ENV-2E1Y: Fluvial Geomorphology: 2004 - 5

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ENV-2E1Y Fluvial Geomorphology 2004 - 5

• Slope Stability and Geotechnics
• Landslide Hazards
• River Bank Stability
• Section 4 - Shear Strength of Soils

N.K. Tovey ?.?.???? ?.?., ?-? ??????????? ????
Landslide on Main Highway at km 365 west of Sao
Paulo August 2002
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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

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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

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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)

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Section 4 - Shear Strength of Soils

• Resolution of Forces

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
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Section 4 - Shear Strength of Soils
Coulomb a French Military Engineer Problem
Why do Military Fortifications Fail?
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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?
F N tan ? ......4.3 ? is the
angle of internal friction
?
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Section 4 - Shear Strength of Soils
Suppose there is some glue between block and
surface Initially - block will not fail until
bond is broken
Block will fail
Block is stable
F C N tan ? ......4.4 C is
the cohesion
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Section 4 - Shear Strength of Soils
F C N tan ?
......4.4 above equation is specified in
forces In terms of stress ? c ? tan ?

• 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 ?

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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 - F
G - G
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Section 4 - Shear Strength of Soils
dense
loose
Peak in dense test is reached at around 1 - 3
strain
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Section 4 - Shear Strength of Soils
Increasing normal stress
dense
?/?
loose
Displacement
Normalising curves to normal stress leads to a
unique set of curves for each soil.
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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
Readings cannot be taken after peak in a stress
controlled test
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Section 4 - Shear Strength of Soils
Dense Test
Loose Test
Medium Dense
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Section 4 - Shear Strength of Soils
Plot volume changes as Void Ratio
loose
Critical void ratio
medium
dense

• All tests eventually come to same Void Ratio

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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

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Section 4 - Shear Strength of Soils

• Distance stress point is from failure line is a
  measure of stability.
• Greater distance
• gt greater stability

Mohr - Coulomb
-ve pwp moves stress point to right
ve pwp
Moves point further from failure line ? greater
stability
Moves point closer to failure line ? less
stability
Slopes near Hadleigh Essex are only stable
because of -ve pwp
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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.

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

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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.

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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

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Section 4 - Shear Strength of Soils
Dense
Loose

• In undrained dense tests pwp goes negative
• In drained dense tests volume increases

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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.

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Section 4 - Shear Strength of Soils

• 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.

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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.

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