1. ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
Slope Stability and Geotechnics
Landslide Hazards
River Bank Stability
Section 4 - Shear Strength of Soils
N.K. Tovey
Н.К.Тови М.А., д-р технических наук
Landslide on Main Highway at km 365 west of Sao Paulo: August 2002

2. ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
Introduction
Seepage and Water Flow through Soils
Consolidation of Soils
Shear Strength ~ 1 lecture
Slope Stability ~ 4 lectures
River Bank Stability ~ 2 lectures
Special Topics
Decompaction of consolidated Quaternary deposits
Landslide Warning Systems
Slope Classification
Microfabric of Sediments

3. Section 4 - Shear Strength of Soils
Definitions:
a normal load or force is one which acts parallel to the normal (i.e. at right angles) to the surface of an object
a shear load or force is one which acts along the plane of the surface of an object
the stress acting on a body (either normal or shear) is the appropriate load or force divided by the area over which it acts.
Stress and Force must NOT be confused

4. Section 4 - Shear Strength of Soils
EQUILIBRIUM
There are three conditions:
the net effect of all forces parallel to one direction must be zero
the net effect of all forces orthogonal (at right angles) to the above direction must be zero
the sum of the moments of the forces must be zero
the first two conditions can be checked by resolving forces (e.g. see Fig. 4.1)

5. Section 4 - Shear Strength of Soils
Resolution of Forces
At Equilibrium:
Resolve forces parallel to P1 :-
P1 = P2 cos 2 + P3 cos 3
Similarly at right angles to P1
P2 sin 2 = P3 sin  P1 P3 P2 3 2

6. Section 4 - Shear Strength of Soils
Coulomb: a French Military Engineer
Problem: Why do Military Fortifications Fail?

7. Section 4 - Shear Strength of Soils
Coulomb: a French Military Engineer
Problem: Why do Military Fortifications Fail?
Is there a relationship between F and N? N F N F
F = N tan 
 is the angle of internal friction 

8. Section 4 - Shear Strength of Soils
Suppose there is some “glue” between block and surface
Initially - block will not fail until bond is broken N F N
Block will fail F
Block is stable  C
F = C + N tan 
C is the cohesion

9. Section 4 - Shear Strength of Soils
F = C + N tan 
above equation is specified in forces
In terms of stress:
 = c +  tan 
Three types of material
granular (frictional) materials - i.e. c = (sands)
 =  tan 
cohesive materials - i.e.  = 0 (wet clays)
 = c
materials with both cohesion and friction
 = c +  tan 

10. Section 4 - Shear Strength of Soils
Stress Point at B
- stable
Stress Point at A
- stable only if cohesion is present
if failure line changes, then failure may occur. F N
F - F
G - G A B

11. Section 4 - Shear Strength of Soils
F - F
Displacement 
dense
loose
Peak in dense test is reached at around 1 - 3% strain

12. Section 4 - Shear Strength of Soils
displacement 
Increasing normal stress
/
dense
loose
Displacement
Normalising curves to normal stress leads to a unique set of curves for each soil.

13. Section 4 - Shear Strength of Soils
Types of Shear Test
Stress controlled test
Strain controlled test (as done in practical)
Failure in stress controlled test
BANG!
Displacement F N N N N N N
Readings cannot be taken after peak in a stress controlled test

14. Section 4 - Shear Strength of Soils
Dense Test
Loose Test 
displacement 
displacement V
displacement V
displacement
Medium Dense

15. Section 4 - Shear Strength of Soils
Plot volume changes as Void Ratio
Void Ratio
displacement
loose
Critical void ratio
medium
dense
All tests eventually come to same Void Ratio

16. Section 4 - Shear Strength of Soils Effects of Water Pressure
 = c +  tan 
Does not allow for water pressure.
Principal of Effective Stress
From Consolidation
Total Stress = effective stress + pore water pressure
or ’ =  u
In terms of stresses involved water cannot take shear
so  = c + (  - u ) tan 
or  = c + ’ tan 
Mohr - Coulomb failure criterion
if pore water pressure = 0 then original equation applies

17. Section 4 - Shear Strength of Soils
Distance stress point is from failure line is a measure of stability.
Greater distance
> greater stability  
Mohr - Coulomb
-ve pwp moves stress point to right A
+ve pwp
Moves point closer to failure line
 less stability
Moves point further from failure line
 greater stability
Slopes near Hadleigh Essex are only stable because of -ve pwp

18. Section 4 - Shear Strength of Soils
The Triaxial Test
Problems with Standard Shear Box
Shear zone is complex
Difficult to get undisturbed samples which are square
Difficult to do undrained or partially drained tests
sands - always will be drained
clays - may be partially drained - depends of strain rate.

19. Section 4 - Shear Strength of Soils
The Triaxial Test
Load
Cell Pressure
Sample in rubber membrane
Porous stone

20. Section 4 - Shear Strength of Soils
The Triaxial Test
Cell pressure can be varied to match that in ground
cylindrical samples can be obtained
sample can be sealed to prevent drainage or to allow partial drainage
can perform both undrained and drained tests

21. Section 4 - Shear Strength of Soils
Drained Test
allow complete dissipation of the pore water pressure.
speed of the test must allow for the permeability of the material.
for clays time is usually at least a week.
measure the volume of water extruded from or sucked into the sample in such tests.
Undrained Test
no drainage is allowed.
measure the pore water pressures during the test.

22. Section 4 - Shear Strength of Soils
Drained Test
response to load and volume change is similar to standard shear box.
Undrained Test
burette is replace by a pore water pressure measuring device.
Since drainage is not required, test can be rapid.
Shear stress will be lower than in drained test if positive pore water pressures develop

23. Section 4 - Shear Strength of Soils
displacement
water pressure
-ve
+ve
Dense
displacement
water pressure
+ve
-ve
Loose
In undrained dense tests pwp goes negative
In drained dense tests volume increases

24. Section 4 - Shear Strength of Soils
4.8 Failure modes in the Triaxial Test.
Loading
its length will shorten as the strain increases
some bulging towards the end.
Over consolidated samples (and dense sands),
usually a very definite failure plane as peak strength is reached.
Normally consolidated clays and loose sands,
failure zone is not visible
usually numerous micro failure zones criss-crossing the bulging region.
Undrained test
orientation of the failure zone is at 45o to the horizontal,
Drained test
orientation will be at (45 + /2), - often not as well defined.

25. Section 4 - Shear Strength of Soils
-ve pwp
+ve pwp e
log 
Water squeezed out
Water sucked in
Critical State Line
Diagram gives an insight into why some slopes appear to fail soon after they have formed, while in other cases they are initially stable, but fail much later.

26. Section 4 - Shear Strength of Soils
4.9 Unifying remarks on the behaviour of soils under shear.
Drained
Some soils expand
Some soils contract
Depends on initial compaction.
Undrained
Some samples +ve pwp develop
Some samples -ve pwp develop
All samples move towards Critical State Line (CSL)
What happens if sample has OCR consistent with CSL?
sample shears with no volume change in dense test
or no pore water change in undrained test.