installed in an oversize shaft which is subsequently filled with straw saw dust as filler to

eliminate uplifting of the pile by heaving soil. Alternatively the pile could be a straight and

the effect of the uplift calculated using Equation 1.47 The friction below the active zone is

utilized in the calculation of the bearing capacity of the pile.

1. 14

Where = the total uplift

D = the diameter of the pile

h = the depth of the pile in the active zone

u = the swelling pressure

f = the coefficient of friction between the pile and the soil

f may be taken as 0.15 while the swelling ;pressure varies between 250 and 500 kN/m2

Figure 1. 13 Pile systems for expansive soils

Straight pile

Sta

ble

zo

ne

Beam

Act

ive

zon

e

Act

ive

Zo

ne

S

tab

le z

on

e

Up

lift

S

kin

fri

ctio

n

Sta

ble

zo

ne

Sk

in f

rict

ion

Under ream pile

Beam

Straight pile

Up

lift

Beam



1.4.2 Foundations on loose sands

Foundations on loose sands are particularly difficult due to the likelihood of collapse in the

event of large storms. The storms result in the realignment of the sand particles and

consequent settlement due to repacking of the sand support. This has resulted in large cracks

in buildings which have been placed on this type of foundation soils. The foundation soils

subsequently loose there bearing capacity and the result is settlement of the foundations. The

superstructure has to absorb the settlement usually with resultant cracks of walls and

structural elements.

A real case story is one of the Garissa teachers college whose buildings were placed

on sand strata. The area is generally dry but when the rain comes, it usually very heavy and

comes in large storms. The performance of the three building types of structures adopted at

Garissa teachers college forms a case study whose findings are used to suggest a construction

procedure for foundations and masonry superstructures on loose sands.

The main teaching bungalow consisted of buildings constructed with a ground beam

which was framed with columns and a concrete roof slab. The masonry was thus reinforced

at the corners with columns and subsequently bound at he top by a ring beam and at the

bottom with a ground beam. These types of buildings were found to have performed well

several years after construction. This type of construction produced a satisfactory type of

constructed and when the buildings were inspected ten years after construction the structural

frames and the infill masonry walls were performing well.

The second type of buildings consisted of three and four and three storied flats. As in

the case of the previous buildings these types of buildings were found to have performed well

ten years after construction

The third type of the buildings was the staff residential bungalows. These were

constructed with a ground beam and masonry walls. The roof of the buildings was a concrete

slab. However as the rains came and went in there stormy characteristics the residential

houses developed cracks in the walls. The cracks were particularly severe in the external

walls and after about 10 years of service and needed attention (Plate 1.1

Based on the satisfactory behavior of the framed structures it was found prudent to

introduce columns at the masonry wall corners in a repair scheme. Plate … It is therefore

recommended for foundations on loose sands the masonry should be reinforced with columns

at the corners. In addition the foundations should be kept as far as is possible free from

percolating water. In this way the in the event of settlement the frame will be able to absolve

the stressed attributable to additional settlement and reduce the severity of the cracks.



Plate 1.1 Cracks in the walls occasioned by settlement of the foundation

Plate 1.2 Introduction of columns to stiffen the walls

1.5 Tutorial examples on chapter one

1) You are responsible for the design of a combined footing to support two columns

as shown in the figure below. The vertical dead loads on column A and B are 500

and 1400KN respectively. The design requires that the resultant of the column

loads acts through the centroid of the footing. In addition the dead loads, columns

A and B also can carry vertical live loads of up to 800 and 1200 KN respectively.

The live loads vary with time, and thus may be present some days and absent

other days. In addition the live load on each column is independent of that on the

other column. Check that the design meets all eccentricity requirements if the

worst possible combination of live loads is imposed



2) A column is carrying a load of 1200kN. The column is located 300mm form the

boundary of wall. Calculate the pressure distribution if the column is founded on a

square base of 1500mm x1500mm. is the foundation safe if the allowable bearing

pressure is estimated at 300kN/m2

3) An internal column is carrying a load of 2400kN. It is located 3000mm from the

column described in Question 1 Design:-

a. a suitable combined base for the two columns

b. A suitable strap footing for the two columns

4) Your client acquires the next plot and you are not limited by the boundary wall.

Calculate the safe bearing pressure below the columns described in questions 1 and 2.

Assume a detailed site investigation has established the following strength parameters.

C’ = 10kN/m2, φ’ =20

o, γsat = 18 kN/m

2, γb= 16 kN/m

2,

4 Four columns are carrying a tower. If the columns are on a square grid of

2.5mssquare, calculate the pressure at each of the four column positions if a raft

foundation of 3 mmx3m is designed to carry the foundation loads estimated at

4000kN, 5000kN, 6000kN and 7000kN


Chapter two: Deep Foundations

Deep foundation can be categorized into three major types. These include

i. Pile foundations

ii. Drilled piers

iii. Caisson foundations.

The ground and structural conditions which require the use of the two types are discussed

under each of the sections dealing with the two types of the foundations.

2.1 Pile foundations

2.1.1 Introduction

Pile foundations are structural members used to transmit surface loads to lower levels in the

soil mass. They are used when soil beneath the level at an appropriate raft or conventional

footing is too weak or too compressible to provide adequate support to the structure load.

The piles have small cross-section area compared to their lengths. The pile materials

generally include timber, steel or concrete. The transfer is by vertical distribution of load

along the pile surface and at the pile end point.

Piles may be used in the following circumstances

a) To transfer loads to a suitable bearing layer when weak strata is ignored and the load

is transferred to an overlying strong bedrock or compact layer.

b) To transfer load through the shaft friction when compact layer is very deep and would

be impractical to reach it

c) To support structures over water where conventional exaction and construction of the

foundation is not possible or very expensive to achieve.

d) To reduce settlement and in particular differential settlement

e) Based on cost. It might prove economical to drive piles down the strata and then

build on top of the piles instead of having to excavate deep layers and then construct

ordinary foundations

f) In structures which have considerable uplift, horizontal and/or inclined forces. This is

especially true for marine and harbor works.

g) To increase the bearing capacity by vibration and compaction of granular layers of

soil.

h) In soils where deep excavations would result in damage of existing buildings.



Piles can be distinguished by the function they are intended to perform or by the material and

construction procedures used in their construction. The various types of piles by function are

shown on Figure 2.1. The main function of the piles is to take the loads by end bearing or by

friction or by combination of the two. Other functions exist and two which can be sited here

include tension piles and fender piles. The tension piles take lateral forces in place of

traditional retaining walls while fender piles also referred to as dolphin piles are marine

structures principally for taking horizontal loads from vessels in the docking areas. Section

2.2 is presentation of piles by their material and construction procedures.

Figure 2. 1 Types of piles by function

2.1.2 Classification of Piles by materials and construction

Piles are constructed in a variety of properties of materials, construction methods and

functions. This makes as simple classification difficult. Notwithstanding theses difficulties

they are classified in accordance with the pile materials and method of construction (Figure

2.2). This classification also identifies the pile materials. The principal timber materials are

timber, concrete and steel.

Soft soil

Hard

strata

End bearing pile Friction pile

Friction

resistance

Tension resistance

Impact from floating

object

Dolphin or fender pile Tension pile

Soft soil

Soft soil

Firm

strata

Combination



Types of piles

Driven piles Bored piles

Large displacement Small displacement Replacement

Preformed. Solid Cast in place formed Steel sections A void is formed

or hollow tubes by driving closed H Piles by excavation.

closed at the end tubular sections Open ended tubes the void is filled

and left in position and then filling the unless a plug forms with concrete

void as the tube is during driving

withdrawn sides may be

Supported or

unsupported

Solid Hollow

The supporting may effected permanently

Pre-cast concrete or Steel or concrete by casing or

Timber. Formed to tubes closed at the Temporarily by casing or drilling mud

required lengths as bottom. Filled or (Betonite) or

units with mechanical unfilled after driving By soil on a continuous auger

Figure 2.2 Principal Types of piles

2.1.3 Driven piles

To install prefabricated and some form of cast in place piles it is necessary to displace soil by

driving the piles. The piling is commonly done by means of a hammer. The hammer

operates between guides or leads by use of lifting cranes. The leads are carried by the cranes

such that they can drive vertical or raking piles. The piling assembly may be mounted on

base suitable for operation on land or on a floating pontoon in the case of piling in the sea.

a) H and

pipe piles

b) RC

Precast pile c) Shell

Pile

d) Cast in-situ

tube withdrawn e) Bored pile



The hammers may be free falling operated by a clutch release mechanism.

Alternatively they are powered by diesel or steam. There are several forms of mechanical

devices and equipment in the market used by piling contractors. In order to reduce the

impact stresses on the hammer and the piles it is normal to strike the pile through a hammer

cushion. The elements of cushion vary but are mainly wood packing in a steel cap or dolly.

The various elements in the cushion not only protect the top of the pile but have a significant

influence on the stress waves developed in the pile during the driving. The rating of a

hammer is based on the gross energy per blow. For a drop hammer the rated energy is the

product of the hammer and the height of fall. The efficiency of the hammer is the defined as

the energy delivered at impact divided by the gross rated energy. Energy having been lost in

the dropping of the hammer to pile. For driving piles to great length the hammers have

energies of between of between 50kNm to over 180kNm.

Piles are installed by impact hammers and driven to a resistance measured by

number of blows required in the final stages of piling. For wood piles the energy would be

limited to about 3 to 4 blows per inch when energy of 15kNm is applied by the hammer. If

the pile is to be driven through heaving strata then, it might be necessary to predrill the

borehole where the pile is to be driven. This eliminates undesirable heaving. Additionally if

the pile is to be driven through dense layers of sand and gravel it is possible to loosen the

hard strata by sending a stream of water jet with specially adapted equipment. The various

types of driven piles are now described.

Timber Piles

Timber piles are made of trunks of timber. The timber should be preserved to prevent decay.

Untreated timber embedded below the ground water table has a long life. If the timber is

exposed to alternating wetting and drying it is subject to decay. These types of piles are not

very common.

Steel Piles

Steel piles (Figure 2.2a) are usually in form of H-Piles and pipe piles. H piles are preferred

where high depth is required while the pipe piles are usually filled with concrete after driving.

In the case of H-Piles the flanges and the web are equal thickness in order to

withstand large impact forces. Steel H piles penetrate the ground more readily than other pile

types because of the relatively small cross-section area. They are subsequently used to reach

stronger bearing stratum at great depth. Steel H piles have also relatively large bearing

capacity of between 500 and 2,000 kN per pile depending on the size of the H section. The

pile H sections are usually 250x250 to 350x350 with varying section thickness.

Pipe piles are of the range of 250mm to 750 mm diameter. The wall thickness is

usually over 2.54mm. In the event that the wall thickness is less than 4.54mm the pile has to

driven with a mandrel. When the thickness of wall is over 2.5mm the pipe acts with any

concrete in carrying the load. Pipe piles are usually driven with the lower end closed with a

plate. In some instances conical driving shoes have been attached. The advantage is not

significant.

Steel piles are subjected to corrosion. The corrosion is minimal when the entire pile

is embedded in natural soil. However, the corrosion can significantly increase in the event of



entrapped oxygen. Zones of water table variation are particularly vulnerable. Severe attacks

are encountered on sea structural sections exposed to high and low water tides where the salt

sprays can significantly cause corrosion. The standard practice is to use piles which have a

factory applied epoxy coating. The most vulnerable sections of the piles should be encased in

concrete.

Hard driving and driving through obstructions causes the piles to twist and bend.

They can easily go out of plumb without the piling team recognizing since the depth is at

depth. Deviations from the vertical of below 10% are usually accepted. A penetration of 2 to

2.5mm per blow should be considered as refusal and further driving would generally cause

deterioration.

Pre-cast Concrete Piles

Pre-cast Concrete Piles (Figure 2.2b) are usually cast in a casting yard and transported to the

construction site. Where hard driving is expected the tip of the pile is fitted with a driving

shoe. They are usually of square or octagonal section. The reinforcement is necessary

within the pile to withstand both handling and driving stresses. It is necessary that the exact

length to be installed be determined accurately. If the required length is underestimated, the

extension can be done only with a lot of difficulties. If the length provided proves to be

longer than needed at the site, the piles have to be cut again with a lot of difficulties.

Pre-stressed concrete piles are used and generally have less reinforcement. The pre-

stressing reduces the incidence of tension cracking during handling and driving. The

difficulties related to the pre-cast concrete piles also apply to the pre-stressed concrete piles

Pre-cast concrete piles have relatively large bearing capacity of between 800 and

2,000 kN per pile. The presence of high concentrations of magnesium or sodium sulphate in

the piled environments causes the piles to deteriorate. The deterioration is in the form of rust

in the reinforcement, cracking and spalling. The best practice is dense concrete of high

quality or the use of pre-stressed piles which are not so much susceptible because tension

cracks are minimized.

Driven cast in place piles

Driven cast in place fall in two categories namely case or uncased type. In the cased type

also known as shell the shell type a corrugated steel or pipe which is driven into the ground.

The driving is terminated when the desired length of the pile has been achieved. The

concrete is poured in the shell and left place. In the shell is then left in place. Figure 2.3

shows the schematic installation of a shell type pile.



`

Figure 2.3 Shell type of pile

In the uncased type a steel tube is driven into the ground and tube is withdrawn upon

concreting. Figure 2.4 shows the schematic installation of a typical driven cast in situ pile

where the casing is withdrawn. The pile illustrated is also known as a Franki pile.

Figure 2.4 Installation of a Franki pile

Difficulties encountered in the installation of driven piles

The installation of driven piles has difficulties due to various factors incidental to the

installation procedures and to the ground encountered at the sites. These difficulties are

varied but the main ones include:-

a) Handling of the preformed sections which could lead to damage of the piles before

installation.

(1)

(2) (3) (4)

(1) A gravel pug is compacted at the

lower end of the pile tube

(2) Pile driven to the required set

(3) Plug broken and a concrete plug

is formed

(4) Core concrete is inserted

(5) Tube is withdrawn as concrete is

placed

(5)

(1)

(2) (3) (4)

(1) RC shells threaded on

mandrel and set in position

(2) Pile driven to the required

set

(3) Mandrel is withdrawn and

top shells above the top of

the pile are removed. A

cage of reinforcement is

introduced

(4) Core concrete is inserted



b) Noise arising from the hammer dropping on to the pile. This can be particularly

undesirable in sites in the busy neighborhoods.

c) Spoiling of the pile in the driving operations include the spoiling of pile heads and or

pile toes. This usually takes place due to overdriving piles when refusal has been

reached. It is usually sufficient to achieve a penetration of 2-2.5 mm per blow in the

last stages of piling.

d) Piles of small cross-section especially H piles driven in boulderly strata could easily

alignment. Vertical piles could end up having bent up shapes and hence lose their

carrying capacity.

2.1.4 Bored piles

Bored piles are also known as cast in place concrete piles (Figures 2.2c-e. The borehole is

effected by various methods using piling equipment. The bore is supported by casing or by

drilling mud (bentonite suspension). At the required depth boring is stopped and the hole is

filled with concrete. If required a cage of reinforcement is placed before concreting is done.

With the use of bored piles larger diameter piles have been installed with corresponding high

bearing capacities. They are constructed in diameters ranging from 300mm to as high as

2400mm. They have been performed to depths of 70 metres and below and can be

constructed vertically or in rakes of up to 1:4. They are thus ideal for many site conditions.

The construction sequence of bored piles depends on the method of construction adopted.

The main construction methods include bored piles with casing support and bored piles with

bentonite support.

Bored piles with casing support

In this type of pile the casing is advanced by a crane and a casing oscillator. The material

below the casing area is excavated and brought up for examination and testing where

necessary. After the depth needed has been achieved the reinforcement cage is inserted

followed by concreting as shown on Figure 2.5

Bored piles with bentonite support

In this type of pile a lead casing is advanced into the soil. The material below the casing area

is excavated and brought up by use of drilling equipment with a bucket which can bail out the

drilled soil. The excavated soil is examined and tested where possible. The drilled hole is

supported by drilling mud After the depth needed has been achieved the reinforcement cage

is inserted followed by concreting as shown on

Figure 2.5



a) With casing

b) With betonite support

Figure 2.5 Installation of a bored pile with drilling mud

Difficulties encountered in the installation of bored piles

The difficulties associated with the installation of bored piles are also varied but the main

ones include:-

i. Poor base preparation after the bearing strata has been reached. Loose particles will

have reached the bottom of the bore and will be difficult to detect or remove. The

base the pile will consequently have a lower bearing capacity than would have been

expected

This installation is suitable in all soils

Install

starter

casing

Advance into the

soil by drilling

and supporting

with bentonite

Insert

reinforcement

cage

Place concrete

with a tremie

pipe and recycle

bentonite

Complete pile

This installation is particularly desirable in gravelly and boulderly conditions

Install casing

using an

oscillator

Advance the

casing and

excavate with grab

Insert

reinforcement

cage

Place concrete with a

tremie pipe as casing

is withdrawn

Complete pile



ii. Poor concreting control where the pile is being cast under artesian conditions. This

usually results from poor shaft control as the concreting continues. The result is

necking of the concrete and/or washout of various sections of the pile. Under ideal

conditions the concreter under tremie conditions should always be placed inside the

wet concrete.

iii. Vibration and movement of the ground in the vicinity of the pile under construction.

It is to be noted that these difficulties are also present in the driven cast in place piles where

the casing is withdrawn as concreting proceeds

2.1.5 Determination of pile load carrying capacity

Determination of load carrying capacity by soil mechanics

Pile design is preceded by extensive site investigation to establish the geotechnical properties

of the soil where the piles will be installed. The parameters obtained in the investigations are

then used in the estimation of the load carrying capacity of the piles. Piles derive their

capacity from base resistance and from side friction. The ultimate load that can be carried by

a pile is then given by Equation 2.1. The terms are explained in Figure 2.6. The accuracy

of the equation depends on the determination of the parameters used in the determination of

Qb and Qs.

Where

= Ultimate Load carrying capacity of the pile

Ultimate Load carrying capacity of the base of the pile

= Ultimate Load carrying capacity of the pile side friction

2. 1

Where

Ab= Area of the pile at the toe of the pile

qf = Ultimate bearing capacity at the toe of the pile

= Surface area of the pile shaft

= Ultimate shearing resistance of the shaft of the pile generally referred to as the

shaft friction

An appropriate factor of safety is applied to the ultimate load. It is prudent to apply different

values for the base and the side friction. This is primarily because the movement needed to

mobilize the friction resistance is much less than the movement needed to mobilize the base

resistance. Initially as the pile is loaded the load is taken by the side friction and as load is

increased the base takes more load. At failure the proportion of load supported by friction

may actually decrease slightly due to plastic flow of the soil near the base of the pile.

Equation 2.2 shows the allowable load when allowing for a factor of safety of 2 and 3 for side

friction and base resistance respectively.



2.2

Figure 2.6 Load distribution of load on a pile

Cohesive soils

Base resistance: The base resistance Qb of piles in cohesive soils is based on the bearing

capacity factor Nc .

2. 3

Where

= bearing capacity factor which is usually taken as 9.0

= undisturbed un-drained shear strength of the soil at the base of the pile

= the cross section area of the pile at the base

In the case of driven piles the clay adjacent to the pile is displaced both laterally and

vertically. Upward movement of the clay results in heave of the ground around the pile and

can cause reduction of the bearing capacity of the pile. The clay in the vicinity of the pile is

completely remolded during driving. Excess pore water pressures are set up during driving.

This pore pressure dissipates in a few months and in any case before significant load is

applied to the pile

In the case of bored pile, the clay area around the pile will be remolded. Additionally

as the water seeps towards the created borehole their softening of the soil in the vicinity of

the pile. Water can also be absolved from the wet concrete when it comes in contact with the

clay. The upshot of this is and subsequent reduction of the pile bearing capacity.

Side resistance is based on the friction mobilized on the surface of the pile. Equation 2.4 and

2.5 shows the estimation of the side friction

Qs

Qb



2. 4

2. 5

Where

= adhesion factor between the pile and the soil

= the average undisturbed shear strength of soil adjoining the pile

= the shaft area which contributes to the friction resistance

Most of the load of a pile installed in a clay soil is derived from the shaft friction and the

problem usually revolves accurate determination of the value of α. For soft clays driving of

piles tend to increase strength around the pile. A value of α equal to 1 can be used. It is

however unlikely that the soil will not in the long run return to its original soft status after

some time. In over-consolidated clays the value varies from 0.3 to 0.6 (Smith and Smith,

1998). A value of 0.45 is usually used for design purposes.

An alternative is approach is to express skin friction in terms of effective stress. The

rationale of this approach is that the area of disturbance during pile installation is relatively

small. The excess pore water pressure induced in the installation process dissipates ahead of

the application of load.

2. 6

Where

Ks = the average coefficient of earth pressure and

= the average effective overburden pressure adjacent to the pile shaft

= the angle of internal friction of the remolded clay. The cohesion intercept of

remolded clay in an drained triaxial test being zero.

Cohesionless soils

Base resistance: The ultimate bearing load carried by a pile depends mainly on the relative

density of the sand in which it is driven. The ultimate bearing capacity at the base of the pile

is given by

Where

= The bearing capacity coefficient.

= The effective overburden pressure at the base of the pile

It is to be noted that the bearing capacity attributable to Nγ usually ignored in pile design as

the value of B is usually small. The values suggested by Berezantzv et al (1961) are often

used and are shown on Figure



Figure 2.7 Bearing capacity factors for use in pile design

Source Berezantzv et al 1961

Side friction: Meyerhof (1959) suggested the average value of friction to be estimated from

Equation 2.6. As can be seen from the Equation the value of fs continues to increase as the

effective overburden increase. However field tests have shown that the maximum value of fs

occurs when the embedded length of the pile is between ten and twenty diameters. In practice

a maximum value of 100 kN/m2 of fs is taken.

2.7

Where Ks = the average coefficient of earth pressure and

= the average effective overburden pressure adjacent to the pile shaft

= the angle of internal friction between the soil and the pile.

Typical values of and Ks are given on Table 2.1 after Smith and Smith (1998) are shown on

Table 2.1. The ultimate load that can be carried by the pile is therefore given by Equation

2.7.

Table 2.1 Typical values of and Ks

Pile material Ks

Loose Dense

Steel 20o 0.5 1

Concrete 0.75φ 1.0 2.0

Wood 0.67 φ 1.5 4.0

Source Smith and Smith (1998)

2.8

Equation 2.8 shows the allowable load when allowing for a factor of safety of 2 and 3 for

side friction and base resistance respectively.

2.9

10

100

25 35 45

Val

ue

of

Nq

φ in Degrees

Nq



Determination of piling parameters from in-situ tests

The above equations pose difficulties with respect to determination of parameters for a

cohesionless soil which is difficult to sample in the field in undisturbed condition for accurate

determination of Nq which depends on the internal angle of friction. The value of the angle of

internal friction between the soil and the pile remains at best an estimate.

Consequently it has been found preferable to use empirical correlations based on the

results of standard penetration and those of the Dutch cone penetration equipment. Meyerhof

(1976) proposed the values given on Table below.

Table 2.2 Pilling parameters from standard penetration tests

Driven piles

Type of soil qb (kN/m2) fs (kN/m

2)

Sands and gravels

Large diameter -

Average diameter -

Non plastic silts

Large diameter -

Average diameter -

Bored piles

Any types of soils

0.67

Source Smith and Smith (1998)

Where N = the uncorrected blow count at the base of the pile

= the average uncorrected value of the blows over the embedded length of

the pile

D = is the embedded length of the pile in the bearing stratum

B = the width or the diameter of the pile.

An alternative to the use of the Standard Penetration tests is to use the Dutch cone test results.

The cone penetration results can be seen in

Figure 2.8. The ultimate base resistance is taken as average value of Cr over a depth of 4d as

shown on

Figure 2.8. The ultimate skin friction can be obtained from Table 2.3.



Figure 2.8 Typical results from a Dutch Cone Test

Table 2.3 Skin friction (fs) values from Dutch cone test results

Type of pile fs kN/m2

Driven piles in dense sand

Driven piles in loose sand

Driven piles in non plastic silts

Where

is the cone resistance along the embedded length of the pile

The allowable bearing load of the pile as before based on the Dutch Cone Test results is

given by Equation 2.9

2.10

2.1.6 Determination of load carrying capacity dynamic methods

Determination of load carrying capacity dynamic methods is applicable to driven piles. The

basis of derivation of dynamic formula is that a relationship exists between the pile capacity

and the driving behavior during the last stages of driving. The energy from the hammer to the

pile is transformed into useful energy and can be represented by Equation 2.10 in the last

stages of the pile driving

2. 11

Where

Cr (kN/m2)

Dep

th (

m)

Est

imat

ed

dep

th

of

the

pil

e

3d

d



M = the mass of the hammer

g = the acceleration of the hammer

h = the drop the hammer

R = the pile capacity

S = the settlement of the hammer as result of the drop h

In practice the above Equation has been modified to take account of several losses which take

place during the driving process. The main losses of energy occur as a result of sound, heat,

friction, quake, losses associated with elastic behavior of the pile and those associated with

the pile head compression. The net energy is equated to the work done in penetrating the

ground by the pile. Figure 2.9 shows the sequence of the pile driving and the

a) Variation of energy upon falling of hammer on to a driven pile

b) Penetration of pile upon falling of hammer on to a driven pile

Figure 2.9 Energy and penetration of a pile during driving

The potential energy of the hammer is Wh. Upon contact with the pile the available energy to

drive the pile into the ground is ef.eiv.Wh, where ef is the efficiency upon falling and eiv is

the efficiency upon impact. The penetration of the pile as shown on Figure 2.9b can be

shown to result in permanent ;penetration attributable to the pile and soil spp aand sso. In

addition there will be elastic penetration sep and ses attributable to the pile and soil

respectively. The work done and the pile resistance equation can now be rewritten as shown

on Equation 2.11.

efWh efeivWh

Wh

(sso+ses)

(sso + spp) +(sep +ses)

Permanent +Elastic penetration

(sso)

(ses)

(sso + spp =set =s)

(sep +ses )=c)

h



2. 12

Where R = The ultimate load capacity of the pile

= the overall efficiency factor

Equation 2.10 is known as Hiley formula. In the field the final stages of the pile are

monitored and recorded as can be seen on

Figure 2.10. It is usual to drive the piles to a minimum set of 2.5mm. Harder driving only

goes to damage the toe of the pile and could reduce the pile capacity in the process. Pile

driving formulas should be used in the piles driven in sand and gravel and in any case should

be calibrated with a load test.

Figure 2.10 Pile driving trace of the final stages

2.1.6 Determination of load carrying capacity pile testing

The load test is the most reliable of all the methods used in the determination of load carrying

capacity of a pile. In this method a full scale test is carried out on a working pile. Essentially

the pile is loaded and a plot of load versus settlement is recorded. From the plot the

allowable load is computed by one of the many formulas available from literature. Full scale

piles are then constructed to the same specification as the test pile

The test is conducted by loading the pile with kentledge load or by use of tension piles

(Figure 2.11). In some piling contracts the working piles cannot be used as tension piles for

testing purposes. This is primarily because in the cause of piling test the tension piles are

lifted slightly. This could lead to weakening of the working piles.

set = s1

set = s3

Elastic comp = c3

Elastic comp = c1

set = s2

Elastic comp = c2



a) Load resisted by kentledge

b) Load resisted by tension piles

Figure 2.11 Methods of testing piles in the field

If the test pile is a purely test pile ahead of the main installation of the pile the maximum load

to be applied is equal to two and half times the estimated safe carrying capacity of the pile.

It is usual to load the pile to 1.5 times the design allowable pile load when a working pile is

tested for ascertaining the integrity of the piles installed.

Maintained load test

The load is applied by maintaining the load in a series of increments. The increments are

usually equal to 20 to 25percent of the design working load of the pile. The subsequent

increments are carried out when the settlement has reduced to less than 0.25mm per hour.

The load is subsequently withdrawn in the same stages as the loading to trace the unloading

curve.

Constant rate of penetration

In this method the load is applied by a constant rate of penetration by a jack in order to

maintain a constant penetration rate (Figure 2.11b). it is usual to maintain penetration rates

of 1.5mm per minute and 0.75mm per minute in the case of sands and clays respectively.

Interpretation of test results

The results are plotted on a load settlement curve as shown on Figure 2.12. In the two

procedures ultimate pile load is taken as the load which achieves a settlement equal to 10

percent the diameter of the pile as is seen in test pile a Figure 2.11b. (BS 8004). The ultimate

pile load could also be reached when the shear failure of the pile soil interface or the pile toe

occurs (Figure 2.12b). The allowable pile load is obtained by dividing the ultimate load by

an appropriate factor of safety. The factor of safety usually ranges from 1.3 to 2.0

Kentledge

Existing ground level Support

Jack

Test pile

Jack

Test pile

Tension pile Tension pile

Existing ground level

Kenteledge



a) Maintained load test results

b) Constant rate penetration test results

Figure 2.12 Pile test load results

The above failure criterion is applicable to normal size piles. In the case of large diameter

piles on rock the ultimate load depends on the capacity of the concrete. This depends on the

stress in the concrete.

2.1.7 Negative skin friction

Negative skin friction is a phenomenon or which occurs in piles when a force develops

between the pile and the adjoining soil in a direction which increases the load on the pile and

or the pile groups. This phenomenon develops when a compressible layer of clay, silt, or

mud etc settles on account of consolidation which may be initiated by ground water lowering

or increase in overburden pressure.

As clay layer settles, piles are dragged into the soil by the consolidating soil and the

overburden soil. The direction of the friction is reversed increases the load on the pile. The

friction generated on the perimeter of the pile due to this dragging is carried by the column

instead of assisting in carrying he pile load. The effect is to reduce the carrying capacity of

the pile. This is the phenomenon known as negative skin friction

Load

Time

Set

tlem

ent

Load

Set

tlem

ent

a

b

Load

Ultimate

load (a)

Ultimate

load (b)

Penetration

Penetration =

0.1 pile diameter



Figure 2.13. The negative skin friction may be estimated from Equation 212 for

single piles and Equation2. For group piles

Figure 2.13 Negative skin friction

2.13

For cohesive soils fs is can be approximated to . while for cohesionless soils fs is

equal to . Where the value of fs is estimated from triaxial testing for cohesive

soils the fs can be taken as 0.5Cu

Where

= the ultimate force generated by the negative friction

= the shearing resistance of the soil

= length embedded above the bottom of the compressible layer

= the pile diameter

= the coefficient of earth pressure at rest

= angle of shearing resistance in terms of effective stress

= average effective overburden pressure

2.1.8 Pile groups

In practice piles are designed and constructed to work in groups. In construction of a group a

pile cap is cat on top of the piles. The cap is usually in contact with the soil on top of the

piles. The bearing capacity of the group is an arithmetic sum of the piles and that of the cap.

Banerjee (1975) showed that the pile cap could support up to 60% of the applied load. If the

cap is clear of the ground surface piles in the group are referred to as free standing piles.

Bearing capacity of groups

Except for the large diameter piles of over 700mm diameter the piles are usually designed in

groups of three or more piles under a column. The minimum under a foundation wall would

be two per typical cross-section. Typical arrangement of the piles is given on Figure 2.14. In

general the ultimate load capacity of the pile group is not the sum of the loads of the piles in

Fill

Compressible clay

Len

gth

of

sett

ling s

oil

=l

l-fi

ll

l-cl

ay



the group. The ration of the ultimate load for the group to the sum of the loads carried by

individual piles is the efficiency factor of the group.

Figure 2.14 Typical arrangement of pile groups

For piles in sand, the group action is complicated by dilatancy and densification

characteristics of the sand. When the spacing of the piles is less than eight times the pile

diameter, group action takes place (Department of Navy, Naval Facilities Engineering

Command, 1982). In dense sand the effect of driving piles is to loosen the sand and hence

the angle of internal friction of the sand in the vicinity of the piles. This results in overall

reduction of the pile bearing capacity. The group efficiency factor is less than one. In loose

sand the effect of driving piles is to increase the density of the sand. The bearing capacity of

the loose sand will therefore be increased. In this case the efficiency factor is more than one.

An efficiency factor of 1.2 is often used. In the case of bored piles in sand the resulting

loosening of sand in the boring operation results in efficiency factors less than 2/3. The

difficulties in the quantification of the design parameters of either loosened or densified sand

strata in piling operations remains a real problem for engineers (Mwea, 1984). Nonetheless

experimental evidence has it that the piles at the centre of a group in sand carry more load

than the piles on the periphery.

For piles in clay the effect of the pile group is to reduce the bearing capacity of the

pile group. This is because the effect of placing piles in a group is to have one large block

taking friction on the sides and base resistance over the block base. The spacing of piles in

clay is of the order of two times the pile diameter to four times the diameter. The efficiency

of the groups range from 0.6 to unity as the pile spacing increases from two diameters to four

diameters. The ultimate load in the case of a pile group is given by Equation 2.13. In the

case where the pile cap rests on the ground the ultimate load should be taken as the less of the

block capacity or the sum of the individual piles on the group.

2. 14

Where = The width of the group

3 – Pile 4 – Pile 5 – Pile

12 – Pile



= Length of the base of the group

= Depth of the group

= Bearing capacity factor of the clay

= The average undrained strength of the undisturbed clay

Whitker (1957) in a series of model tests showed that block failure as a group in clays occurs

when the spacing of the piles is not more than 1.5d apart. General practice is however to

space the piles at between 2 and 3d. In such cases the efficiency of the group is

approximately 0.7.

Settlement of groups

The settlement o a group of piles can be estimated by assuming that the entire load acts at a

depth as an equivalent raft. In clays the raft is assumed to be located at a depth of 2/3 D

where D is the depth of the pile group. The load is at spread of 1:4 from the underside of the

pile cap to allow for friction transfer. After the assumed depth of the raft the load is

distributed at a spread of 1:2 (Error! Reference source not found.a). Immediate settlement

nd consolidation settlement can then be estimated for the layers of soil below 2/3D by

application of normal methods.

For groups in sand the equivalent raft is at a depth of 2/3Db from depth 2/3D. The

spread from the perimeter of the piles is 1:4 followed by a spread of 1:2 Error! Reference

ource not found.b). The settlement of the underlying sand stratum is then gotten from

application of standard penetration data and or the cone penetration resistance

Figure 2.15 Equivalent raft concept for piles

2.2 Drilled piers and Caisson Foundations

2.2.1 Drilled piers

The term drilled pier foundations is used in a number of situations which to refer to deep

foundations which method of construction is fundamentally different from that of piles. A

large shaft performed in soil and then filled with concrete may be termed as a drilled pier.

ACI (1972) refers to all shafts where a person may enter and work as a drilled piers. In this

definition all shafts larger than 750mm diameter can be referred to as drilled piers. Figure

D 2/3D 1:4

1:2

1:4

1:2

2/3Db Db

Clay stratum Sand stratum

Position of equivalent raft Position of equivalent raft

2/3D



*** shows typical piers used in practice. In general drilled piers are used where the soil has

a low bearing capacity and it is necessary large loads to firmer stratum and the following

conditions preclude the use of smaller piles.

i. Pile vibrations are not acceptable.

ii. Pile members are too small for the loads.

iii. A large bearing end is needed for higher load capacity

2.2.2 Caisson Foundations

The term caisson is also used to refer to box type structures consisting of many cells built in,

concrete or steel or combination of both. They are built wholly or partly at higher ground and

sunk to final position. They are used to transmit large loads through water and soil to firm

strata. They are used in large bridges, shore protection structures. They are generally used

under the following conditions.

i. The soil contains large boulders which would otherwise obstruct the penetration of

piles and or construction of cast in place piles.

ii. A massive substructure is needed to extend below the river bend to provide resistance

against floating objects and scour.

iii. Foundation is subjected to very large lateral forces.

Caissons may be divided into three categories

i. Open caissons

ii. Pneumatic caissons

iii. Box caissons or floating caissons

Open caissons

An open caisson essentially consists of a box open at the top and bottom ( Figure 2.16). the

soil is removed from the caisson by grabbing, dredging from inside the caisson. The sinking

of the caisson proceeds by the caissons self weight assisted by cutting edges of the walls.

When the desired level has been reached concrete is poured under onto the base of the

caisson by tremie pipe. In some cases the caisson has been pumped out. But in most of the

cases the caisson has been left in place. The bearing capacity of the soil below is usually

determined by normal bearing equations.

Straight pier Underreamed pier

Pier socketed Into

Rock



The concrete seal at the bottom is placed as a plug at the bottom of the caisson but

later serves as a permanent base of the caisson. Its thickness can be obtained from the

equations below

For circular caissons

√

For rectangular caissons

√

Where

= thickness of the seal

σo = contact pressure or hydrostatic pressure

R = radius of the caisson in the case of circular caisson

fc = the allowable concrete stress in tension (0.1 to 0.2cube strength)

b= width or the short side of the caisson in the case of a rectangular caisson

l= length or the long side of the caisson in the case of a rectangular caisson

β = coefficient which depends on the l/b ratio

Figure 2.16 Open Caissons

Pneumatic caissons

Pneumatic caissons provide an airtight enclosure (Figure 2.17). In effect water is prevented

from getting into the enclosure and the workers can excavate and pour concrete under dry

conditions. The reliability of the quality in this case is better in so the mechanical ventilation

is carried out to the strictest of the specifications. Pneumatic caissons are costly and should

be considered only with the following conditions in mind:

i. Premium pay because of associated health hazards

ii. Overall safety requirements are high

Cutting edge

Ground surface

Water level

Circular open caisson Box caisson



iii. Much of the effort is towards making the work environment suitable for the workers

When the excavation has reached the desired stratum the concrete is sent down to the

working chamber carefully to fill any weak points on the exposed strata. After this initial

filling the area is filled except a small portion of the chamber below the roof of the chamber.

This final portion is filled with grout which also fills any spaces which might have been left

behind during the concreting.

The seal design and estimation of the bearing capacity is the same as that of the open

caissons

Figure 2.17 Pneumatic caissons

Box caissons

Open caissons are usually cast on the ground and then towed to the site. They area then

lowered to a prepared ground. They are carefully aligned on place and then made stable by

placement of ballast. The design and construction of box caissons do not bring any new

design requirements. The ground upon which the caisson is being laid needs to have been

exhaustively investigated to ascertain the foundation depth and any likely difficulties likely to

be encountered. After the caisson is in place it may be filled with either sand concrete or

sand. The caisson should be checked against stability as it is floated to the final place of the

intended foundation.

Design of caissons

The caissons will be designed to resist vertical loads including superstructures, own weight

minus buoyancy forces. The lateral forces will typically include forces due to wind,

earthquake, earth and water pressures, and traction from traffic and pressure from current

flow.

The forces acting on a caisson must be estimated as accurately as can be to enable a

safe design. There are many methods adopted by various geotechnical engineers but the for

stability of the caisson the following combination of forces will suffice

Compressed air in

working chamber



i. All forces are resolved into

ii. A single vertical force

iii. Two horizontal forces in the direction across and along the caisson.

It has been found out that analysis of the caisson in a direction transverse to the direction of

the axis is more critical. From Figure ***-* the three equations of static equilibrium are

solved. This are

W = Base reaction + skin friction

Q = Passive pressure created on BF – Passive pressure on DE – Base friction

Q (H+D) = Moment of all the forces

Qmax =Area ABC-Area FEC

Qmax =1/2 γD2 (Kp - Ka)- ½*2* D (Kp - Ka)*D1

Moments about O:

Qmax (H+D)=1/2 γD2 (Kp-Ka)D*1/3- ½*2* D (Kp-Ka)*D*D1*1/3

From structural

analyses

Q

From geotechnical

analyses

h

D

O

Q



Therefore D1 and Qmax can be calculated and necessary adjustments of the caisson are

made depending on values of Kp and Ka

2.4 Examples of Piling Schemes

Sutong bridge in China

Sutong bridge in China (Plate 1), which has a centre span of 1088m, designed in an area of

high winds and likely to be hit with massive earthquakes (Bitener et al, 2007). The

foundation strata presented the designers with particularly difficult task. The soils at the site

consisted of firm to stiff clay extending to 45 metres below the sea bend. This clay strata was

underlain with a medium to very dense coarse sands, silty sands and occasional loam layers

matrix to a to of 250 metres below the sea bed where the basement rock was encountered.

The designed pile groups covered a plan area of 113.8x48.1m. The design consisted

of 2.8 and 2.5 diameter piles. Permanent casings were installed to a depth of 40 metres. The

overall depth of the piles was of the region of 110 metres. The shafts were designed to mainly

be carried by friction since the displacement needed to mobilize the end bearing is two to

three times that needed to mobilize the skin friction The tips of the pile shafts were however

grouted to increase the bearing capacity of the piles. This procedure densifies the soil below

the shaft and any debris left during the drilling operations. The increased the pile capacity

end bearing capacity is of the order of 20%.

Plate 1 of the Sutong Bridge in China (1088 m center span)

The Nyali bridge in Mombasa

This is a pre-stressed concrete bridge founded on seabed which had coral deposits, sand and

clay soils matrix proved to a depth of 100metres below the sea bend. The designers

depended on the skin friction for the centre piers. The design consisted of 2.0metre diameter

shafts drilled down to depth of 50 metres. On plan the piles have a rectangular layout of 3x8

piles per pier.

2.5 Tutorial examples on chapter two

1) A single pile 0.6 m diameter is bored into sand strata six meters thick overlying a clay

stratum of infinite depth. Detailed investigations have established that N value in the



sand zone increases with depth (n=3Z). The undrained cohesion increases with depth

(Cu = 5+4Z). Assuming the adhesion factor α = 0.35, determine

a) An equation for the estimation of pile working load if the pile is to terminate

in the sand zone.

b) An equation for the estimation of the pile working load if the pile is to

terminate in the clay zone.

2) A precast reinforced concrete pile measured 450mm x450mm. The pile was driven to

a depth of 15 metres to a set of 3mm by a drop hammer of 2.5 tones freely through 1.5

metres. The piling arrangement was changed to have a 4.2 tone hammer falling

through 2 metres. Assuming the same resistance with the new hammer, determine the

set achieved if the following information is also available.

2.5 tone hammer 4.2 tone hammer

Overall efficiency factor 0.5 0.35

Elastic compression of pile 4mm 4mm

Elastic compression of soil 4.5mm 5.0mm

3) A pile under test has started showing considerable settlement under load of seventy

tones. The pile diameter is 500mm and a length of 8.5metres in stiff clay. Assuming

below the 8.5metres the clay was soft clay and did not contribute to any resistance

evaluate the magnitude of the unit shear along its skin. (Answer 10.5tones per m2).

4) A 500mm diameter bored pile is to be made in stiff clay to a depth of 20metres. The

un-drained strength of the clay varies with depth as shown in the following table

Depth 4 6 8 142 16 20 24

Cu (kN/m2) 78 86 102 132 157 184 212

Determine the maximum load that may be applied to the pile. The following factors

may be taken.

Adhesion factor α = 0.45

Overall factor of safety = 2

Nc for piles is usually taken as = 9

(Answer 1025kN).


Chapter Three: Retaining Walls

3.1 Introduction

Retaining walls are used to retain soils between two different elevations in areas of terrain

possessing undesirable slopes or in areas where the landscape needs to be shaped severely

and engineered for more specific purposes like hillside farming or roadway overpasses. The

most important consideration in proper design and installation of retaining walls is to

recognize the tendency of the retained material to move. This creates lateral earth pressure

behind the wall which depends on the angle of internal friction (φ) and the cohesive strength

(c) of the retained material, as well as the direction and magnitude of movement the retaining

structure undergoes.

Earth pressures will push the wall forward or overturn it if not properly taken into account.

Any groundwater behind the wall that is not dissipated by a drainage system causes

hydrostatic pressure on the wall. If the wall is not designed to retain water, a proper drainage

system behind the wall in order to limit the pressure to the wall's design value is needed.

Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability

of the material behind the wall.

3.2 Types of retaining walls

3.2.1 Gravity walls

Gravity walls (Figure depend on their mass (stone, concrete or other heavy material) to resist

pressure from behind and may have a 'batter' setback to improve stability by leaning back

toward the retained soil. For short landscaping walls, they are often made from mortarless

stone or segmental concrete units (masonry units). Dry-stacked gravity walls are somewhat

flexible and do not require a rigid footing in frost areas.

Tall gravity retaining walls are increasingly built as composite walls such as reinforced earth

with precast facing; gabions; crib walls; or soil-nailed walls

Retaining walls


Figure 1.14 Different types of retaining Walls

3.2.2 Cantilevered retaining walls

Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place

concrete or mortared masonry (often in the shape of an inverted T). These walls consist of a

cantilever stem, cantilever heel and toe

For high walls in excess of eight meters designing counterfort on the back of the wall, or

buttress in the front, improves their strength resisting high loads. This type of wall uses less

material than a traditional high cantilever wall when designed carefully. The horizontal load

is taken by spanning horizontally

Mass Stone Wall

Reinforced earth wall

Crib Wall

Road way

Road

way

Gabion Mattress Wall

- 51- Retaining Walls


3.2.3 Sheet pile wall

Sheet pile retaining walls are usually used in soft soils. Sheet pile walls are made out of steel,

vinyl or wood planks which are driven into the ground. They are usually driven 1/3 height

above ground, 2/3 below ground. This however may be altered depending on the

environment. Taller sheet pile walls will need a tie-back anchor, placed in the soil a distance

behind the face of the wall that is tied to the wall, usually by a cable or a rod. Anchors are

then placed behind the potential failure plane in the soil.

3.2.4 Bored pile

Bored pile retaining walls are built by assembling a sequence of bored piles, preceded by

excavating away the excess soil. Depending on the project, the bored pile retaining wall may

include a series of earth anchors, reinforcing beams, soil improvement operations and

Basement

wall

Original

ground Ground

Floor

Upper

Basement

Lower

Basement

Bridge Abutment waalls Basement Wall

Ground Floor

Upper basement

Lower Basement



shotcrete reinforcement layer. This construction technique tends to be employed in scenarios

where sheet piling is a valid construction solution, but where the vibration or noise levels

generated by a pile driver are not acceptable.

3.2.4 Anchored

An anchored retaining wall can be constructed in any of the aforementioned alternatives but

also includes additional strength using cables or other stays anchored in the rock or soil

behind it. The anchors are driven into the material with boring; anchors are then expanded at

the end of the cable, either by mechanical means or often by injecting pressurized concrete,

which expands to form a bulb in the soil. Technically complex. This method is very useful

where high loads are expected, or where the wall itself has to be slender and would be too

weak to retain the soil

3.3 Design of retaining walls

The Design of any Retaining Wall is concerned with

The stability of the retaining wall is due to its self weight and the dead weight on top of the

heel. The wall is designed to obtain an acceptable factor of safety with respect to

a. Overall slope stability failure of the soil around the wall

b. Overturning.

c. Sliding.

d. Ensuring that allowable soil bearing pressures is not exceeded at the base of the

wall. This is critical at the toe of the wall

These design stability failure modes are shown on Figure

Figure 3.1 Retaining wall failure modes

a) Overall slope stability failure b) Overturning

c) Sliding d) Overturning



The design steps of a retaining wall

i. Start with an assumed geometry of the wall. The first trial experience but the

following dimensions are generally good for the start in the case of a cantilever

retaining wall

a. The footing width, to be about 0.4 to 0.7 of the height of the wall

b. The toe projection is with 1/3 to 1/4 of the height of the wall.

c. The footing thickness and the stem width at the footing is1/10 to 1/14H of the

height of the wall.

ii. Compute overturning moments, calculated about the front (toe) bottom edge of the

footing.

iii. Compute resisting moments based upon the assumed footing width, calculated about

the front edge of the footing.

iv. An overturning factor of safety (resisting moments/ overturning moments) of at least

1.5 is considered safe.

v. Check sliding. A factor of safety with respect to sliding of 1.5 is considered safe.

Calculate the eccentricity of the total vertical load. Is it within or outside the middle-

third of the footing width?

vi. Calculate the soil pressure at the toe and heel. If the eccentricity, e, is > B/6 (B =

width of footing) it will be outside the middle third of the footing width (not

recommended!), and because there cannot be tension between the footing and soil, a

triangular pressure distribution will be the result. if this condition cannot be

avoided, then adjust the wall dimensions

vii. Design the stem. Start at the bottom of the stem where moments and shears are

highest. Then, for economy, check up the stem to determine if the bar size can be

reduced or alternate bars dropped. The thickness of the stem may vary, top to

bottom. The minimum top thickness for reinforced concrete walls is usually 150mm

to properly place the concrete200mm at the bottom.

viii. Design footing for moments and shears.



Example

Design a cantilever retaining wall to retain earth for a height of 4 meters. The backfill is

horizontal. The density of the retained soil is 18kN/m3. The safe bearing capacity is

200kN/m2. The angle of friction for the backfill is 350while that of the base is 40

0

i) Assumed geometry

Assume a depth of foundation of 1.2m. Therefore total height is 5.2m.

Total height for stability = 5.2+.32 Try 5.52m

Width of the base, 4*5.52 to, 7*5.52: 2.208 to 3.864 Try 3.0m

Thickness of the base 1/10 to 1/14H 0.552 to 0.392 Try 450mm

Width of the toe of the base 1/3 to 1/4B 1,0m to 0,75m Try 750mm

Width of the heel of the base =3-.75-.45 Try 1800mm

hs = height of slope 1.8*tanβ Try 320mm

Thickness of stem at base 1/10 to 1/14H 0.552 to 0.394 Try 450mm

Thickness of stem at top 200 to 400mm Try 200mm

Thickness of heel =3-.75-.45 Try 1800mm

Stability analysis

Note that all the loads and actions are per metre length of the retaining wall

Assume that Pa is the Rankine lateral force and has two components of the vertical force and

horizontal force

From ECE 2406

H T

1.2m

Pa

W4 β=100

β=100

4.0m

0.32m

H=5.52m

W1

W2

W3

W5

H/3=1.84m

0.2m

B=3m

1.8 .45

.75



√ √

Cos 10o = 0.98

Cos 35o = 0.82

√

√ =

=

= 0.195

Stability Computations

Take moments about the Toe (T)

Area Force Lever arm Moment

W1 = .5*1.8*0.32 *1*18 = 5.184 2/3*1.8+.45+.75=2.4 12.4416

W2 = 1.8*4 *1*18= 129.6 1/2*1.8+.45+.75=2.1 272.16

W3 = 0.2*4 *1*24= 19.2 1/2*0.2+.25+.75=1.1 21.12

W4 = ½*0.25*4 *1*24= 12 2/3*0.25+0.75=0.917 11

W5 = 0.25*4 *1*24= 12 2/3*0.25+0.75=0.917 11

Area lever arm

moment

dimentions density Force Dimensions LA Moment

W1 = 0.50 1.80 0.32 18.00= 5.18 1.20 0.45 0.75 2.40= 12.44

W2 1.00 1.80 4.00 18.00=129.60 0.90 0.45 0.75 2.10= 272.16

W3 1.00 0.20 4.00 24.00=19.20 0.10 0.45 0.75 1.30= 24.96

W4 0.50 0.25 4.00 24.00=12.00 0.17 0.75 0.92= 11.00

W5 1.00 1.20 1.80 24.00=51.84 1.50 1.50= 77.76



Design steps of a restrained retaining wall

i. Start with assumed geometry of the wall

ii. Compute all applied loads

iii. Select restraint – level and base of stem design assumptions: pinned - pinned; pinned

fixed; or fixed - fixed. Then based on statics determine the reactions at the top and

at the base of the wall.

iv. If a floor slab is present at the top of the footing, check its adequacy to sustain this

lateral sliding force.

v. Design the stem. If the stem is assumed pinned at the base and at the top, the

maximum moment will be a positive moment near mid-height—usually the same

material (concrete or masonry) and thickness will be used for the full height. Some

degree of fixity at the top of the wall even with a pinned

vi. Design the footing. If the stem is assumed fixed at the base check the soil pressure

and design for the moments and shears. If the stem is assumed pinned at the footing

interface, try to centre the footing under the wall to prevent eccentricity.

vii. Check sliding. If a restraining floor slab is not present, a key or adjusting the footing

width or depth may be required

3.2.5 Examples on retaining walls

A retaining wall is needed to retain a highway as shown in the figure below. Design a

suitable wall if it is to support a 10kN/m2

surcharge as shown. The backfill is made up of

compacted granular material for density 20kN/m3

and Ø = 350

Assume an allowable bearing

pressure of 300 kN/m2. The

strata at the base has a Ø= 40

0

University of Nairobi –FCE 511 Geotechnical Engineering IV -57-

Chapter Four : Site Investigation

4.1 Introduction

Site investigations are also referred to as soil exploration. It consists of investigating the

condition on which construction is planned. From site investigation it should be possible to

obtain information for the following geotechnical engineering activities

i. Design of new foundations

ii. Modification of existing foundations

iii. Location of materials of construction of roads, runways, etc

iv. Identification of materials needed for the construction of pavement structures for

roads, runways etc

v. Identification of ground to be excavated in the construction of various facilities

including water pipe lines, building foundations, earthworks in cut areas etc

The site investigation should form a part of a coordinated chain of design from inception of

the project through preliminary to the final detailed design of a civil engineering project. It

should indeed continue post construction monitoring of the completed schemes. Because of

the diversity of civil engineering schemes a set of standard procedures is not possible for all

site investigations. The varying civil engineering schemes require a variety of options in

breadth and detail needed for the various schemes. The objectives for which a site

investigation is carried out also differ with various schemes. The main objectives of carrying

out a site investigation are now presented

i) Suitability of site for particular works

In the case of option of site for particular works a detailed site investigation should be able to

enable determination of the most suitable site. Thus it is possible to shift a bridge from one

location which would call for expensive deep foundations to one where ordinary shallow

foundations would be sufficient.

ii) Adequate and economic design

A site investigation leads to safe structures during and after construction. Additionally

sufficient information is obtained for quantifying the excavations needed in the preparation of

the bills of quantities. This should minimizes the possibility of cost overruns due to

unexpected ground conditions being met at construction time.

- 58- Site Investigation


iii) Planning construction

By identifying different materials along the construction paths and their locations a

systematic procedure of carrying out the works is evolved. In the case of road works

materials from the cut areas are analyzed for use in the fill areas. It is then possible to

proceed with construction of the fills and cuts methodically with minimum haulages and

waste of materials.

iv) Prediction in changes in structure

Carefully and well executed site investigations should enable the prediction of the likely

settlement of structures under construction. Equally important is the ability to predict the

effect of excavations on the neighboring structures.

v) Safe structural design of large structures

Heavy modern structures require more detailed site investigations. Today we are seeing

higher buildings, larger bridges and installations sensitive to settlement. Structures and civil

engineering schemes are being put up very quickly. Immediate and consolidated settlement

is taking place when the works are commissioned. Further settlement takes place during the

useful life of the civil engineering installation. Accurate estimation of the settlement regime

is particularly important considering that clients are becoming more and more sensitive to the

performance of structures and the argument that cracks are minor and do not pose any

danger to the structure is no longer good.

4.1.2 Planning a site investigation

Table 4.1 shows a schematic way in which various activities with respect to site investigation

can be performed at various stages of a project. It is clear from the table that site

investigation should not be treated as an afterthought but rather should grow with the project

from conceptual initial design to eventual post construction period.


Table 4.1 Stages of a site investigation

Phase Pre-construction Construction Post Construction

Stage

Conceptual

Initial design Preliminary design Detailed design

Supervision of

construction

Operation &

Maintenance

Main activity

Conceptual

design Design Alternatives Detailed Site Construction control -Performance

Site

investigation

activity

Define Scope of

SI

Desk study of SI –

Review of existing

data Preliminary trial

pits

Detailed investigations

-Boreholes

-Trial pits etc

Laboratory and field

tests

Field observations

– field densities

- field moisture contents -

Monitoring and

checking performance –

- pore water pressures

Settlement

Inclinations

SI Reports

Terms of

reference and

bid documents

i) Preliminary SI

investigation report

ii) Cost estimate of SI

Detailed design report

-SI report As built SI report -

-Maintenance reports

-Performance reports

-Research reports

Foundation Engineering I: - ECE 2406 60

4.2 Preliminary and detailed stage site investigations

4.2.1 Preliminary stage site investigations

This should lead to information needed for the design of the various alternatives at the

preliminary stage of the study. The activities in this stage can be summarized as follows:

i) A study of any existing site investigation reports for the area or in the neighborhood

should form the basis of this stage of investigations.

ii) A study of geographical a geological maps of the site in the case of large sites.

Topographical characteristics should lead to useful information such faulty areas.

Heavily forested areas are an indication of deep rooted top soils.

iii) A site inspection of the existing buildings and any existing structures. Any signs of

distress which can be related to the settlement of the foundations. Any information

from archives, previous records held by the local authorities.

iv) Inspection of the soil profiles, in cut areas, old used quarries. Structured questions to

local people with regard to the geotechnical information being sought yields

considerable information. Such questions are:

a) What is the depth of the pit latrines in the area?

b) At what depth murram encountered?

c) At what depth was water struck?

v) Aerial survey of the site could give useful information with regard to land formations

and soil profiles.

vi) Seismic refractions could be carried out at this stage of investigations. Usually a

specialist is needed to interpret the results.

vii) Preliminary trial pits

Geophysical methods

Geophysical methods involve sending of seismic or electrical waves through the ground. The

determination of the soil strata is based on the fact that the velocity or the resistance seismic

wave transmission or resistance to electrical flow differs with different rock types and soils.

The method allows the boundaries of the soils to be determined seismic refraction is

described below

Seismic refraction is conducted by having a source of seismic waves (Figure 4.1). The

seismic waves are induced by detonating a small explosive or by striking a metal plate hard.

Waves are subsequently emitted in all directions, through the air, and through the soil in all

directions. Seismic wave transducers called geophones are placed radially from the

epicenter. A circuit connects the geophones and the detonator for accurate determination of

time. A direct wave will reach the geophone first since it is the shortest distance covered.

When there is a dense stratum at depth a refracted wave will travel along the top of the bed

rock. As it travels it leaks energy to the surface which can be picked by the geophone.


Figure 4.1 Seismic refraction – arrangement of equipment

For short distances the direct waves reach the geophones first. For longer distances

the refracted wave reaches first though the distances is longer than t he surface direct

distance. This is so because the speed of the wave in the dense material is higher than that in

the overburden material of less density. The geophone has a mechanism which records the

first wave and ignores the others. This enables a plot of arrival time versus the distance.

The first section of the graph represents the direct wave measurements while the second

section represents the refracted wave measurements (

Figure 4.2). The inverse of these curves are the velocities of the seismic waves. The

general types of the rocks are determined by geophysics from the knowledge of velocity

versus rock type. It is also used in the determination of depth to water table and thicknesses

of multiple strata. The depth D to the bedrock can be estimated from the formula.

Figure 4.2 Time versus distance for seismic waves

√

4.2.2 Detailed stage site investigations

At this stage the aim is to obtain data for use in the final design of the works. The

investigation is carried out by use of trial pits, sounding and boring. The extent of the use of

these methods depends on the type of the project at hand and the geotechnical parameters

being sought.

The trial pits

Tim

e

Distance d

Geophones

Seismic source


The pit and shaft technique supplies the most detailed and reliable data on he existing soil

conditions. Once the trial pit has been dug stratification of the soil should be done usually in

the field. In addition as much information should be recorded. This information includes

i. Depth to ground water table.

ii. Field assessment of the bearing capacity.

iii. Depth of the various strata encountered in the trial pit.

iv. The encountered soils should classified by visual inspection

a. Coarse grained soils should be described with adjectives such as angular,

rounded with traces of fines etc

b. Fine grained soils should be studied to indicate whether they are loamy, of low

plasticity, whether they are sandy clays etc

c. All soils should be described indicating their color and odour if any. Decaying

organic matter if encountered should be mentioned.

v. Obtain undisturbed samples when you can for the different layers of strata

encountered. These samples can then be taken to the laboratory for tests

For large sites the pits should then be surveyed and located in a grid system for incorporation

into the site investigation report.

Sounding tests

These are basically are penetration tests carried out to supplementing trial pits and borings.

The penetration resistance is measured and related to the bearing capacity. They are widely

used in site investigations. They consist of the cone penetrometer already presented in

chapter 1. The other commonly used penetration equipment is the dynamic cone

penetrometer used in the estimation of the California bearing ratio (CBR) of road pavement

layers. This enables the design of the pavement layers to be carried out

Boring methods

When a deep stratum has to be investigated it will usually be necessary to perform boring

operations to ascertain the strata below the ground to be used in the support of the proposed

structures. Several boring methods are available and are summarized as follows

Percussion drilling consists of a derrick, a power unit and a winch carrying a light steel cable

which passes thorough a pulley. The unit can be towed by a vehicle after the assembly is

folded. The assembly drops a chisel on the ground and strata being drilled


Figure 4.3 Schematic presentation of a drilling chisel

The excavation is effected by the drilling chisel. The drilling rods provide the necessary

weight for the penetration the strata. Further weight may be added when need arises. The

winch raises and lowers the chisel and its attachments

Below the water table the loosened soil forms slurry. Above the water table water is

introduced to form the slurry. Periodically the slurry is bailed out by a shell or a bailer to

make progress into the soil. In boreholes which are liable to collapse the borehole must be

cased. In some cases the casings slide on their own weight. On completion of the job, the

casing is jacked out.

Percussion drilling is usually done in diameters of 150mm to 300mm. the borehole

depth investigated by this drilling method can be up to 50 to 60 metres. This method of

drilling can be done on virtually all types of soils including those with boulders and cobbles.

The rig is versatile enough to place mechanical augers and penetrating testing equipments at

appropriate depths.

Power operated augers are usually on vehicles. Downward pressure is applied by pressure

or dead weight. The augurs are 75-300mm diameters. Augers are usually used in self

supporting soils. Casing is usually not needed since the augers have to be removed before

driving. In full flight augers the rod and the helix cover the entire length being investigated.

The augur is then brought up. The soil is ejected by reverse rotation. The likely hood of soil

from different strata being mixed up is very high. In the short flight augur the auger is

advanced into the soil and then raised. The soil is also ejected by reverse rotation.

Rod

Chisel


Figure 4.4 full flight and short flight augurs

The continuous flight augurs are sometimes fitted with a hollow stem which is plugged

during the drilling operations. When samples are needed the plug and the rods are removed

and a sampler is introduced for the recovery of a sample. The sample may be undisturbed

depending on the sampler utilized. The flight augurs are not suitable for use in loose soils

which are likely to collapse as the augur is inserted and removed from the hole.

Hand and portable augers are usually operated by persons by turning the handle of the

augur. The hand augers are typically of 75 – 300mm diameters. The soil is locked in the

auger and frequent removal is needed to ensure that the augur does not get stack in the soil.

Undisturbed samples may be obtained by introduction of small diameter tubes which are

hammered into the strata under investigation. This method is suitable for self supporting

soils. It is not possible to penetrate coarse granular soils.

Figure 4.5 schematic representation of a hand augur

Wash boring is a method of boring where water is pumped through boring rods and released

through narrow holes in the chisel attached at eth lower ends of the boring arrangement

(Figure ****).

Full flight augur Short flight augur


Figure 4.6 schematic representation of a hand augur

In this method the soil is loosened and broken by water jet. This is aided by the up an down

movements of the chisel. An attachment to the rods called a tiller enable the rotation on the

drilling bit. The drilling winch is able to raise and lower the chisel and hence get the

chopping action of the chisel.

This method is suitable for most soils but progress is slow if the particles of coarse

gravel larger particles are present. The accurate identification of the soil types is difficult.

The method cannot be used to recover soil samples for testing. However tube samplers can

be advanced into the borehole for obtaining relatively undisturbed samples.

Rotary drilling is done by use of drilling bits that cuts and grinds the subsoil or rock at the

bottom of the borehole. Water is usually pumped down hollow rods passing under pressure

through to the drilling tools. This cools and lubricates the bits. The fluid also provides

support for the borehole where there is no casing.

Two methods of rotary drilling are available. The first is open drilling where the soils

and rocks are broken within the diameter of the hole. Subsequently the tubes are removed and

tube samplers and testing continues below the borehole. This advances the drilling. The

second method is known as core drilling and involves creation of an annular hole in the

material and intact rock enters the drilling core. This advances the drilling and enables

samples to be retrieved from the borehole. The sample is then subjected to immediate field

description and taken to the laboratory for various tests. Typical core diameters range from

41mm to 165mm. The method is fast, but in large gravelly soils the speed is slowed by

rotation of the bit without advancement into the ground.

Water from pump

To sump

Drilling bit

Tiller


4.2.3 Sampling

Disturbed samples

Disturbed samples are recovered from trial pits and along drilling tools where there is no

attempt to retain the soil constituents. Disturbed samples should however be collected

carefully and placed in airtight tins or jars or in plastic sampling bags. The samples should be

labeled to give the borehole or trial pit identification number, depth of recovery and field

description should be done. The disturbed samples are used for identification tests namely

Field moisture content, PI, grading, compaction and CBR.

Undisturbed sample – cohesive soils

Undisturbed samples are recovered from trial pits and along drilling tools where there is an

attempt to retain the soil constituents. Such a sample is taken in an airtight container with

wax at both ends to prevent moisture from escaping during transportation to the laboratory.

In trial pits the samples can be obtained by pressing a sampler into the ground at the

appropriate depth. The sampler is typically 100mm diameter by 150mm long. In the hand

augur a 38mm sampling tube with a length of 200mm is fitted to the rod after the removal of

the augur. The tube is pressed into the soil and given half a turn to break the soil. The

sampler is then removed and the ends are waxed. In boring rigs a 105mm diameter sampler is

introduced to the borehole to recover a 100mm diameter sample. The sample is usually

381mm long and is fitted with a cutting shoe of about 110mm diameter. The sample is driven

by a falling weight. Any entrapped air or water is expelled from the top through a non return

valve. For soft clays thin walled samplers are preferred to minimize disturbance.

Inevitably there will be some disturbance in the process of retrieving soil samples

from the ground. The least disturbance is for shoes samples cut from the floor of trial pits.

Sample tubes, inserted by pressing, jacking or steady hammering produce some form of

disturbance depending on the thickness of the sampler walls. The degree of disturbance is

related to the area ratio of the sampler tube as given by Equation ****** In general good

samplers have and area ratio not exceeding 25%. Area ratios less than 10% are very good

and are used for very sensitive soils.

x100%

Figure 4.7 Typical sample tubes

De

Di

Di

De

Sampler tubes Sampler tubes fitted with a cutting shoe


Undisturbed sample – cohesionless soils

Various methods have been employed to obtain undisturbed sand samples. These include

freezing, chemical application, and use of compressed air (Smith and Smith, 1998).

Whatever method is employed eventual disturbance occurs as the soil is transported to the

laboratory for testing. In light of these difficulties it is prudent to assess the engineering

properties of cohesionless soils through field testing such as penetration.

Quality class for soil sampling

Table ** below based on Rowe (1972) shows the quality classes for soil samples obtained

from various site investigation operations.

Table 4.2 Quality class for soil sampling

Quality

class

Method of sampling Use of sample

5 Material brought up by drilling tools an no attempt is made

to retain all the soil constituents

Rough sequence of

strata

4 As for 5 but all soil constituents are retained as far as

possible. Bulk an jar samples. Plastic bag samples

Sequence of strata and

remolded properties

3 Pressed or driven thin or thick walled samplers with water

balance in very permeable soils

As above and

examination of soil

fabric

2 As for class 3 above but with water balance all the time As 3 and γ, n, mv, cu, c’

θ’

1 Thin walled piston samplers with water balance As 2 and cv and k

Borehole logs

Borehole logs summarizes all the laboratory an field tests carried out on samples representing

the various strata encountered in the boring operations. All ground conditions encountered at

the site are also included. The log enables a rapid accurate assessment of the soil profile on a

vertical scale. The details of the various strata encountered including all their geological

formation details which can be inferred are given. The details captured should include the

depth to which ground water was encountered. The description is based on particle

distribution and plasticity based visual inspection and feel. Soil color should also be

recorded.


Courtesy of Norken Engineering Consultants

Figure 4.8 Borehole logs


4.2.4 Scope of Site Investigation

Spacing of the trial pits and or boreholes

The scope of site investigation is dependent on the effect of the construction on the ground.

The scope should be commensurate with the needed geotechnical parameters. Table 4.3

shows the suggested minimum number of borings for the various structures.

Table 4.3 Recommended spacing of investigation trial pits and boreholes

Project Type of soil/Distance between borings Minimum no

Uniform Average Erratic

Multistory 45 30 15 4

1 to 2 storeys 60 30 15 3

Bridge piers and

abutments

30 30 15 1 – 2 per unit

For highways and runways during preliminary design the subgrade soils along the proposed

alignment should be sampled at 1000metres and the samples should be tested to establish the

in-situ CBR, grading and plasticity of the materials. At this stage the material site should be

investigated at 60 meter intervals. In the detailed stage the subgrade is sampled at 500meters

while the material sites are sampled at 30metres.

Depth of investigation

The depth should be such as to capture the geotechnical information needed for the design of

the facility. Equally important is to capture the information needed in the quantification of

the bill of quantities to ensure an accurate specification of the works is carried out. The

recommended depths below the formation of investigation for the various civil engineering

schemes is shown on Table 4.4 based on Figure 4.9 below.

Table 4.4 Depth of investigation

Project Depth In rock Parameters to be established

Column foundations 1.5B-3B 1.5-3m C, θ, N, RQD,TCR

Raft foundations 1.5B 1.5-3m C, θ, N. RQD,TCR

Bridge piers and

abutments

1.5B-3B 1.5-3m C, θ, N, RQD,TCR

Earthworks in fill for

highways

0.5L 0.50m PI, CBR for fill material

Strength of support

Earthworks in cut

highways

0.5H 0.50m Establish the type of excavated

material and strength of support

Pipe works D 0 Investigate type of excavated material

and strength of support


a) Structural foundations

b) Highway earthworks

c) Pipe works

Figure 4.9 Scope of foundation investigations

4.2.5 Site Investigation Reports

List of suitable headings

Title page

Gives the title of the project at a glance

Abstract

The abstract should be approximately 200 words. It is a very important element of the

project and should be prepared with care. It must convey the essence of the site investigation

and all the important findings without ambiguity.

List of contents

Guides the reader to the various chapters

Field work

A brief and complete description of what was done in the field. Boreholes, and trial pits

performed, field testing etc. Actual procedures of standard tests need not be repeated. A

L

L

H

In cut H

D

L

In fill

Retaining walls

Piled foundations

B

B

Raft foundations

B

Column

foundations

B


mention of the tests performed is sufficient. New procedures and peculiar fieldwork should

be explained.

Laboratory work

A brief and complete description of what was done in the laboratory work carried out . as in

the case of field testing actual procedures of standard tests need not be repeated. A mention

of the tests performed is sufficient. New procedures and peculiar laboratory equipment and

procedures should however be explained

Site description and geology

An engineering summary of the nature of the site an its geology, including aspects such

excavated areas and what was found, stability of natural slopes, drainage etc

Engineering properties of soils an rocks

A summary of the results of field and laboratory tests and other observations made at the site

Discussion

A reasoned discussion of what design and construction problems are likely to be encountered

in relation to the site and its geological situations.

Recommendations and conclusions

A brief but clear statement of the recommended geotechnical parameters investigated. The

treatment of the various aspects of design should come out clearly and without doubt. Values

of use in design and construction should be summarized viz, allowable bearing capacity,

estimated settlement, suitable types of foundations, construction requirements namely

grouting, compaction etc

References

A list of the books, papers, referred to in the work

Appendices

Appendix A – should contain site plan, borehole logs, photographs, etc

Appendix B – should contain tables of results of field and laboratory test those not included

in Appendix A

Appendix C – Any special or unusual test procedures adopted in the investigation

References:

Craig FR, 1987, Soil mechanics, Van Nostrand Reinhold (International) London

Bowles JE , 1982, Foundation Engineering, McGraw-Hill international book company,

Tokyo.

Tomlinson MJ and Boorman R (1986), Foundation and construction, Longman scientific and

technical, England

Franklin JA and Dussealt MB (1989) Rock Engineering, McGraw-Hill international editions,

London

Chen FH (1975) Foundations on expansive soils, Elsevier scientific Publishing Company


Chapter Six : Shoring and underpinning

Syllabus Shoring and underpinning

6.1 Shoring

Shoring is the process of supporting a building, a structure, or trench with props when in

danger of collapse or during repairs or alterations. Shoring comes from shore a timber or

metal prop.

Buildings

Raking Shores consist of one or more timbers sloping between the face of the structure to be

supported and the ground. The most effective support is given if the raker meets the wall at

an angle of 60 to 70 degrees. A wall-plate is typically used to increase the area of support.

Foundations

Shoring is commonly used when installing the foundation of a building. A shoring system

such as piles and lagging or shotcrete will support the surrounding loads until the

underground levels of the building are constructed.

Trenches

During excavation, shoring systems provide safety for workers in a trench and speeds up

excavation. It is designed to prevent collapse Concrete structures shoring, in this case also

referred to as falsework, provides temporary support until the concrete becomes hard and

achieves the desired strength to support loads.

a) Sketch of a timber double raking

shore. Projected centre lines of

floors and shores meet.

b) Sketch of a timber single flying

shore between adjacent buildings.

Figure 6.1 Examples of Shoring

https://en.wikipedia.org/wiki/File:Double-raking-shore.png

- 73- shoring and underpinning


c) Schematic sketch of a modern steel

trench shore being lowered into a

trench.

d) Traditional trench shoring or

Timbering.


6.2 Underpinning

Underpinning is the process of strengthening the foundation of an existing building or other

structure. Underpinning may be necessary for a variety of reasons:

• The original foundation is not strong or stable enough.

• The usage of the structure has changed in which case additional load is being

transmitted to the foundation.

• The properties of the soil supporting the foundation may have changed or were

mischaracterized during design.

• The construction of nearby structures necessitates the excavation of soil supporting

existing foundations.

• To increase the depth or load capacity of existing foundations to support the addition of

another storey to the building (above or below grade).

• Natural causes have caused the structure to move, thereby requiring stabilisation of

foundation soils and/or footings.

Underpinning may be accomplished by extending the foundation in depth or in breadth

so it either rests on a more supportive soil stratum or distributes its load across a greater

area. Use of micro piles and jet grouting are common methods in underpinning.

Mass Concrete Underpinning

Mass concrete underpinning method is an old tradition established over the years. This

underpinning method strengthens an existing structure's foundation by digging boxes by hand

underneath and sequentially pouring concrete in a strategic order. The final result is basically

a foundation built underneath the existing foundation. This underpinning method is generally

applied when the existing foundation is at a shallow depth but reports of fifteen meter depths

have been made. Heavy machinery is not called for in this method due to the tight nature of

the boxes being dug.

a) Mass concrete below the wall b) Sequence of operations

Figure 6.2 Underpinning a wall

Beam and base underpinning

The beam and base method of underpinning is a more technically advanced adaptation of

traditional mass concrete underpinning. A reinforced concrete beam is constructed below,

Ground floor

1 2 3

1

2

3

1 2 3

1

2

3

- 75- Shoring and underpinning

Foundation Engineering I: - ECE 2406

above or in replacement of the existing footing. The beam then transfers the load of the

building to mass concrete bases, which are constructed at designed strategic locations. Base

sizes and depths are dependent upon the prevailing ground conditions. Beam design is

dependent upon the configuration of the building and the applied loads.

Figure 6.3 Beam and base new foundations

Mini-piled underpinning

Mini-piles have the greatest use where ground conditions are very variable, where access is

restrictive, where environmental pollution aspects are significant, and where structural

movements in service must be minimal. Mini-piled underpinning is generally used when the

loads from the foundations need to be transferred to stable soils at considerable depths in

excess of 5 m. Mini-piles may either be augured or driven steel cased, and are normally

between 150 mm and 300 mm in diameter. Piling rigs for this type of underpinning are

designed to operate in with limited headroom and limited space. The equipment is capable of

constructing piles to depths of up to 15m.

Figure 6.4 Piled underpinning

Beam

New wall or series of columns

New base

Ground floor

Ground floor

Piles driven or bore

to firm ground


Chapter Seven : Excavation , bracing, ground water, dewatering

techniques.

Syllabus: Excavation, bracing ground water, dewatering techniques

7.1 Excavation and bracing

Ordinarily excavations in most cases will proceed without support. However in deep

excavations it will be necessary to support the sides in order to protect the workers Bracing

is usually done by installing a support of struts and piles. In very soft and loose soils the

piling is done first. This is then followed by installation of struts as the excavation is done.

As the depth increases the soil starts to yield before the strut is installed. Because of the

support being granted by the supports the Rankine conditions are not met in the force

generation. Figure 7 shows a braced excavation.

The pressure on the struts for design purposes is empirically determined from the empirical

formulas shown on the Figure

Figure a) is the strutted excavation

Figure b) shows the measured loads in sand excavations

Figure c) shows the Estimate of lateral load in sand excavations.

The pressure is rectangular with the maximum value being 0.65*Ka*ɣH.

Figure d) Estimate of lateral load in clay excavations where the stability number ɣH/Cu is

less than 4.

Where the pressure varies between 0.2 ɣH and 0.4 ɣH. Note the large variation

Figure e) Estimate of lateral load in clay excavations where the stability number ɣH/Cu is

greater than 4.

The pressure is rectangular with the maximum value being 1.0*Ka*ɣH.

M is usually taken as 1 but may be taken as low as 0.4 for the very soft clays

Note the large variation



Figure 7. 1 Strutted excavation

7.2 Ground water and dewatering techniques

Dewatering involves controlling groundwater by pumping, to locally lower groundwater

levels in the vicinity of the excavation.

Sump pumping

The most common and simple form of dewatering is sump pumping. In this case

groundwater is allowed to enter the excavation where it is then collected in a sump and

pumped away by robust solids handling pumps. Sump pumping can be effective in many

circumstances, but seepage into the excavation can create the risk of instability and other

construction problems.

Wellpoints

Wellpoint dewatering is widely used for excavations of shallow depths, especially for

pipeline trench excavations. A typical wellpoint system consists of a series of small

diameters wells (known as wellpoints) connected via a header pipe, to the suction side of a

suitable wellpoint pump. The pump creates a vacuum in the header pipe, drawing water up

out of the ground. For long pipeline trenches, horizontal wellpoints may be installed by

special trenching machines. Wellpoints are typically installed in lines around the excavation,

and are pumped by diesel or electrically powered pumps, with associated header mains, water

discharge pipes, power supply generators, electrical controls and monitoring systems.



Figure 7.2 Dewatering from wellpoints

Source (www.groundwaterenginneering.com

Shallow Wells

Shallow wells comprise surface pumps which draw water through suction pipes installed in

bored wells drilled by the most appropriate well drilling and or bored piling equipment. The

limiting depth to which this method is employed is about 8 m. Because wells are

prebored,this method is used when hard or variable soil conditions preclude the use of a

wellpoint system. Since the initial cost of installation is more compared to wellpoints it is

preferred in cases where dewatering lasts several months or more. Another field of

application is the silty soils where correct filtering is important.

Deep wells

A deep well system consists of an array of bored wells pumped by submersible pumps.

Pumping from each well lowers the groundwater level and creates a cone of depression or

drawdown around itself. Several wells acting in combination can lower groundwater level

over a wide area beneath an excavation. Because the technique does not operate on a

suction principle, large drawdowns can be achieved, limited only by the depth of the wells,

and the hydrogeological conditions.

The wells are generally sited just outside the area of proposed excavation, and are pumped

by electric submersible pumps near the base of each well. Water collection pipes, power

supply generators, electrical controls and monitoring systems are located at the surface.



Figure 7.3 Dewatering for a rectangular foundation from deep wells

Source (www.groundwaterenginneering.com)

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