1. EAG 345 – GEOTECHNICAL ANALYSIS
(iv) Determination of shear strength parameters of soils (2)
By: Dr Mohd Ashraf Mohamad Ismail

2. 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

3. 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

4. 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

5. 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)

6. Unconsolidated- Undrained test (UU Test)
Step 3: During application of axial load
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

7. 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]

8. Typical values for parameter B
f (saturation)
Degree of saturation

9. Typical values for parameter A

10. Relation between effective and total stress criteria
Three identical saturated soil samples are sheared to failure in UU triaxial tests. Each sample is subjected to a different cell pressure. No water can drain at any stage. At failure the Mohr circles are found to be as shown t s s3 s1

11. Relation between effective and total stress criteria
Three identical saturated soil samples are sheared to failure in UU triaxial tests. Each sample is subjected to a different cell pressure. No water can drain at any stage. At failure the Mohr circles are found to be as shown t s s3 s1
We find that all the total stress Mohr circles are the same size, and therefore fu = 0 and t = su = cu = constant

12. Relation between effective and total stress criteria
Because each sample is at failure, the fundamental effective stress failure condition must also be satisfied. As all the circles have the same size there must be only one effective stress Mohr circle t s s3 s1

13. Relation between effective and total stress criteria
The different total stress Mohr circles with a single effective stress Mohr circle indicate that the pore pressure is different for each sample.
As discussed previously increasing the cell pressure without allowing drainage has the effect of increasing the pore pressure by the same amount (Du = Dsc) with no change in effective stress.
The change in pore pressure during shearing is a function of the initial effective stress and the moisture content. As these are identical for the three samples an identical strength is obtained.

14. Significance of undrained strength parameters
It is often found that a series of undrained tests from a particular site give a value of fu that is not zero (cu not constant). If this happens either
the samples are not saturated, or
the samples have different moisture contents
If the samples are not saturated analyses based on undrained behaviour will not be correct
The undrained strength cu is not a fundamental soil property. If the moisture content changes so will the undrained strength.

15. 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

16. 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

17. 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

18. 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

19. Unconfined Compression Test (UC Test)
s1 = sVC + Ds
s3 = 0
Confining pressure is zero in the UC test

20. Unconfined Compression Test (UC Test)
s1 = sVC + Dsf
s3 = 0
Shear stress, t
Normal stress, s qu
Note: Theoritically qu = cu , However in the actual case qu < cu due to premature failure of the sample

21. Other laboratory shear tests
Direct simple shear test
Torsional ring shear test
Plane strain triaxial test

22. Other laboratory shear tests
Direct simple shear test
Torsional ring shear test
Plane strain triaxial test

23. Direct simple shear test
f = 80 mm
Soil specimen
Porous stones
Spiral wire in rubber membrane
Direct shear test
Direct simple shear test

24. Other laboratory shear tests
Direct simple shear test
Torsional ring shear test
Plane strain triaxial test

25. Torsional ring shear test
Peak
Residual t
Shear displacement tf s’
f’max
f’res
The ring shear test is suited to the relatively rapid determination of drained residual shear strength because of the short drainage path through the thin specimen, and the capability of testing one specimen under different normal stresses to quickly obtain a shear strength envelope.
The test results are primarily applicable to assess the shear strength in slopes that contain a preexisting shear surface, such as old landslides, and sheared bedding planes, joints, or faults.

26. Torsional ring shear testsN
Preparation of ring shaped undisturbed samples is very difficult. Therefore, remoulded samples are used in most cases

27. Other laboratory shear tests
Direct simple shear test
Torsional ring shear test
Plane strain triaxial test

28. Plane strain triaxial test
s’1, e1
s’2, e2
s’3, e3
Plane strain test
s’2 ≠ s’3
e2 = 0
s’1
s’2
s’3
Rigid platens
Specimen

29. Drained and undrained conditions
DRAINED condition occurs when there is no change in pore water pressure due to external loading
In a drained condition, the pore water can drain out of the soil easily, causing volumetric strains in the soil
UNDRAINED condition occurs when the pore water is unable to drain out of the soil
In undrained condition the rate of loading is much quicker than the rate at which the pore water is able to drain out of the soil
As a result, most of the external loading is taken by the pore water, resulting in an increase in the pore water pressure.
The tendency of soil to change in volume is suppressed during undrained loading.

30. Drained and undrained conditions
The existence of either a drained or an undrained condition in a soil depends on: ((i)soil types; fine-grained or coarse grained, (ii) geological formation and (iii) rate of loading)
For a rate of loading associated with a normal construction activity, saturated coarse grained soils (e.g. sands and gravel) experience drained conditions and saturated fine-grained soils (e.g. silts and clays) experience undrained conditions
If the rate of loading is fast enough (e.g. during an earthquake), even coarse-grained soils can experience undrained loading, often resulting in liquefaction.

31. Example of undrained loading (Liquefaction)
When loading is rapidly applied and large enough such that it does not flow out in time before the next cycle of load is applied, the water pressure may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contact between grains are the means by which the weight of the buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depth. This loss of soil structure causes it to lose all of its strength and it may be observed to flow like a liquid.

32. Example of undrained loading (Liquefaction)
Uplift of sewerage during Niigata earthquake 2004
Collapse of flat house during the 1964 Niigata earthquake, Japan.

33. Drained and undrained conditions
The shear strength of a fine-grained soil under undrained condition is called the undrained shear strength and denoted as su.
su is the radius of the Mohr’s circle of total stress:
The undrained shear strength depends only on the initial void ratio or the initial water content of the soil τ
σ, σ’
Total stress circle

34. Drained and undrained conditions
The undrained shear strength is not a fundamental soil parameter.
Its value depends on the values of the initial confining stresses. τ
An increase in initial confining stresses causes a decrease in void ratio and an increase in undrained shear strength
σ, σ’

35. Selection of shear strength parameter – Drained or undrained ?
CU with pore water pressure measurement
When designing a geotechnical structure, both undrained and drained conditions must be considered to determine which one is more critical

36. Drained and Undrained shear strength
Condition
Drained
Undrained
Excess porewater pressure
Not zero; could be positive or negative
Volume change
Compression
Positive excess porewater pressure
Expansion
Negative excess porewater pressure
Consolidation
Yes, fine grained soil No
Yes
Yes, but lateral expansion must occur so that the volume change is zero
Analysis
Effective stress (EFA)
Total stress (TSA)
Design parameters
Homework: Do reading from page 243 – 245 (Section 7.8) – Soil mechanics and foundations