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Shear Strength of Soil

Shear Strength of Soil. COSC 323: Soils in Construction. Shear Failure. Shear Failure. Shear Failure. Shear Strength due to Sliding Friction. Normal Force N. Shearing Force S. Failure Plane. Shear Strength due to sliding friction. Normal Force. Sliding Force.

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Shear Strength of Soil

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  1. Shear Strength of Soil COSC 323: Soils in Construction

  2. Shear Failure

  3. Shear Failure

  4. Shear Failure

  5. Shear Strength due to Sliding Friction Normal Force N Shearing Force S Failure Plane

  6. Shear Strength due to sliding friction Normal Force Sliding Force

  7. Shear Strength due to sliding friction Normal Force Sliding Force

  8. Shear Strength due to Sliding FrictionNormal Stress vs. Shear Stress

  9. Shear Strength due to Sliding Friction ________________ (s) ______________________ (F) _________________ (s)

  10. Coulomb Equation ________________ ________________ (Strength gained from the ionic bound between grain particles) Effective intergranular __________________ (perpendicular to the shear plane) ______________________ (Strength gained from internal friction resistance) Coefficient of friction

  11. Shear strength of soil • Soil cannot resist __________ • Soil can resist compression. • For excessive compression failure occurs in the form of ________ along the internal surface within the soil • Structural Strength of soil = f ( soil’s ______ strength) • _______________ • Soil’s ability to resist sliding • Important for • foundation design • Lateral earth pressure calculations • Slope stability analysis

  12. Methods of investigating shear strength • ____________________________ (for cohesive soil) • _____________________ • _____________________ • Vane test (for soft clay) • Standard penetration test (for cohesionless soil) • Penetrometer test

  13. Unconfined compression test SoilSpecimen

  14. Direct Shear Test Normal Load Sharing Force Soil Specimen Sharing Force Normal Load Sharing Force Sharing Force

  15. Direct Shear Test Normal Load Sharing Force Soil Specimen _____________ = Normal load / the specimen’s cross-sectional area _____________ = Shearing Force / the specimen’s cross- sectional area The graph can be used to determine the given soil’s shear strength for any load Problem: Shear failure is forced to occur along a _____________ plane, which is not necessarily the weakest plane of the soil specimen tested. Test with different Normal Load Shear Stress f c Normal Stress f: angle of internal friction c: cohesion

  16. Triaxial Compression Test Wrap the specimen with rubber membrane

  17. Triaxial Compression Test Enclose the specimen in a chamber filled with ___________

  18. Triaxial Compression Test Apply a specific ___________ using water s3 s3 s3 s3

  19. Triaxial Compression Test Apply a ___________ and increase until the specimen fails Use different lateral pressure, conduct the same experiment s3 s3 s3 s3

  20. Triaxial Compression Test s3 : _______________ Dp : _____________ at failure axial = (Load at failure / cross-sectional area) s3 s3 s3 s3

  21. Triaxial Compression Test s3 : Minor principal stress Dp : Deviator stress at failure axial = (Load at failure / cross-sectional area) s1 = s3 + Dp; _______ ______________ How to obtain cohesion and angle of internal friction? s3 s3

  22. Triaxial Compression Test s3 ___=s3+Dp s3 s3 s3 s3 s3 ___=s3+Dp After Before

  23. Triaxial Compression Test First Test Shear Stress A B Normal Stress ___ ___ ___

  24. Triaxial Compression Test Second Test Shear Stress A B C D Normal Stress (s3)1 (Dp)1 (s1)1 ___ ___ ___

  25. Triaxial Compression Test Shear Stress ________________ A B C D Normal Stress (s3)1 (Dp)1 (s1)1 (s3)2 (Dp)2 (s1)2

  26. Triaxial Compression Test Shear Stress __ Strength Envelop __ A B C D Normal Stress (s3)1 (Dp)1 (s1)1 (s3)2 (Dp)2 (s1)2

  27. Example • Gien • Triaxial compression tests on three specimens of a soil sample were performed. Each test was carried out until the specimen experienced shear failure. The test data are tabulated as follows: • Required • The soil’s cohesion and angle of internal friction

  28. Example

  29. Example 8 6 4 2 0 2 4 6 8 10 12 14

  30. Example 8 6 4 2 0 2 4 6 8 10 12 14

  31. Example 8 6 4 2 0 2 4 6 8 10 12 14

  32. Example 8 6 4 2 0 2 4 6 8 10 12 14

  33. Example 8 ____ 6 4 ___ 2 0 2 4 6 8 10 12 14

  34. Variations in Shear Test Procedures • Unconsolidated Undrained (UU) – Q test • Consolidated Undrained (CU) • Consolidated Drained (CD) – S test

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