Shear Strength of Soils Duration 17 min 04

Shear failure Soils generally fail in shear embankment strip footing failure surface mobilised shear

Shear failure surface The soil grains slide over each other along the failure surface.

Shear failure At failure, shear stress along the failure surface ( ) reaches the

Mohr-Coulomb Failure Criterion pe o l e nv e e r u l fai

Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional. f f

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

Mohr Circles & Failure Envelope X Y Y X Soil elements at different locations

Mohr Circles & Failure Envelope The soil element does not fail if the Mohr

Mohr Circles & Failure Envelope As loading progresses, Mohr circle becomes larger… GL c

Orientation of Failure Plane Failure plane oriented at 45 + /2 to horizontal Y

Mohr circles in terms of & ’ v’ v X h = X u

Envelopes in terms of & ’ Identical specimens initially subjected to different isotropic stresses

Triaxial Test Apparatus piston (to apply deviatoric stress) failure plane O-ring impervious membrane soil

Types of Triaxial Tests deviatoric stress ( ) Under all-around cell pressure c Is

Types of Triaxial Tests Depending on whether drainage is allowed or not during v

For unconsolidated undrained test, in terms of total stresses, u = 0 Granular soils

CD, CU and UU Triaxial Tests Consolidated Drained (CD) Test v no excess pore

CD, CU and UU Triaxial Tests Consolidated Undrained (CU) Test v pore pressure develops

CD, CU and UU Triaxial Tests Unconsolidated Undrained (UU) Test v pore pressure develops

1 - 3 Relation at Failure 1 3 X X soil element at

Stress Point v h X t stress point ( v- h)/2 h v s

Stress Path During loading… t Stress path is the locus of stress points Stress

Failure Envelopes t failure tan-1 (sin ) c c cos stress path s During

Pore Pressure Parameters A simple way to estimate the pore pressure change in undrained

Pore Pressure Parameters B-parameter B = f (saturation, . . ) For saturated soils,

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Shear Strength of Soils Duration: 17 min: 04 sec N. Sivakugan 1

Shear failure Soils generally fail in shear embankment strip footing failure surface mobilised shear resistance At failure, shear stress along the failure surface reaches the shear strength. 2

c and are measures of shear strength. Higher the values, higher the shear strength. 7

Mohr Circles & Failure Envelope X Y Y X Soil elements at different locations X ~ failure Y ~ stable 8

Mohr Circles & Failure Envelope The soil element does not fail if the Mohr circle is contained within the envelope GL c Y c c Initially, Mohr circle is a point 9 c+

Mohr Circles & Failure Envelope As loading progresses, Mohr circle becomes larger… GL c Y c c. . and finally failure occurs when Mohr circle touches the envelope 10

Orientation of Failure Plane Failure plane oriented at 45 + /2 to horizontal Y 45 + /2 GL 45 + /2 c Y c 90+ c c+ 11

Mohr circles in terms of & ’ v’ v X h = X u h ’ effective stresses h ’ v’ h + X total stresses u v 12 u

Envelopes in terms of & ’ Identical specimens initially subjected to different isotropic stresses ( c) and then loaded axially to failure f c c uf Initially… Failure c, in terms of At failure, 3 = c; 1 = c+ f c’, ’ in terms of ’ 3 ’ = 3 – u f ; 1 ’ = 1 - u f 13

Triaxial Test Apparatus piston (to apply deviatoric stress) failure plane O-ring impervious membrane soil sample at failure porous stone perspex cell water cell pressure pore pressure or back pressure pedestal SIVA Copyright© 2001 volume change 14

Types of Triaxial Tests deviatoric stress ( ) Under all-around cell pressure c Is the drainage valve open? yes Consolidated sample SIVA no Unconsolidated sample Copyright© 2001 Shearing (loading) Is the drainage valve open? yes no Drained Undrained loading 15

Types of Triaxial Tests Depending on whether drainage is allowed or not during v initial isotropic cell pressure application, and v shearing, there are three special types of triaxial tests that have practical significances. They are: Consolidated Drained (CD) test Consolidated Undrained (CU) test Unconsolidated Undrained (UU) test SIVA Copyright© 2001 16

For unconsolidated undrained test, in terms of total stresses, u = 0 Granular soils have no cohesion. c = 0 & c’= 0 For normally consolidated clays, c’ = 0 & c = 0. 17

CD, CU and UU Triaxial Tests Consolidated Drained (CD) Test v no excess pore pressure throughout the test v very slow shearing to avoid build-up of pore pressure Can be days! not desirable v gives c’ and ’ Use c’ and ’ for analysing fully drained situations (e. g. , long term stability, very slow loading) SIVA Copyright© 2001 18

CD, CU and UU Triaxial Tests Consolidated Undrained (CU) Test v pore pressure develops during shear Measure ’ v gives c’ and ’ v faster than CD ( preferred way to find c’ and ’) SIVA Copyright© 2001 19

CD, CU and UU Triaxial Tests Unconsolidated Undrained (UU) Test v pore pressure develops during shear = 0; i. e. , failure envelope is Not measured horizontal ’ unknown v analyse in terms of gives cu and u v very quick test Use cu and u for analysing undrained situations (e. g. , short term stability, quick loading) SIVA Copyright© 2001 20

1 - 3 Relation at Failure 1 3 X X soil element at failure 1 3 21

Stress Path During loading… t Stress path is the locus of stress points Stress path is a convenient way to keep track of the progress in loading with respect to failure envelope. SIVA Copyright© 2001 23

Pore Pressure Parameters A simple way to estimate the pore pressure change in undrained loading, in terms of total stress changes ~ after Skempton (1954) 1 Y 3 u = ? Skempton’s pore pressure parameters A and B 25

Pore Pressure Parameters B-parameter B = f (saturation, . . ) For saturated soils, B 1. A-parameter at failure (Af) Af = f(OCR) For normally consolidated clays Af 1. For heavily overconsolidated clays Af is negative. 26