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Earth Structures Lecture-1 25-06-2012 Dr. Zia-ur-Rehman Email: gzia718@ hotmail.com Cell: 0300-4927158
Earth StructuresLecture-1 25-06-2012
Dr. Zia-ur-RehmanEmail: gzia718@ hotmail.com
Cell: 0300-4927158
Course OutlineFailure Mechanisms in Natural and Artificial Slopes. Stability Analysis for slopes in Cohesive, Non-Cohesive and C-phi soils. Use of stability charts. Steady state seepage problems in Earth Structures. Influence of surcharge, submergence and tension crack on Stability. Numerical Integration Analysis by Fellenius Method and Bishop's Simplified Method.
Principles of Design and Stability Analysis of Earth and Rock Fill Dams under Drained and Un-drained conditions: Stress Distribution and Deformation within the Dam and Foundation Strata. Effect of earthquakes on slope stability. 22
Books
Slope StabilityUS Army Corps of Engineers
3
Geotechnical EngineeringPrinciples and Practices of Soil Mechanics and Foundation Engineering
By V N S Murthy
Basic Soil MechanicsBy Roy Whitlow
Soil Strength and Slope StabilityByJ. Michael DuncanStephen G. Wright
3
Sessional Marks
Four parameters
•Attendance•Presentation and Report•Assignments•Tests
44
Slope
The slope is defined as the ratio of the altitude change to the horizontal distance between any two points on the line.
An earth mass the surface of which makes an angle with the horizontal direction.
55
Types of Slopes
Man made slopesNatural slopesNatural slopes are those that exist in nature and are formed by natural causes. Such slopes often exist in hilly areas.
The sides of cuttings, the slopes of embankments constructed for roads, railway lines, canals etc. and the slopes of earth dams constructed for storing water are examples of man made slopes.
66
Types of Slopes-Extent
Infinite slopesThe term infinite slope is used to designate a constant slope of infinite extent. The long slope of the face of a mountain is an example of this type.
Finite slopesFinite slopes are limited in extent. The slopes of embankments and earth dams are examples of finite slopes. The slope length depends on the height of the dam or embankment.
77
Causes of Failure of Slopes Gravitational forceThe component of gravity that acts in the direction of probable motion.
88
Causes of Failure of Slopes Force due to seepage water The seepage occurring within a soil mass causes seepage forces, which have much greater effect than is commonly realized.
99
Causes of Failure of Slopes Erosion of the surface of slopes due to flowing water Erosion on the surface of a slope may be the cause of the removal of a certain weight of soil, and may thus lead to an increased stability as far as mass movement is concerned. On the other hand, erosion in the form of undercutting at the toe may increase the height of the slope, or decrease the length of the incipient failure surface, thus decreasing the stability.
1010
Causes of Failure of Slopes Sudden lowering of water adjacent to a slopeWhen there is a lowering of the ground water or of a free water surface adjacent to the slope, for example in a sudden drawdown of the water surface in a reservoir there is a decrease in the buoyancy of the soil which is in effect an increase in the weight. This increase in weight causes increase in the shearing stresses that may or may not be in part counteracted by the increase in shearing strength, depending upon whether or not the soil is able to undergo compression which the load increase tends to cause. If a large mass of soil is saturated and is of low permeability, practically no volume changes will be able to occur except at a slow rate, and in spite of the increase of load the strength increase may be inappreciable.
1111
Causes of Failure of Slopes Forces due to earthquakesShear at constant volume may be accompanied by a decrease in the intergranular pressure and an increase in the neutral pressure. A failure may be caused by such a condition in which the entire soil mass passes into a state of liquefaction and flows like a liquid. A condition of this type may be developed if the mass of soil is subject to vibration, for example, due to earthquake forces.
1212
Drained/Un-drained Conditions Drained Condition
Drained is the condition under which water is able to flow into or out of a mass of soil in the length of time that the soil is subjected to some change in load. Under drained conditions, changes in the loads on the soil do not cause changes in the water pressure in the voids in the soil, because the water can move in or out of the soil freely when the volume of voids increases or decreases in response to the changing loads.
Un-drained Condition
Undrained is the condition under which there is no flow of water into or out of a mass of soil in the length of time that the soil is subjected to some change in load. Changes in the loads on the soil cause changes in the water pressure in the voids, because the water cannot move in or out in response to the tendency for the volume of voids to change.
1313
Drained/Un-drained Conditions Drained Condition
Drained signifies a condition where changes in load are slow enough, or remain in place long enough, so that water is able to flow in or out of the soil, permitting the soil to reach a state of equilibrium with regard to water flow. The pore pressures in the drained condition are controlled by the hydraulic boundary conditions, and are unaffected by the changes in loads.
Un-drained Condition
Undrained signifies a condition where changes in loads occur more rapidly than water can flow in or out of the soil. The pore pressures increase or decrease in response to the changes in loads.
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Total Stress and Effective Stress
Stress is defined as force per unit area.
Total stress is the sum of all forces, including those transmitted through interparticle contacts and those transmitted through water pressures, divided by the total area. Total area includes both the area of voids and the area of solid.
Effective stress includes only the forces that are transmitted through particle contacts. It is equal to the total stress minus the water pressure.
1515
Driving and Resisting Forces
When slope is not stable. Slope stability is based on the interplay between two types of forces, driving forces and resisting forces. Driving forces promote downslope movement of material, whereas resisting forces prevent movement. When driving forces overcome resisting forces, the slope is unstable and results in mass wasting.
1616
Driving and Resisting Forces
The main driving force in most land movements is gravity.
The main resisting force is the material's shear strength.
1717
Driving Forces
Water plays a key role in producing slope failure. In the form of rivers and wave action, water erodes the base of slopes, removing support, which increases driving forces.
Water can also increase the driving force by loading, i.e., adding to the total mass that is subjected to the force of gravity.
1818
Resisting Forces
Resisting forces act oppositely of driving forces. The resistance to downslope movement is dependent on the shear strength of the slope material.
Shear strength is a function of cohesion (ability of particles to attract and hold each other together) and internal friction (friction between grains within a material).
1919
Measuring Shear Strength
In the LaboratoryDirect shear test Unconfined compression testTriaxial compression test In the FieldVane shear test Standard Penetration test (SPT)Cone Penetration test (CPT)Pressuremeter test (PMT)
2020
Direct Shear Test Device
2121
Direct Shear Test Device
2222
Unconfined Compression Test
σ3=0
σ1
2323
Triaxial Compression Test
2424
Vane Shear Test
2525
SPT
2626
SPT
2727
SPT
2828
CPT
2929
PMT
3030
Dummy cone (1)
Claming nuts (4)
Clamping rings (3)
Pre
ssur
emet
er
mod
ule
(2)
Electronics box (15)
Pressure transducer (11)
Gas-electricseparator (10)
58.6 Barsmax.
10 Barsmax.
(9)
(12)
Pressure source (14)
Pressure hoseand electrical cable (7)
Push rods (8)
12V
To datalogger
A/D convertor (18)Portable
computer (19)
230 V
Battery power(Handy Mains) (17)
12 V Battery (16)
Friction reducer/adaptor (5)
(6)
Jack
Ground surface
Pressureregulators (13)
(a)(b)
+-
HET
Pressure
PMT
3131
PMT
3232
