Shear Strength of Soils Younus Geology deptt CNU

Shear Strength of Soils Younus, Geology deptt. , CNU 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.

Shear failure surface The soil grains slide over each other along the failure surface. No crushing of individual grains. Younus, Geology deptt. , CNU 3

Shear failure At failure, shear stress along the failure surface ( ) reaches the shear strength ( f). Younus, Geology deptt. , CNU 4

Mohr-Coulomb Failure Criterion pe o l e nv e e r u l fai cohesion f c friction angle f is the maximum shear stress the soil can take without failure, under normal stress of . Younus, Geology deptt. , CNU 5

Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional. f f tan c c f Younus, Geology deptt. , CNU co nt ive hes ne mpo co frictional component 6

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

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

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 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

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

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

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

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

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

v h Stress Point X t stress point ( v- h)/2 h v s ( v+ h)/2 Younus, Geology deptt. , CNU 16

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. Younus, Geology deptt. , CNU 17

Failure Envelopes t failure tan-1 (sin ) c c cos stress path s During loading (shearing)…. Younus, Geology deptt. , CNU 18