ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Slope Stability and Geotechnics Landslide Hazards River...
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Transcript of ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Slope Stability and Geotechnics Landslide Hazards River...
ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
Slope Stability and Geotechnics
Landslide Hazards
River Bank Stability
Section 4 - Shear Strength of Soils
N.K. ToveyН.К.Тови М.А., д-р технических наук
Landslide on Main Highway at km 365 west of Sao Paulo: August 2002
• Introduction• Seepage and Water Flow through Soils• Consolidation of Soils
ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
• 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
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
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)
Section 4 - Shear Strength of Soils
• Resolution of Forces
Section 4 - Shear Strength of Soils
P1
P3
P2
23
At Equilibrium:
Resolve forces parallel to P1 :-
P1 = P2 cos 2 + P3 cos 3
...........4.1
Similarly at right angles to P1
P2 sin 2 = P3 sin 3 ...........4.2
Section 4 - Shear Strength of Soils
Coulomb: a French Military Engineer
Problem: Why do Military Fortifications Fail?
Section 4 - Shear Strength of Soils
Coulomb: a French Military Engineer
Problem: Why do Military Fortifications Fail?
N
F
F = N tan ......4.3
is the angle of internal friction
F
N
Is there a relationship between F and N?
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
F = C + N tan ......4.4
C is the cohesion
F
N
C
Block will fail
Block is stable
• Three types of material– granular (frictional) materials - i.e. c = 0 (sands)
= tan – cohesive materials - i.e. = 0 (wet clays)
= c
– materials with both cohesion and friction = c + tan
Section 4 - Shear Strength of Soils
F = C + N tan ......4.4
above equation is specified in forces
In terms of stress:
= c + tan
• Stress Point at B - stable
• Stress Point at A- stable only if
cohesion is present
• if failure line changes, then failure may occur.
Section 4 - Shear Strength of Soils
F
N
F - F
G - G
B
A
Section 4 - Shear Strength of Soils
F
N
F - F
NNNNNNNN
Displacement
dense
loose
Peak in dense test is reached at around 1 - 3% strain
Section 4 - Shear Strength of Soils
displacement
Increasing normal stress
Displacement
/ dense
loose
Normalising curves to normal stress leads to a unique set of curves for each soil.
• Types of Shear Test– Stress controlled test– Strain controlled test (as done in practical)
Section 4 - Shear Strength of Soils
Failure in stress controlled test
Displacement
F
Readings cannot be taken after peak in a stress controlled test
NN
NNNNBANG!
Section 4 - Shear Strength of Soils
displacement
displacement
V
displacement
V
displacement
Dense Test Loose Test
Medium Dense
• All tests eventually come to same Void Ratio
Section 4 - Shear Strength of Soils
Plot volume changes as Void Ratio
Void Ratio
displacement
medium
Critical void ratio
loose
dense
= 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
Section 4 - Shear Strength of SoilsEffects of Water Pressure
• Distance stress point is from failure line is a measure of stability.
• Greater distance
> greater stability
Section 4 - Shear Strength of Soils
Mohr - Coulomb
A-ve pwp moves stress point to right
Moves point further from failure line
greater stability
Moves point closer to failure line
less stability
+ve pwp
Slopes near Hadleigh Essex are only stable because of -ve pwp
• 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.
Section 4 - Shear Strength of Soils
The Triaxial Test
Section 4 - Shear Strength of Soils
The Triaxial Test
Load
Cell Pressure
Sample in rubber
membrane
Porous stone
• 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
Section 4 - Shear Strength of Soils
The Triaxial Test
• 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.
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
Section 4 - Shear Strength of Soils
Section 4 - Shear Strength of Soils
• In undrained dense tests pwp goes negative
• In drained dense tests volume increases
displacement
wat
er p
ress
ure
-ve
+ve
displacement
wat
er p
ress
ure
+ve
-ve
DenseLoose
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.
• 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.
Section 4 - Shear Strength of Soils
e
log
-ve pwp
+ve pwp
Water squeezed out
Critical State Line
Water sucked in
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.
Section 4 - Shear Strength of Soils