1. Soil Mechanics Topic – Triaxial shear test(CD, CU, UU tests)
Created by
Patel Jay K.
Guided By
Prof. Kumar Trivedi

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

3. When soil is loaded, shearing stresses induced in it
When soil is loaded, shearing stresses induced in it. When shearing stresses reach limiting value, shear deformation take place, leading to failure of soil mass.
Failure may be in the form of sinking of a footing or movement of a wedge of soil behind retaining wall or slide in an earth embankment.

4. Shear failure of soils Embankment Strip footing Failure surface
Mobilized shear resistance

5. Shear failure of soils
Retaining wall

6. Shear failure of soils Mobilized shear resistance Retaining wall
Failure surface
Mobilized shear resistance
Retaining wall

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

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

9. Triaxial compression test
The Triaxial shear test introduced by casagrande and terzaghi in 1936, is far the most popular and extensively used shear strength test, both for field application as well as for the purpose of research. As the name suggest, the specimen is subjected to three compressive stresses in mutually perpendicular direction.

10. Stages and types of tests
Consolidation stage in which normal stress applied the specimen and is allowed to consolidate
Shear stage in which stresses is applied to specimen to shear it.
Types of tests
Unconsolidated un-drained test (UU test)
Consolidated Un-drained test (CU test)
Consolidated drained test (CD test)

12. Specimen preparation (undisturbed sample)
Sampling tubes
Sample extruder

13. Specimen preparation (undisturbed sample)
Edges of the sample are carefully trimmed
Setting up the sample in the triaxial cell

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

15. Specimen preparation (undisturbed sample)
Proving ring to measure the deviator load
Dial gauge to measure vertical displacement

16. Consolidated- drained test (CD Test)
Total, s =
Pore water pressure, u
Effective, s’ +
Step 1: At the end of consolidation s3
s’3 = s3
Drainage
Step 2: During axial stress increase
s3 + sd s3
s1 = s3 + sd = s’1
s3 = s’3
Drainage
Step 3: At failure
s3 + sd s3
s1f = s3 + sd = s’1f
s3 = s’3
Drainage

17. How to determine strength parameters c and f
Mohr – Coulomb failure envelope
Shear stress, t
s or s’
s3c
s1c
s3b
s1b
(s2)b
s3a
s1a
(s2)a

18. Failure envelopes
For OC Clay, c ≠ 0 t
s or s’ f s3 s1
(s2) c sc OC NC

19. 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

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

21. Some practical applications of CD analysis for clays
3. Excavation or natural slope in clay t
t = In situ drained shear strength

22. Important points about Triaxial CD test
Since u = 0 in CD tests, s = s’
Therefore, c = c’ and f = f’
CD test simulates the long term condition in the field. Thus, c and f should be used to evaluate the long term behavior of soils

23. = + Consolidated- Undrained test (CU Test) s3 s3 + sd s3 s3 + sd s3
Total, s =
Pore water pressure, u
Effective, s’ +
Step 1: At the end of consolidation s3
s’3 = s3
Drainage
Step 2: During axial stress increase
s3 + sd s3
s’1 = sVC + sd - ud
s’3 = s3 - ud
No drainage ud
Step 3: At failure
s3 + sd s3
s’1 = sVC + sd - udf
s’3 = s3 - udf
No drainage
udf

24. Failure envelope f’ fcu s or s’ Shear stress, t s3b s1b (Dsd)fa s’3b
Mohr – Coulomb failure envelope in terms of effective stresses f’ C’
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. Unconsolidated- Undrained test (UU Test)
Step 1: Immediately after sampling
Step 2: After application of hydrostatic cell pressure
s’3 = s3 - uc
sC = s3
No drainage uc = +
Step 3: During application of axial load
s’1 = s3 + Dsd - uc ud
s’3 = s3 - uc ud
s3 + sd s3
No drainage =
uc ± ud +

26. Important points about Triaxial UU test
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
UU test simulates the short term condition in the field. Thus, UU test can be used to analyze the short term behavior of soils

27. Failure Envelope t s or s’ c s’3 s’1 s3a s1a s3b s1b s2
Failure envelope, f = 0 t
s or s’ c
s’3
s’1
s3a
s1a s2
s3b
s1b

28. 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

29. 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

30. Some practical applications of UU analysis for clays
3. Footing placed rapidly on clay deposit
t = In situ undrained shear strength

31. Thank you