N'K' Tovey

1
Landslide Hazards
Types of Landslide Why Landslides occur

• N.K. Tovey
• ?.?.????

Landslide just west of Maracas Beach, Trinidad
December 2002
2
Mans Influence (Agriculture /Development)
Cut / Fill Slopes
Construction
Drainage
Pumping
Hydrology (rainfall)
Earthquakes
Geology
Ground Water
Ground Loading (Consolidation)
Erosion/Deposition
Glaciation
Weathering
Surface Water
Material Properties (Shear Strength)
Geochemistry
Stability Assessment
Slope Profile
Landslide Preventive Measures
Slope Management
Design
Landslide Warning
Landslide
Cost
Build
Temporarily Safe
No Danger
Consequence
3
Landslides Types of Landslide

• Cut Slopes
• Fill Slopes
• Retaining Walls
• Hybrids Cut/Retaining Wall / Fill/Retaining
  Wall
• Natural Slopes - is there a better word?

4
Landslides Types of Slope

• Cut Slopes
• Fill Slopes
• Retaining Walls

• Hybrids Cut/Retaining Wall / Fill/Retaining
  Wall
• Natural Slopes - is there a better word?

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Cut Slopes and Fill Slopes
Failure of Natural Slope cut slope and
retaining wall unaffected
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Is there such a thing as a Natural Slope?
Deep seated landslide unaffected by anthropogenic
activity

• ?
• slopes where there has been no anthropogenic
  activity, or where there is such activity it
  causes small changes to the geometry of the slope
  so that the Factor of Safety is largely
  unaffected.

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Relationship between mobilizing resisting forces
Shear Force
Normal Force
Force (S) required to move block is proportional
to Normal Force (N) On a slope N depends on
weight and ? N W cos ? S also
depends on weight and ? N W sin ?
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Properties 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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Properties 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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Properties 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 0
  (sands)
• ? ? tan ?
• cohesive materials - i.e. ? 0 (wet clays)
• ? c
• materials with both cohesion and friction
• ? c ? tan ?

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Properties 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
Implication Vertical slopes stable only to a
height of 2c/? Where ? is unit weight ?g
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Properties of Soils Effects of Packing and Water
Dense grains have to rise up to slip over each
other. Sample must EXPAND
Loose grains can slip over each other easily
Water filling voids forces grains apart pwp
Water partly filling voids causes suction -
pwp
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Properties of Soils
dense
loose
Peak in dense test is reached at around 1 - 3
strain
What happens if residual strength is used
compared to peak strength?
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Properties of Soils
dense
loose
Dense / overconsolidated soils expand on
shearing Loose / normally consolidated soils
contract on shearing Eventually a common void
ratio and shear strength
What Shear Strength should be used?
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Properties of Soils Effects of water

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

Mohr - Coulomb
C
-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
? c (? - u) tan ?
A
Slopes near Hadleigh, Essex are only stable
because of -ve pwp
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Properties of Soils Effects of previous history
Consolidation and Rebound
What happens if slope movement does not allow
volume change? If dense, sample tries to expand
gt - ve pwp gtgtgtgt more stable If loose, sample
tries to contract gt ve pwp gtgtgtgt less stable
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Types of Slope Failure
Water table
Infinite Slope
Failure Surface
Extensive Slope of nearly constant angle Many
slopes approximate to this

• Strata are parallel to surface
• Failure Surface is parallel to surface
• Water Table is parallel to surface
• Analysis is relatively straight forward assuming
  a block sliding and relevant properties.

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Types of Slope Failure Finite Slopes
?

• Straight Line failure
• Only applicable for slopes ABOVE water table
• Postulate failure mechanism
• Need to test for minimum factor of safety.

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Types of Slope Failure problem of cracks
?

• Cracking at surface at crest
• Reduces length of shear resistance
• Allows water to fill crack and cause
  destabilising pressure
• In dry summers keep crest damp to prevent crack
  formation!!!

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Types of Slope Failure

• Backward tilting surface at crest of failure
  10o.

• Summer water table
• Winter Water Table

• Slope failure
• Toe failure
• Deep seated Base failure

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Types of Slope Failure

• If water is under pressure then debris from
  landslide becomes fluid. Controlled disaster
  becomes a major disaster - Aberfan

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Types of Slope Failure Progressive Failure
dense

• Bulging must occur before failure takes places

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Types of Slope Failure Method of Analysis

• Divide up slope into slices and sum up the
  stability of all slices.
• Detailed analysis allows for estimating
  inter-slice forces
• Can be ignored as first approximation as these
  are conservation assumptions
• Leads to lower Fs than actual

S
W
N
24
Slope Failure Remedial Action

• Create berms with longitudinal drains to remove
  surface water

• Reprofile Slope remove material at middle top
• Add weight to toe
• Lower water table

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Types of Slope Failure

• Analysis of safety involves
• Soil Sampling
• Field Surveying
• Location of Water Table
• Laboratory Testing of samples
• Analysis of stability
• Some parts of analysis are conservative -
• Other parts over estimate factor of safety and
  may give false sense of security

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Appraising Test Data
strength
1 2 3 4 5
Test No.

• Several tests on soil samples from a slope
• What value should be used in analysis

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Errors in Interpretation of Field Data

• Two Boreholes
• Both hit solid rock
• Fs 1.4

• Slope Failed
• Inappropriate Failure mechanism