1 N.K. Tovey Н.К.Тови Landslide just west of Maracas Beach, Trinidad December 2002 Types of...

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1 N.K. Tovey Н.К.Тови Landslide just west of Maracas Beach, Trinidad Types of Landslide Why Landslides occur Landslide Hazards
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Transcript of 1 N.K. Tovey Н.К.Тови Landslide just west of Maracas Beach, Trinidad December 2002 Types of...

Page 1: 1 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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N.K. Tovey

Н.К.Тови

Landslide just west of Maracas Beach, Trinidad December 2002

Types of Landslide

Why Landslides occur

Landslide Hazards

Page 2: 1 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

Page 3: 1 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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Landslides: Types of Landslide

• Cut Slopes

• Fill Slopes

• Retaining Walls

• Hybrids: Cut/Retaining Wall / Fill/Retaining Wall

• “Natural” Slopes - is there a better word?

Page 4: 1 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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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?

Page 5: 1 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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Cut Slopes and Fill Slopes

Fill Slope

Retaining Wall

“Natural” Slope

Cut Slope

Failure of “Natural Slope” – cut slope and retaining wall unaffected

Page 6: 1 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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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

Page 7: 1 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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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

Page 8: 1 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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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?

Page 9: 1 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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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

Page 10: 1 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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• 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

Page 11: 1 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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• 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

Page 12: 1 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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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

Page 13: 1 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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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?

Page 14: 1 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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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

Page 15: 1 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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• 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

Page 16: 1 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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-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

Page 17: 1 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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• 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

Page 18: 1 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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• 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

Page 19: 1 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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• 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

Page 20: 1 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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• 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.

Page 21: 1 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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Types of Slope Failure

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

Page 22: 1 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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Types of Slope Failure: Progressive Failure

• Bulging must occur before failure takes places

Displacement

dense

Page 23: 1 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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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

Page 24: 1 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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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

Page 25: 1 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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• 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

Page 26: 1 N.K. Tovey Н.К.Тови Landslide just west of Maracas Beach, Trinidad December 2002 Types of Landslide Why Landslides occur Landslide Hazards.

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.

Page 27: 1 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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Errors in Interpretation of Field Data

• Two Boreholes

• Both hit solid rock

• Fs ~1.4

• Slope Failed

• Inappropriate Failure mechanism