1

N.K. Tovey

Н.К.Тови

Landslide just west of Maracas Beach, Trinidad December 2002

Types of Landslide

Why Landslides occur

Landslide Hazards

2

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

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

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?

5

Cut Slopes and Fill Slopes

Fill Slope

Retaining Wall

“Natural” Slope

Cut Slope

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

6

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

7

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

8

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?

9

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

10

• 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

11

• 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

12

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

13

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?

14

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

15

• 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

16

-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

17

• 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

18

• 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

19

• 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

20

• 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.

21

Types of Slope Failure

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

22

Types of Slope Failure: Progressive Failure

• Bulging must occur before failure takes places

Displacement

dense

23

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

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

25

• 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.

27

Errors in Interpretation of Field Data

• Two Boreholes

• Both hit solid rock

• Fs ~1.4

• Slope Failed

• Inappropriate Failure mechanism