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8/10/2019 geotechnical engineering_Chapter 1 - Soil Strength and Stiffness
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CEGB 333GEOTECHNICAL ENGINEECHAPTER 1: SOIL STRENGTH AND STI
MISS INTAN NOR ZULIANA BIN
INTAN@
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WEEK 1: COURSE OUTLINE
COURSE OBJECTIVE:
To introduce concept of soil failure models and its behaviours. To provide an understanding in analyses of soil problems associ
structures and its application to civil engineering. To enhance students technicaljudgments in solving complex so
engineering problems.
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WEEK 1: COURSE OUTLINE (SYLLAMODULE 1: SOIL STRENGTH AND STIFNESS
Stress-strain, Mohr-Coulomb Failure Theory and Strength ofSoil.
Soil Stiffness and Elasticity. Soil Deformation: Elastic (linear and Non-Linear), Perfectly-
Plastic
and Elasto-Plastic (strain-Hardening and Strain-Softening)models.
Stress Paths.
MODULE 2: CRITICAL STATE SOIL MECHANICS
Critical State Concept.
State Boundary Surface. Critical State Line and Stress Paths. Soil Yielding.
MODULE 3: STRESSES AND DISPLACEMENTS OF SOIL MASS
Stresses in Soil Mass due to Applied Loading: Point Load, LineLoad, Uniform pressures (strip, circular and rectangular areas)
and Linearly-increasing pressures.
Influence Chart for vertical Stress.
Elastic Displacements.
MODULE 4: LATERAL EARTH PRESSUR
Types of Retaining Walls. Rankines Theory of Earth Pressure. Coulombs Theory of Earth Pressure.
MODULE 5: CONSOLIDATION SETTLEM
Method of Consolidation SettlementDimensional Method, Skempton-Bje
Path Method.
Degree of Consolidation and TerzaghDimensional Consolidation.
Coefficient of Consolidation: Log-Tim
Methods. Compression Ratios and Secondary C
MODULE 6: STABILITY OF SLOPES
Mass Movement and Landslides. Taylors Stability Number Method. Method of Slices for Circular Slip: Con
(Fellenius/Swedish) and Bishops Sim
Translational Slide on Infinite Slope.
Slope with Plane Failure Surface (Cul
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WEEK 1: COURSE OUTLINE
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WEEK 1: COURSE OUTLINE
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WEEK 1: COURSE OUTLINE
QUIZ 1: WEEK 4 DURING CLASS TIME TEST 1: WEEK 7 18TH JULY 2014. FRIDAY. 3PM-4PM. VENUE (TBA
TEST 2: WEEK 11 14TH AUGUST 2014. WEDNESDAY. 6PM-7PM. V GROUP PROJECT: DEADLINE BY 5PM THE END OF WEEK 14. IND
ASSESSMENT ON REPORT WRITING AND ORAL PRESENTATION
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CO1-PO1a:Ability to comprehend the stress-strain behaviouApply fundamental knowledge of mathematics, science and civil eprinciples in solving complex problems (Comprehend (C1,C2))
This chapter covers :
Stress-strain, Mohr-Coulomb Failure Theory and St Soil Stiffness and Elasticity.
Soil Deformation: Elastic (linear and Non-Linear), PPlastic
and Elasto-Plastic (strain-Hardening and Strain-Sofmodels.
Stress Paths.
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INTRODUCTION
Soil consists of solid particles with continuous voids filand/or water.
Soil particles and water can be considered as incomprematerials unlike air which is highly compressible.
Thus this will change the volume of the soils due torearrangement of soil particles into new positions throrolling, sliding , with regards of changes in inter-partic
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MOVEMENT OF WATER IN SOIL
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EFFECTIVE STRESS
The effective stress concept (Terzaghi, 1943) is mainly to saturated soil mass. Where there are only 2 phase; s
liquid.
The principle of effective stress only applies to fully satsoil.
The effective stress consists of 2 other stresses: the toand power water pressure.
the total normal stress () on a plane being the force parea acts in a normal direction across the plane by assusoil in a single solid phase material.
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EFFECTIVE STRESS
The pore water pressure (u) is a pressure of the water fthe voids between soil particles. the effective normal stress () represents the stress tr
through soil skeletal due to interparticle forces.
Compression and shear strength are the function of efstress. Effective stress is the stress that controls engineering
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EFFECTIVE STRESS
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WEEK 1: STRESS-STRAIN
Soil behaviour is observes through the study of stress- s
Stress is being force per unit area. Whilst strain is deformation in a unit length, area, or vol
L
LOADy
y
xx
xy
Shear stressNormal stress
STRESSSTRAIN
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STRESS-STRAIN
In principle, soils behave like other solids when subject tloading, but there are significant differences to, say, ste
Except for some partially cemented types, soils cannot sustai When loaded, soils will generally undergo a change in volume
pore fluid pressure.
Saturated soils can only undergo a change in volume as pore wsqueezed out (or lost by drying, etc.); the rate of water loss (dcontrolled by the permeability of the soil.
Some (hard or still) soils will exhibit brittle failure by shearing,simply distort plastically.
Once a shear slip has occurred the problem changes from onemechanics to one of rigid body mechanics.
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MOHR COULOMB THEORY
Mohr (1900) presented a theory for rupture in materialcontended that a materials fails because of a critical co
normal stress and shear stress, and not from either manormal or shear stress alone. Thus the functional relati
between normal stress and shear stress on a failure plaexpressed in the form
f = f() Equation 1.1
where
f= shear stress on the failure plane
= normal stress on the failure plane
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The failure envelope defined by equation 1.1 is a curvemost soil mechanics problems , it is sufficient to appro
shear stress on the failure plane as a linear function of stress (Coulomb, 1776). This relation can be written as
f = c + tan
Where
c = cohesion = angle of internal friction
The preceding equation is called the Mohr-Coulomb failucriterion.
MOHR COULOMB THEORY
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SHEAR STRENGTH ENVELOPE
FAILURE ENVELOPE
c
Y
XY
X
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STRESS-STRAIN BEHAVIOUR
rr
a
r
= 1-
3
TRIAXIAL TEST
33
1
1
1A
1
VC
3 < 1, therefore:
3 = minor principal stress
1 = major principal stress
1C
1B
1Aexpansion
contraction
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STRESS-STRAIN BEHAVIOUR
Dense sand / OC clay
loose sand / NC clay
Dense sand Loose sand
stressesstresses
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STRENGTH OF SOIL
Shear strength model as MC failure criterion is used to determistrength parameters of soil; friction angle, and cohesion, c.
Shear strength parameters are not constant and depending on
Initial state of the soil (stress history) Type of loading (drained or undrained)
Shear strength parameters are used to define ultimate strengthsoil.
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STRESS-STRAIN BEHAVIOURS
Elastic strain
hardening/softenin
Plastic flow
elastic-perfectly plastic
rigid-perfectly plastiLinearly elastic
Typical soil model
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SOIL STIFFNESS & ELASTICITY
Young Modulus, Y
Soil A
High Y
Stiffness?
Soil B
Low Y
Stiffness?
Higherstiffness?
Youngs modulus is the stress needed to
compress the solid to shorten in a unit strain.
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ANALYSIS OF STRESS PATH In an elastic body the deformation caused by a c
loading is predictable from the value of E and tchange in load.
The final value of strain is not affected by intermvariations in the pattern of loading, but only wit
overall change. Soil masses, however, demonstrate elasto-plast
behaviour, so that the exact pattern of loading ounloading may significantly affect the final resu
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ANALYSIS OF STRESS PATH
In an analysis of elasto-plastic behaviour it is instrplot the stress change that take place throughoutentire pattern of loading.
Diagrams or graphs of stress changes are referredstress path diagrams.
The will take a number of forms dependent on tyanalysis required.
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STRESS PATHS
Describes a change in the stress state of a soil.
Is a line drawn through the stress points for successive sstates. Can be linear or curve depending on the loading pattern Stress path is a convenient way to keep track of the pro
in loading with respect to failure envelope.
important to show the stress/strain/volumetric behaviosoil. Can be shown in / space, 1/3 space ,t/s space and
space.
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Loose or soft soils in compression generally exhibit strain-hardecharacteristics i.e. they contract and become stiffer. The shear bsoil is more complex and much depends on the density.
In compact sands and overconsolidated clays a brittle failure in shear slip is likely to occur at peak stress.
In loose or soft soils contraction takes place up to the yield poincontinuous shearing occurs at constant or decreasing ultimate svery large strain (>1m) occur, e.g. in hillside or embankment lanultimate stress may further decrease to a lower residual stress wform of strain softening behaviour.
Stress path in / space
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/ space
Idealized types of stress-strain behaviors: (a) nonlinear elastic
Model, (b) linear elastic model, and (c) elasto-plastic model
Stress path in / space
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Various types of elastoplastic behaviors: (a) strain hardening, (b)
perfectly plastic, (c) strain softening, and (d) combination of a to c.
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Stress path in 1/3 space
For many problems and in the interpretation of shear tcomparisons are often required between drained and u
behaviour and between effective stresses and total strStress paths plotted on principal stress axes may be us
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Stress path in 1/3 space
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Stress path in t/s space
Stress path can be conveniently represented bycircle and this can also be related to a failure critThe coordinated of the maximum shear stress pMohr circle are given by :
s = (1 + 3)
t = (1 - 3)
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Stress path in t/s space
Stress path in q/p space
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Stress path in q /p space
While the stress path methods
described above are useful inproblem involving plane strain, theyare somewhat limited in a general
sense since they cannot easilyrepresent true triaxial conditions.
If the mean stress p and thedeviator stress q are used insteadof s and t then the plane stain,biaxially symmetrical and true
triaxial stress states can be
represented with equal facility.