1. Kuat Geser Tanah (Shear Strength) - Triaxial Test - (Courtesy of COSC 323: Soils in Construction)
oleh:
A. Adhe Noor PSH, ST., MT
Staf Pengajar Program Studi Teknik Sipil
Jurusan Teknik Fakultas Sains dan Teknik
Universitas Jenderal Soedirman

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

19. Some practical applications of CD analysis for clays
3. Excavation or natural slope in clay t
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

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

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

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

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

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

25. CU tests Strength parameters c and f obtained from CD 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

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

27. Some practical applications of CU analysis for clays
1. Embankment constructed rapidly over a soft clay deposit
Soft clay t
t = in situ undrained shear strength

28. Some practical applications of CU analysis for clays
2. Rapid drawdown behind an earth dam t
Core
t = Undrained shear strength of clay core

29. Some practical applications of CU analysis for clays
3. Rapid construction of an embankment on a natural slope
t = In situ undrained shear strength t
Note: Total stress parameters from CU test (ccu and fcu) can be used for stability problems where,
Soil have become fully consolidated and are at equilibrium with the existing stress state; Then for some reason additional stresses are applied quickly with no drainage occurring

30. Unconsolidated- Undrained test (UU Test)
Data analysis
s3 + Dsd s3
No drainage
Specimen condition during shearing
sC = s3
No drainage
Initial specimen condition
Initial volume of the sample = A0 × H0
Volume of the sample during shearing = A × H
Since the test is conducted under undrained condition,
A × H = A0 × H0
A ×(H0 – DH) = A0 × H0
A ×(1 – DH/H0) = A0

31. Unconsolidated- Undrained test (UU Test)
Step 1: Immediately after sampling
Step 2: After application of hydrostatic cell pressure
s’3 = s3 - Duc
sC = s3
No drainage
Duc = +
Increase of pwp due to increase of cell pressure
Duc = B Ds3
Skempton’s pore water pressure parameter, B
Increase of cell pressure
Note: If soil is fully saturated, then B = 1 (hence, Duc = Ds3)

32. Unconsolidated- Undrained test (UU Test)
Step 3: During application of axial load
s’1 = s3 + Dsd - Duc Dud
s’3 = s3 - Duc Dud
s3 + Dsd s3
No drainage =
Duc ± Dud +
Increase of pwp due to increase of deviator stress
Dud = ABDsd
Increase of deviator stress
Skempton’s pore water pressure parameter, A

33. Unconsolidated- Undrained test (UU Test)
Combining steps 2 and 3,
Duc = B Ds3
Dud = ABDsd
Total pore water pressure increment at any stage, Du
Du = Duc + Dud
Du = B [Ds3 + ADsd]
Skempton’s pore water pressure equation
Du = B [Ds3 + A(Ds1 – Ds3]

34. Unconsolidated- Undrained test (UU Test)
Total, s =
Neutral, u
Effective, s’ +
Step 1: Immediately after sampling
s’V0 = ur
s’h0 = ur
-ur
Step 2: After application of hydrostatic cell pressure
s’VC = sC + ur - sC = ur
s’h = ur sC
No drainage
-ur + Duc = -ur + sc
(Sr = 100% ; B = 1)
Step 3: During application of axial load
s’V = sC + Ds + ur - sc Du
s’h = sC + ur - sc Du
sC + Ds sC
No drainage
-ur + sc ± Du
Step 3: At failure
s’hf = sC + ur - sc Duf = s’3f
s’Vf = sC + Dsf + ur - sc Duf = s’1f sC
sC + Dsf
No drainage
-ur + sc ± Duf

35. Unconsolidated- Undrained test (UU Test)
Total, s =
Neutral, u
Effective, s’ +
Step 3: At failure
s’hf = sC + ur - sc Duf = s’3f
s’Vf = sC + Dsf + ur - sc Duf = s’1f
-ur + sc ± Duf sC
sC + Dsf
No drainage
Mohr circle in terms of effective stresses do not depend on the cell pressure.
Therefore, we get only one Mohr circle in terms of effective stress for different cell pressures
s’3
s’1
Dsf t s’

36. Unconsolidated- Undrained test (UU Test)
Total, s =
Neutral, u
Effective, s’ +
Step 3: At failure
s’hf = sC + ur - sc Duf = s’3f
s’Vf = sC + Dsf + ur - sc Duf = s’1f
-ur + sc ± Duf sC
sC + Dsf
No drainage
Mohr circles in terms of total stresses
Failure envelope, fu = 0 t
s or s’ cu
s’3
s’1
s3a
s1a
Dsf
s3b
s1b ub ua

37. Unconsolidated- Undrained test (UU Test)
Effect of degree of saturation on failure envelope t
s or s’
S < 100%
S > 100%
s3a
s1a
s3b
s1b
s3c
s1c

38. Some practical applications of UU analysis for clays
1. Embankment constructed rapidly over a soft clay deposit
Soft clay t
t = in situ undrained shear strength

39. Some practical applications of UU analysis for clays
2. Large earth dam constructed rapidly with no change in water content of soft clay
t = Undrained shear strength of clay core t
Core

40. Some practical applications of UU analysis for clays
3. Footing placed rapidly on clay deposit
t = In situ undrained shear strength
Note: UU test simulates the short term condition in the field. Thus, cu can be used to analyze the short term behavior of soils

41. Example Given Required
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
Specimen
Number
Minor Principal Stress
(kips/ft2)
Deviator Stress at Failure 1
1.44
5.76 2
2.88
6.85 3
4.32
7.50

42. Example Specimen Number Minor Principal Stress (kips/ft2)
Deviator Stress at Failure
Major Principal Stress (kips/ft2) 1
1.44
5.76
7.2 2
2.88
6.85
9.73 3
4.32
7.50
11.82

43. Example 8 6 4 2 2 4 6 8 10 12 14

44. Example 8 6 4 2 2 4 6 8 10 12 14

45. Example 8 6 4 2 2 4 6 8 10 12 14

46. Example 8 2 6 4 4 2 2 4 6 8 10 12 14

47. THE END