Shear Strength sivakugan

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  • 1.Shear Strengthof Soils N.Sivakugan Duration: 17 min: 04 sec

2. Shear failure

  • Soils generally fail inshear

strip footing embankment At failure, shear stress along the failure surface reaches the shear strength. failure surface mobilised shear resistance 3. Shear failure The soil grains slide over each other along the failure surface. No crushing of individual grains. failure surface 4. Shear failure At failure, shear stress along the failure surface ( ) reaches the shear strength ( f ). 5. Mohr-Coulomb Failure Criterion c failure envelope cohesion friction angle fis the maximum shear stress the soil can take without failure, under normal stress of . f 6. Mohr-Coulomb Failure Criterion Shear strength consists of two components:cohesiveandfrictional . f f c ftan c cohesive component frictional component 7. c andare measures of shear strength. Higher the values, higher the shear strength. 8. Mohr Circles & Failure Envelope X Y Soil elements atdifferent locations X Y ~ failure ~ stable X Y 9. Mohr Circles & Failure Envelope Y Initially, Mohr circle is a point c c + The soil element does not fail if the Mohr circle is contained within the envelope GL c c 10. Mohr Circles & Failure Envelope Y c GL As loading progresses, Mohr circle becomes larger .. and finally failure occurs when Mohr circle touches the envelope c c 11. Orientation of Failure Plane Y c GL c + 90+ Failure plane oriented at45 + /2to horizontal c c 45 + /2 45 + /2 Y 12. Mohr circles in terms of& = + v h v h X X X v h v h u u total stresses effective stresses u 13. Envelopes in terms of& Identical specimens initially subjected to different isotropic stresses ( c ) and then loaded axially to failure f Initially Failure u f At failure, 3 = c ; 1= c + f 3 = 3 u f; 1 = 1- u f c, c, in terms of in terms of c c c c 14. Triaxial Test Apparatus piston (to apply deviatoric stress) pedestal perspex cell cell pressure back pressure pore pressure or volume change porous stone impervious membrane O-ring water soil sample at failurefailure plane 15. Types of Triaxial Tests Under all-around cell pressure c Shearing (loading) Is the drainage valve open? Is the drainage valve open? deviatoric stress ( ) yes no yes no C onsolidatedsample U nconsolidatedsample D rainedloading U ndrainedloading 16. Types of Triaxial Tests Depending 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: Consolidated Drained(CD) test ConsolidatedUndrained(CU) test UnconsolidatedUndrained(UU) test 17. Granular soils have no cohesion. c = 0 & c = 0 For normally consolidated clays, c = 0 & c = 0. For unconsolidated undrained test, in terms of total stresses, u= 0 18. CD ,CUandUUTriaxial Tests

  • no excess pore pressure throughout the test
  • very slow shearing to avoid build-up of porepressure

Consolidated Drained (CD) Test

  • gives c and

Can be days! not desirable Use c and for analysingfully drained situations (e.g., long term stability,very slow loading) 19. CD ,CUandUUTriaxial Tests

  • pore pressure develops during shear
  • faster than CD ( preferred way to find c and )

Consolidated Undrained (CU) Test

  • gives c and

Measure 20. CD ,CUandUUTriaxial Tests

  • pore pressure develops during shear
  • very quick test

Unconsolidated Undrained (UU) Test

  • analyse in terms of gives c uand u

Not measured unknown =0 ; i.e., failure envelopeis horizontal Use c uand ufor analysing undrained situations (e.g., short term stability,quick loading) 21. 1 - 3Relation at Failure X soil element at failure 3 1 X 3 1 22. Stress Point h v stress point t s ( v - h )/2 ( v + h )/2 stress point X v h 23. Stress Path Stress path is the locus of stress points Stress path Stress path is a convenient way to keep track of the progress in loading with respect to failure envelope. During loading t s 24. Failure Envelopes c cos tan -1 (sin ) failure During loading (shearing). stress path t s c 25. Pore Pressure ParametersA simple way to estimate the pore pressure change in undrained loading, in terms oftotal stresschanges ~ after Skempton (1954) Skemptons pore pressure parameters A and B Y 1 3 u =? 26. Pore Pressure ParametersFor saturated soils, B1. A-parameter at failure (A f ) For normally consolidated clays A f 1. B-parameter B = f (saturation,..) A f= f(OCR) For heavily overconsolidated clays A fis negative.