ENV-2E1Y: Fluvial Geomorphology: 2004 - 5

Slope Stability and Geotechnics

Landslide HazardsRiver Bank Stability

Section 4 - Shear Strength of SoilsN.K. Tovey

Н.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 ......4.4above 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 SoilsPlot 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 SoilsThe 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

Dense Loose

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