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

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