Shear strength

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

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

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
CD test
CU test
UU test

4. Unconsolidated- Undrained test (UU Test)
Data analysis
C = 3
C = 3
No drainage
Initial specimen condition
3 + d
3
No drainage
Specimen condition
during shearing
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 – H) = A0 × H0
A ×(1 – H/H0) = A0 z
A
A


1
0

5. Unconsolidated- Undrained test (UU Test)
Step 1: Immediately after sampling
0
0
= +
Step 2: After application of hydrostatic cell pressure
uc =B 3
C = 3
C = 3 uc
’3 =3 - uc
’3 =3 - uc
No drainage
Increase of pwp due to increase
of cell pressure
Increase of cell pressure
Skempton’s pore water
pressure parameter, B
Note: If soil is fully saturated, then B = 1 (hence, uc = 3)

6. Unconsolidated- Undrained test (UU Test)
Step 3: During application of axial load
3 + d
3
No drainage
ud =ABd
uc ± ud
= +
Increase of pwp due to increase of
deviator stress
Increase of deviator stress
Skempton’s pore water
pressure parameter, A
'
1 3 d c du u       m
'
3 3 c du u    m

7. Unconsolidated- Undrained test (UU Test)
Combining steps 2 and 3,
uc =B 3 ud =ABd
u=uc + ud
Total pore water pressure increment at any stage, u
u=B [3 + Ad]
Skempton’s pore
water pressure
equation
u=B [3 + A(1 – 3]

8. Typical values for parameter B
Degree of
saturation
f (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

13

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

13
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
t

1313
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  f  c n' tan '

13. • 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 (u = c) 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.
Relation between effective and total stress criteria

14. • 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.
Significance of undrained strength parameters

15. 3b 1b
Unconsolidated- Undrained test (UU Test)
Effect of degree of saturation on failure envelope
3a 1a3c 1c
t
 or ’
S < 100% S > 100%

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

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

18. 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
Some practical applications of UU analysis for clays

19. Unconfined Compression Test (UC Test)
1 = VC + 
3 = 0
Confining pressure is zero in the UC test

20. Unconfined Compression Test (UC Test)
1 = VC + f
3 = 0
Shearstress,t
Normal stress, 
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.  Direct simple shear test
 Torsional ring shear test
 Plane strain triaxial test
Other laboratory shear tests

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

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

25. Torsional ring shear test
Peak
Residual
t
Shear displacement
tf
’
f’max
f’res

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

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

28. Plane strain triaxial test
’1, 1
’2, 2
’3, 3
Plane strain test
’2 ≠ ’3
2 = 0
’1
’2
’3
’2
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:
τ
σ, σ’ 1 f
 3 f

us
   1 3
2
f f
us
 

• The undrained shear strength
depends only on the initial void
ratio or the initial water content of
the soil
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
σ, σ’
• An increase in initial
confining stresses causes a
decrease in void ratio and
an increase in undrained
shear strength
1us
2us
 1 f
 3 f
  1 f
 3 f


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
Compression Yes Yes, but lateral expansion must occur
so that the volume change is zero
Analysis Effective stress (EFA) Total stress (TSA)
Design parameters
0:
uS' '
(or )cs pf f
Homework: Do reading from page 243 – 245 (Section 7.8) – Soil mechanics and foundations