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N.K. Tovey
Н.К.Тови
Landslide just west of Maracas Beach, Trinidad December 2002
Types of Landslide
Why Landslides occur
Landslide Hazards
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Landslide
Consequence
Remedial Measures
Remove ConsequenceSafe at the moment
Cost
Build
Landslide Warning
No Danger Temporarily Safe
Design
LandslidePreventive Measures
Stability Assessment Slope Profile
GeologyErosion/DepositionGlaciationWeatheringGeochemistry
Cut / Fill SlopesConstructionDrainage Pumping
Man’s Influence (Agriculture /Development)
Earthquakes
Material Properties (Shear Strength)
Ground Loading(Consolidation)
Slope Management
Hydrology (rainfall)
Ground Water
Surface Water
GIS
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Landslides: Types of Landslide
• Cut Slopes
• Fill Slopes
• Retaining Walls
• Hybrids: Cut/Retaining Wall / Fill/Retaining Wall
• “Natural” Slopes - is there a better word?
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Fill Slope
Retaining Wall
“Natural” Slope
Cut Slope
• Cut Slopes
• Fill Slopes
• Retaining Walls
Landslides: Types of Slope
• Hybrids: Cut/Retaining Wall / Fill/Retaining Wall
• “Natural” Slopes - is there a better word?
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Cut Slopes and Fill Slopes
Fill Slope
Retaining Wall
“Natural” Slope
Cut Slope
Failure of “Natural Slope” – cut slope and retaining wall unaffected
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Is there such a thing as a “Natural Slope?”
•?
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.
Landslides triggered by
anthropogenic activity
Deep seated landslide unaffected
by anthropogenic activity
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N
S
=WN
S
W
Relationship between mobilizing & resisting forces
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
S1
N1
S2
N2
S3
N3
N
S
W
W
Normal Force
Shear Force
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Properties 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?
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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
Properties of Soils
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• 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
F = C + N tan
above equation is specified in forces
In terms of stress:
= c + tan
Properties of Soils
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• Stress Point at B
- stable
• Stress Point at A
- stable only if cohesion is present
• if failure line changes, then failure may occur.
Properties of Soils
F
N
F - F
G - G
B
A
Implication: Vertical slopes stable only to a height of 2c/Where is unit weight = ρg
stable
stable
just stable
stab
le
unst
able
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Properties of Soils – Effects of Packing and Water
Loose: grains can slip over each other easily
Dense: grains have to rise up to slip over each other.
Sample must EXPAND
Water filling voids forces grains apart +pwp
Water partly filling voids causes suction - pwp
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F
N
F - F
NNNNNNNN
Displacement
dense
loose
Peak in dense test is reached at around 1 - 3% strain
Properties of Soils
What happens if residual strength is used compared to peak strength?
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Properties of Soils
Displacement
dense
loose
Displacement
Change in volume
Displacement
volume
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?
At critical voids ratio
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• Distance stress point is from failure line is a measure of stability.
• Greater distance
> greater stability
• Fs = CA / BA
Properties of Soils – Effects of water
Mohr - Coulomb
B
-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
C
A = c + ( - u) tan
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-ve pwp
+ve pwp
Water squeezed
out
Water sucked in
e
log
Properties of Soils – Effects of previous history
e
log
Critical State Line
Sedimentation / glaciation/ anthropogenic loading
Erosion / unloading
Consolidation and Rebound
Void ratio (e) = volume of voids
volume of solids
Dense / heavily overconsolidated
loose / lightly overconsolidated
What happens if slope movement does not allow volume change? If dense, sample tries to expand
> - ve pwp >>>> more stableIf loose, sample tries to contract
> +ve pwp >>>> less stable
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• 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.
Extensive Slope of nearly constant angle
Many slopes approximate to this
Water table
Types of Slope Failure
Infinite Slope
W
Failure Surface
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• Straight Line failure
• Only applicable for slopes ABOVE water table
• Postulate failure mechanism
• Need to test for minimum factor of safety.
Types of Slope Failure: Finite Slopes
W
Fs
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• 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!!!
Types of Slope Failure – problem of cracks
W
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• Slope failure
• Toe failure
• Deep seated Base failure
Types of Slope Failure
• Summer water table
• Winter Water Table
• Backward tilting surface at crest of failure ~ 10o.
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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
• Bulging must occur before failure takes places
Displacement
dense
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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 actualS
N
W
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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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• 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
Types of Slope Failure
Appraising Test Data
• Several tests on soil samples from a slope• What value should be used in analysis
stre
ng
th
1 2 3 4 5
Test No.
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Errors in Interpretation of Field Data
• Two Boreholes
• Both hit solid rock
• Fs ~1.4
• Slope Failed
• Inappropriate Failure mechanism