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ECGD 4122 – Foundation Engineering Lecture 4

Faculty of Applied Engineering and Urban Planning. Civil Engineering Department. 2 nd Semester 2008/2009. ECGD 4122 – Foundation Engineering Lecture 4. Content. Shear Strength Subsoil Exploration. Mohr-Coulomb Failure Criterion. Total Stresses:

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ECGD 4122 – Foundation Engineering Lecture 4

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  1. Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 4

  2. Content • Shear Strength • Subsoil Exploration

  3. Mohr-Coulomb Failure Criterion Total Stresses: Coulomb approximation for shear stress on the failure plane:

  4. Mohr-Coulomb Failure Criterion

  5. Shear Strength Measurement Based on Mohr’s circle criteria, two ways are possible to cause failure:

  6. Shear Strength Measurement First: Increase the normal stress in one direction

  7. Shear Strength Measurement Second: Apply shear directly

  8. 1 3=0 Shear Strength Measurement Direct Shear Uniaxial Compression

  9. Unconfined compression test is used when  = 0 assumption is valid Triaxial compression is a more generalized version The soil sample is first compressed isotropically and then sheared by axial loading Shear Strength Measurement 1 3 Triaxial Compression

  10. Direct Shear Test

  11. Direct Shear Test

  12. Direct Shear Test

  13. Direct Shear Test

  14. Direct Shear Test

  15. Direct Shear Test

  16. Direct Shear Test

  17. Unconfined Compression Test For clay soils Cylindrical specimen No confining stresses (i.e. 3 = 0) Axial stress= 1 1 3= 0 Uniaxial Compression

  18. 1 3=0 Unconfined Compression Test Uniaxial Compression

  19. Unconfined Compression Test Data

  20. Unconfined Compression Test Data

  21. Unconfined Compression Test Data

  22. 1 3=0 Uniaxial Compression Unconfined Compression Test Max. shear plane  Horiz. plane  1

  23. Unconfined Compression Test

  24. Triaxial Compression Test

  25. Triaxial Compression Test

  26. Triaxial Compression Tests Unconsolidated Undrained (UU-Test); Also called “Undrained” Test Consolidated Undrained Test (CU- Test) Consolidated Drained (CD-Test); Also called “Drained Test”

  27. Consolidated Undrained Triaxial Test for Undisturbed Soils

  28. Shear Strengthin terms of Total Stress Shear Strength in terms of effective stress Shear strength in terms of total stress hydrostatic pore pressure

  29. Shear StrengthTotal Stress -  = 0 condition Shear strength in terms of total stress For cohesive soils under saturated conditions,  = 0.

  30. Mohr-Coulomb Failure Criterion Shear Strength,S  = 0 C Normal Stress, 

  31. Subsoil Exploration • Boring • Sampling • Standard Penetration Test (SPT) • Vane Shear Test • Cone Penetration Test (CPT) • Observation of Water Table

  32. Boring Boring Spacing

  33. Boring ASTM Method

  34. Boring ASTM Method for Calculating Depth of Boring

  35. Boring Methods • Auger • 3-5 m deep • Hand-driven

  36. Boring Methods • Flight Auger • 1-3 steps • Tractor • mounted

  37. Boring Methods • Rotary drilling • Wash boring

  38. Sampling Common Sampling Methods • Standard Split Spoon • The sampler is driven into the soil at the bottom of the borehole by means of hammer blows.

  39. Sampling

  40. Sampling

  41. Sampling

  42. Sampling Common Sampling Methods • Standard Split Spoon • The number of blows required for driving the sampler through three 152.4 mm (6”) intervals is recorded. The sum of the number of blows required for driving the last two 152.4 mm (6 in.) intervals is defined as the standard penetration number (N).

  43. Standard Penetration Number

  44. Standard Penetration Number

  45. Vane Shear Test

  46. Vane Shear Test

  47. Vane Shear Test

  48. Cone Penetration Test • Measures: • The Cone Resistance • The Frictional Resistance

  49. Cone Penetration Test

  50. Cone Penetration Test

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