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- 1. Shear Strength of Soils N. Sivakugan Duration: 17 min: 04 sec
- 2. Shear failure <ul><li>Soils generally fail in shear </li></ul>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 f is 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: cohesive and frictional . f f c f tan c cohesive component frictional component
- 7. c and are measures of shear strength. Higher the values, higher the shear strength.
- 8. Mohr Circles & Failure Envelope X Y Soil elements at different 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 at 45 + /2 to 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 failure failure 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 onsolidated sample U nconsolidated sample D rained loading U ndrained loading
- 16. Types of Triaxial Tests Depending on whether drainage is allowed or not during <ul><li>initial isotropic cell pressure application, and </li></ul><ul><li>shearing , </li></ul>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
- 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 , CU and UU Triaxial Tests <ul><li>no excess pore pressure throughout the test </li></ul><ul><li>very slow shearing to avoid build-up of pore pressure </li></ul>Consolidated Drained (CD) Test <ul><li>gives c’ and ’ </li></ul>Can be days! not desirable Use c’ and ’ for analysing fully drained situations (e.g., long term stability, very slow loading)
- 19. CD , CU and UU Triaxial Tests <ul><li>pore pressure develops during shear </li></ul><ul><li>faster than CD ( preferred way to find c’ and ’) </li></ul>Consolidated Undrained (CU) Test <ul><li>gives c’ and ’ </li></ul>Measure ’
- 20. CD , CU and UU Triaxial Tests <ul><li>pore pressure develops during shear </li></ul><ul><li>very quick test </li></ul>Unconsolidated Undrained (UU) Test <ul><li>analyse in terms of gives c u and u </li></ul>Not measured ’ unknown = 0 ; i.e., failure envelope is horizontal Use c u and u for analysing undrained situations (e.g., short term stability, quick loading)
- 21. 1 - 3 Relation 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 Parameters A simple way to estimate the pore pressure change in undrained loading, in terms of total stress changes ~ after Skempton (1954) Skempton’s pore pressure parameters A and B Y 1 3 u = ?
- 26. Pore Pressure Parameters For saturated soils, B 1. 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 f is negative.

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