Triaxial Shear Test

1. Triaxial Shear Test CEP 701 - Soil Engineering Laboratory

2. Steel Concrete Soil Compressive strength Shear strength Tensile strength Presence of pore water Complex behavior Strength of different materials

3. Embankment Strip footing Failure surface Mobilized shear resistance Shear failure of soils Soils generally fail in shear At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.

4. Retaining wall Shear failure of soils Soils generally fail in shear

5. Mobilized shear resistance Failure surface Shear failure of soils Soils generally fail in shear Retaining wall At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.

6. failure surface Shear failure mechanism The soil grains slide over each other along the failure surface. No crushing of individual grains.

7.       Shear failure mechanism At failure, shear stress along the failure surface () reaches the shear strength (f).

8.  failure envelope Friction angle Cohesion f c   Mohr-Coulomb Failure Criterion(in terms of total stresses)  f is the maximum shear stress the soil can take without failure, under normal stress of .

9.  ’ failure envelope Effective cohesion Effective friction angle f c’ ’ ’ Mohr-Coulomb Failure Criterion(in terms of effective stresses) u = pore water pressure f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.

10.  f ’f tan ’ frictional component ’ cohesive component c’ c’ ’f ' Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional. c and  are measures of shear strength. Higher the values, higher the shear strength.

11. s’1 s’ s’3 s’3 t q s’1 Soil element Mohr Circle of stress Resolving forces insandtdirections,

12. t s’ Mohr Circle of stress

13. t (s’, t) q s’ PD = Pole w.r.t. plane Mohr Circle of stress

14. Failure surface Y Y X X Soil elements at different locations X ~ failure Y ~ stable Mohr Circles & Failure Envelope  ’

15.  c c Y c+  Initially, Mohr circle is a point Mohr Circles & Failure Envelope The soil element does not fail if the Mohr circle is contained within the envelope GL c

16. As loading progresses, Mohr circle becomes larger…  c c .. and finally failure occurs when Mohr circle touches the envelope Mohr Circles & Failure Envelope GL Y c

17. (s’, tf) (90 – q) q Therefore, 90 – q + f’ = q q = 45 + f’/2 PD = Pole w.r.t. plane Orientation of Failure Plane Failure envelope f’ s’

18. v v’ u h h’ + u X X X t effective stresses total stresses s or s’ h’ v’ h v u Mohr circles in terms of total & effective stresses =

19. v v’ u h h’ + u X X X Failure envelope in terms of effective stresses Failure envelope in terms of total stresses t f’ f effective stresses total stresses c’ c s or s’ h’ v’ h v u Failure envelopes in terms of total & effective stresses = If X is on failure

20. t ’v = s’1 Failure envelope in terms of effective stresses ’h = s’3 effective stresses (s’1 - s’3)/2 X c’ f’ X is on failure ’3 ’1 s’ (s’1+ s’3)/2 c’ Cotf’ Therefore, Mohr Coulomb failure criterion with Mohr circle of stress

21. Mohr Coulomb failure criterion with Mohr circle of stress

22. Laboratory tests on specimens taken from representative undisturbed samples Field tests • Most common laboratory tests to determine the shear strength parameters are, • Direct shear test • Triaxial shear test • Vane shear test • Torvane • Pocket penetrometer • Fall cone • Pressuremeter • Static cone penetrometer • Standard penetration test Determination of shear strength parameters of soils (c, forc’, f’) Other laboratory tests include, Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test

23. A representative soil sample z z svc + Ds svc shc shc shc shc svc svc + Ds After and during construction Before construction Laboratory tests Field conditions

24. svc + Ds shc shc Traxial test 0 svc svc + Ds svc shc shc 0 0 t Direct shear test svc 0 t svc Representative soil sample taken from the site Step 1 Set the specimen in the apparatus and apply the initial stress condition Laboratory tests Simulating field conditions in the laboratory Step 2 Apply the corresponding field stress conditions

25. Piston (to apply deviatoric stress) Failure plane O-ring impervious membrane Soil sample Soil sample at failure Porous stone Perspex cell Water Cell pressure Pore pressure or volume change Back pressure pedestal Triaxial Shear Test

27. Sampling tubes Sample extruder Triaxial Shear Test Specimen preparation (undisturbed sample)

28. Setting up the sample in the triaxial cell Edges of the sample are carefully trimmed Triaxial Shear Test Specimen preparation (undisturbed sample)

29. Sample is covered with a rubber membrane and sealed Cell is completely filled with water Triaxial Shear Test Specimen preparation (undisturbed sample)

30. Proving ring to measure the deviator load Dial gauge to measure vertical displacement Triaxial Shear Test Specimen preparation (undisturbed sample)

31. deviatoric stress ( = q) c Step 2 Step 1 c c c c c+ q c Under all-around cell pressure c Shearing (loading) yes no yes no Consolidated sample Undrained loading Drained loading Unconsolidated sample Types of Triaxial Tests Is the drainage valve open? Is the drainage valve open?

32. Step 2 Step 1 Under all-around cell pressure c Shearing (loading) Is the drainage valve open? Is the drainage valve open? CD test UU test yes no yes no Consolidated sample Undrained loading Drained loading Unconsolidated sample CU test Types of Triaxial Tests

33. = + Total, s Effective, s’ Neutral, u sVC s’VC =sVC shC s’hC =shC 0 Drainage Drainage Drainage sVC + Ds sVC + Dsf s’V =sVC +Ds = s’1 shC shC s’h =shC = s’3 0 0 s’Vf =sVC +Dsf= s’1f s’hf =shC = s’3f Consolidated- drained test (CD Test) Step 1: At the end of consolidation Step 2: During axial stress increase Step 3: At failure

34. s1 = sVC + Ds s3 = shC Consolidated- drained test (CD Test) Deviator stress (q or Dsd) = s1 – s3

35. Time Expansion Volume change of the sample Compression Consolidated- drained test (CD Test) Volume change of sample during consolidation

36. Deviator stress, Dsd Dense sand or OC clay (Dsd)f Loose sand or NC Clay (Dsd)f Axial strain Expansion Dense sand or OC clay Volume change of the sample Axial strain Compression Loose sand or NC clay Consolidated- drained test (CD Test) Stress-strain relationship during shearing

37. Deviator stress, Dsd s1 = s3 + (Dsd)f (Dsd)fc Confining stress = s3c s3 Confining stress = s3b Confining stress = s3a (Dsd)fb Axial strain (Dsd)fa Shear stress, t f Mohr – Coulomb failure envelope s or s’ s3c s3a s3b s1a s1b s1c (Dsd)fa (Dsd)fb CD tests How to determine strength parameters c and f

38. Strength parameters c and f obtained from CD tests CD tests Therefore, c = c’ and f = f’ Since u = 0 in CD tests, s = s’ cd and fd are used to denote them

39. fd Mohr – Coulomb failure envelope Shear stress, t s or s’ s3a s1a (Dsd)fa CD tests Failure envelopes For sand and NC Clay, cd = 0 Therefore, one CD test would be sufficient to determine fd of sand or NC clay

40. NC OC t f s or s’ c s3 s1 sc (Dsd)f CD tests Failure envelopes For OC Clay, cd ≠ 0

41. Soft clay t Some practical applications of CD analysis for clays 1. Embankment constructed very slowly, in layers over a soft clay deposit t = in situ drained shear strength

42. t Core t = drained shear strength of clay core Some practical applications of CD analysis for clays 2. Earth dam with steady state seepage

43. t Some practical applications of CD analysis for clays 3. Excavation or natural slope in clay t = In situ drained shear strength Note: CD test simulates the long term condition in the field. Thus, cd and fd should be used to evaluate the long term behavior of soils

44. = + Total, s Effective, s’ Neutral, u sVC s’VC =sVC shC s’hC =shC 0 Drainage s’V =sVC +Ds ± Du = s’1 sVC + Ds sVC + Dsf No drainage shC shC s’h =shC ± Du= s’3 ±Du s’Vf =sVC +Dsf± Duf = s’1f No drainage s’hf =shC ± Duf = s’3f ±Duf Consolidated- Undrained test (CU Test) Step 1: At the end of consolidation Step 2: During axial stress increase Step 3: At failure

45. Time Expansion Volume change of the sample Compression Consolidated- Undrained test (CU Test) Volume change of sample during consolidation

46. Deviator stress, Dsd Dense sand or OC clay (Dsd)f Loose sand or NC Clay (Dsd)f Axial strain + Loose sand /NC Clay Du Axial strain - Dense sand or OC clay Consolidated- Undrained test (CU Test) Stress-strain relationship during shearing

47. Deviator stress, Dsd (Dsd)fb s1 = s3 + (Dsd)f Confining stress = s3b Confining stress = s3a s3 (Dsd)fa Axial strain Total stresses at failure Shear stress, t fcu Mohr – Coulomb failure envelope in terms of total stresses s or s’ ccu s3b s3a s1a s1b (Dsd)fa CU tests How to determine strength parameters c and f

48. s’1 = s3 + (Dsd)f -uf s’3= s3 -uf Mohr – Coulomb failure envelope in terms of effective stresses Shear stress, t f’ ufb s or s’ ufa C’ s’3b s’1b s3b s3a s’3a s1a s1b s’1a (Dsd)fa (Dsd)fa CU tests How to determine strength parameters c and f uf Effective stresses at failure Mohr – Coulomb failure envelope in terms of total stresses fcu ccu

49. Strength parameters c and f obtained from CD tests CU tests Shear strength parameters in terms of effective stresses are c’ and f’ Shear strength parameters in terms of total stresses are ccu and fcu c’ = cd and f’ = fd

50. Mohr – Coulomb failure envelope in terms of effective stresses f’ fcu Mohr – Coulomb failure envelope in terms of total stresses Shear stress, t s3a s1a s or s’ s3a s1a (Dsd)fa CU tests Failure envelopes For sand and NC Clay, ccu and c’ = 0 Therefore, one CU test would be sufficient to determine fcu and f’(= fd) of sand or NC clay