# Shear Strength of Soils

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30-Dec-2015Category

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Shear Strength of Soils

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Shear failureSoils generally fail in shearstrip footingembankmentAt failure, shear stress along the failure surface reaches the shear strength.

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Shear failureThe soil grains slide over each other along the failure surface.No crushing of individual grains.

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Shear failureAt failure, shear stress along the failure surface () reaches the shear strength (f).

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Mohr-Coulomb Failure Criterioncfailure envelopecohesionfriction anglef is the maximum shear stress the soil can take without failure, under normal stress of .

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Mohr-Coulomb Failure CriterionShear strength consists of two components: cohesive and frictional.

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c and are measures of shear strength.Higher the values, higher the shear strength.

Mohr Circles & Failure EnvelopeXYSoil elements at different locationsXY~ failure~ stable

Mohr Circles & Failure EnvelopeYInitially, Mohr circle is a pointcc+The soil element does not fail if the Mohr circle is contained within the envelopeGL

Mohr Circles & Failure EnvelopeYcGLAs loading progresses, Mohr circle becomes larger.. and finally failure occurs when Mohr circle touches the envelope

Orientation of Failure PlaneYcGLc+90+Failure plane oriented at 45 + /2 to horizontal

Mohr circles in terms of & =+vhvh

Envelopes in terms of & Identical specimens initially subjected to different isotropic stresses (c) and then loaded axially to failurefInitiallyFailureufAt failure,3 = c; 1 = c+f3 = 3 uf ; 1 = 1 - ufc, c, in terms of in terms of

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Triaxial Test Apparatuspiston (to apply deviatoric stress)pedestalperspex cellcell pressureback pressurepore pressure orvolume change

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Types of Triaxial TestsUnder all-around cell pressure cShearing (loading)Is the drainage valve open?Is the drainage valve open?deviatoric stress ()yesnoyesnoConsolidated sampleUnconsolidated sampleDrained loadingUndrained loading

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Types of Triaxial TestsDepending on whether drainage is allowed or not during initial isotropic cell pressure application, and shearing,there are three special types of triaxial tests that have practical significances. They are:ConsolidatedDrained (CD) testConsolidated Undrained (CU) testUnconsolidated Undrained (UU) test

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Granular soils have no cohesion. c = 0 & c= 0For normally consolidated clays, c = 0 & c = 0.For unconsolidated undrained test, in terms of total stresses, u = 0

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CD, CU and UU Triaxial Tests no excess pore pressure throughout the test very slow shearing to avoid build-up of pore pressureConsolidated Drained (CD) Test gives c and Can be days! not desirableUse c and for analysing fully drainedsituations (e.g., long term stability, very slow loading)

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CD, CU and UU Triaxial Tests pore pressure develops during shear faster than CD (preferred way to find c and )Consolidated Undrained (CU) Test gives c and Measure

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CD, CU and UU Triaxial Tests pore pressure develops during shear very quick testUnconsolidated Undrained (UU) Test analyse in terms of gives cu and u Not measured unknown

= 0; i.e., failure envelope is horizontalUse cu and u for analysing undrained situations (e.g., short term stability, quick loading)

1- 3 Relation at Failure

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Stress Pointhvstress point

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Stress PathStress path is the locus of stress pointsStress pathDuring loadingStress path is a convenient way to keep track of theprogress in loading with respect to failure envelope.

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Failure Envelopesc cos tan-1 (sin )failureDuring loading (shearing).stress path

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Pore Pressure Parameters A simple way to estimate the pore pressure change in undrained loading, in terms of total stress changes ~ after Skempton (1954)Skemptons pore pressure parameters A and B

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Pore Pressure Parameters For saturated soils, B 1.A-parameter at failure (Af)For normally consolidated clays Af 1.B-parameterB = f (saturation,..)Af = f(OCR)For heavily overconsolidated clays Af is negative.

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