geotechnical engineering_Chapter 1 - Soil Strength and Stiffness

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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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.

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geotechnical engineering_Chapter 1 - Soil Strength and Stiffness