ecg503 week 4 lecture note chp2

7/26/2019 Ecg503 Week 4 Lecture Note Chp2

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GEOTECHNICAL ENGINEERING

ECG 503

LECTURE NOTE 03

TOPIC : 2 0 SLOPE RISK

ENGINEERING

31 JULY 2008


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LEARNING OUTCOMES

Learning outcomes:

At the end of this lecture/week the students would

be able to:

Conduct slope risk assessment analysis.

Determine the various of slope protection and

rehabilitation works.


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Types of slopes

Slope materials

Types of failures

Stability analysis

SLOPE

STABILITY

Classification of landslides

Cut Slope

Earth fill / embankment

Total stress vs.

effective stress

Shear strength

Plastic Equilibrium

Factor of safety

Methods

Infinite slopes

Finite slopes

OUTLINE soils

Slope instability

Increased in stress

Decreased in strength

Rocks

Residual soils


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TYPES OF LANDSLIDES


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TYPES OF LANDSLIDES (2)


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TPYE OF MOVEMENT

TYPE OF MATERIAL

Bedrock

Engineering soils

Predominantly coarse Predominantly fine

Falls Rock fall Debris fall Earth fall

Topples Rock topple Debris topple Earth topple

Slides

Rotational Rock slump Debris slump Earth slump

Translational

Few unitsRock block

slide

Debris block

slideEarth block slide

Many

unitsRock Slide Debris slide Earth slide

Lateral spreads Rock spread Debris spread Earth spread

FlowsRock flow

(deep creep)

Debris flow

(soil creep)

Earth flow

(soil creep)

Complex Combination of two or more principal types of movement

Classification of Landslides (Varnes, 1978)


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TYPES OF SLOPE1. Natural slopes

2. Engineered slopes

Cut slopes

Fill slopes: Road and

railway embankmentsinvolving compacted

soils

Retaining structures

Rock slopes Bogot (Colombia)Geogrid reinforced steep road

embankment

Engineered Cut Slope

Altered by excavation activities Slope failure of a roadembankment (fill slope)

Rock slope in Hong Kong
http://www.dot.ca.gov/dist05/projects/bigsur/gallery/detract/images/cut_slope_jpg.jpg


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FACTORS AFFECTING SLOPE STABILITY

1. Pore pressure increased

2. Erosion

3. Soil stratification

4. Soil degradation and weathering

5. Presence of tension cracks

6. Vegetative cover

7. Change of stress condition

8. Rise of the groundwater table9. Rock mass discontinuities

10. Rock mass state

Among the factors which may contribute to slope

sliding movements are :-


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Main items required to evaluate the

stability of a slope

1.Shear strength of soils

2.Slope geometry

3.Pore pressures or seepage forces

4.Loading and environmental conditions.


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Slope failure in natural

slope

TYPES OF SLOPE (continue)

The Wolf Mountainlandslide, a large

slump-debris flow

with an estimated

volume of 500,000

cubic yards,

occurred on May18, 1997 . The

debris flows

overran a 400-ft

stretch of U.S.

Highway 26-89, a

primary highway

leading to Jackson, and to

Yellowstone

National Park .


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Rock fall

Potential rock fall hazard


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Slope failure in residual soil formation involving rock fall

Bukit Lanjan

Kuala Lumpur

28th Nov 2003

Surcharge


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Rock slopes

Sheet jointing in

granite (Hong Kong)

Rock slide by undercutting

sheet joints (Hong Kong)


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Slope failure at Bukit Antarabangsa

Condominium (15/5/1999)


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Mud flow

Tragedy at Pos Dipang


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Tanah runtuh di Kampong Pasir, Hulu Klang

pada 31 Mei 2006 involved 4 lifes.


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ROCK FALL


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EXAMPLE OF

DEBRIS FLOW


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EXAMPLE OF EARTH FLOW


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EXAMPLE OF AN

EARTH SLUMP

(Rotational )


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EXAMPLE OF A SOIL CREEP


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EAST COAST EXPRESSWAY

169 kilometer

STEP

TURFING

TREE PLANTING
http://www.llmnet.gov.my/bm/Bina/lpt/lawatan%20Tim%20Menteri%2019_07_04%20065.jpghttp://www.llmnet.gov.my/bm/Bina/lpt/lawatan%20Tim%20Menteri%2019_07_04%20065.jpghttp://www.llmnet.gov.my/bm/Bina/lpt/lawatan%20Tim%20Menteri%2019_07_04%20065.jpghttp://www.llmnet.gov.my/bm/Bina/lpt/lawatan%20Tim%20Menteri%2019_07_04%20065.jpghttp://www.llmnet.gov.my/bm/Bina/lpt/lawatan%20Tim%20Menteri%2019_07_04%20065.jpghttp://www.uitm.edu.my/conference/stss_04/index.htm


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CUT SLOPE

NORTH SOUTH EXPRESSWAY

848 kilometer
http://www.llmnet.gov.my/bm/Operasi/PLUS/highwayplus2.JPGhttp://www.llmnet.gov.my/bm/Operasi/PLUS/highwayplus2.JPGhttp://www.llmnet.gov.my/bm/Operasi/PLUS/highwayplus2.JPGhttp://www.llmnet.gov.my/bm/Operasi/PLUS/highwayplus2.JPGhttp://www.llmnet.gov.my/bm/Bina/lpt/lebuhraya%20LPT%20(2).jpg


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EAST COAST EXPRESSWAY

169 kilometer

TURFING

DRAIN
http://www.llmnet.gov.my/bm/Bina/lpt/lebuhraya%20LPT%20(2).jpghttp://www.llmnet.gov.my/bm/Bina/lpt/lebuhraya%20LPT%20(2).jpghttp://www.llmnet.gov.my/bm/Bina/lpt/lebuhraya%20LPT%20(2).jpghttp://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPG


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KL - KARAK HIGHWAY60 kilometer

TERRACING

TURFING

EXISTING TREE

CASCADED DRAIN

V SHAPED DRAIN
http://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk3.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk1.JPG


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KL - KARAK HIGHWAY

60 kilometer

SLOPE CUTTING

EXISTING TREE
http://www.llmnet.gov.my/bm/Operasi/KLK/klk1.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk1.JPGhttp://www.llmnet.gov.my/bm/Operasi/KLK/klk1.JPGhttp://www.llmnet.gov.my/bm/Operasi/SAE/Kesas2.JPG


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SHAH ALAM EXPRESSWAY

34.5 kilometer

SOIL NAILING

GUNITING

SURFACE DRAINAGE
http://www.llmnet.gov.my/bm/Operasi/SAE/Kesas2.JPGhttp://www.llmnet.gov.my/bm/Operasi/SAE/Kesas2.JPGhttp://www.llmnet.gov.my/bm/Operasi/SAE/Kesas2.JPGhttp://www.llmnet.gov.my/bm/Operasi/SAE/Kesas2.JPG


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COVERED BY CREEPERS GUNITING & TURFING

GUNITING GEOSYNTHETIC SHEET


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R.C WALL GUNITING

SOIL NAILING GUNITING

Normal question asked


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Normal question asked.

1. Why slopes fail during rainy season ?

2. Why slope failure due to rainfall is a shallow type slope failure ?

3. What is the important of unsaturated soil mechanics in slope stability ?

4. Why the distribution of the negative pore water pressure above GWT is ignore in the

conventional slope stability analysis ?

5. Why the stability analysis using the conventional method indicates slope failed i.e.

FOS < 1 whereas the slope is still standing ?

6. How does geotechnical engineer modelled slope failure due to rainfall infiltration

before understanding the unsaturated soil mechanics ?

7. Why slope that has been standing safely for quite a while suddenly fails ?

8. How does infiltration affect the stability of slope ?

9. Does soil shear strength continuously increase with suction ?

10. Can the conventional method of slope stability analysis modelled shallow type ofslope failure due to infiltration ?

11. Is the Highland Towers tragedy involved the subject of stability of slope in effect of

infiltration ?

12. Does the slope cover using shotcrete or plastic sheet help to stabilize the slope ?

PREVIOUS PRACTICE OF MODELLING SLOPE


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Geotechnical engineers used to model slope failure due to rainfall infiltration by

considering the GWT to rise. Whereas GWT in tropical residual soil is far below the

slope surface. The assumption does not depicts what actually happen in the field.

This happened because the mode of failure is not well understood besides the lack

of knowledge in the mechanics of unsaturated soils in the past.

1. Effective stress decrease due to

buoyancy effect of the

submerged section of the slice.

2. Therefore the shear resistance

decrease.

3. Thence, FOS decrease.

4. But still the shallow type of

slope failure cannot bemodelled.

PREVIOUS PRACTICE OF MODELLING SLOPE

FAILURE DUE TO RAINFALL INFILTRATION


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FEATURES OF A SLOPE FAILURE


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TERMINOLOGY


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SLOPE MATERIALS

Types of geologic soil deposits

Typical residual soil profile

(Little, 1969)

MODES OF SLOPE FAILURE


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MODES OF SLOPE FAILURE

Toe circle

Slope circle

Slope failureBase failure

Shallow slope failure

Typical of

slope failure

due to rainfallinfiltration

Translational slides

Theoretical model


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Theoretical model

for slope stability

Effective stress analysis ordinary slice method (Fellenius, 1927)

The theory doesnt work for rainfall

induced failure, therefore the factors

involved in the theoretical model need

to be changed or improved.

From theoretical model, the behaviour can be anticipated

before it actually happen.

Disturbing factor

Resisting factor

Mechanism of rainfallEffective stress and shear strength


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Rain

Mechanism of rainfall

induced slope failure

International Seminar on Civil and Infrastructure Engineering, UiTM 13 & 14 June 2006 (ISCIE 06)

A

At point A, effective

stress increase duringinfiltration. Effective

stress increase should

be elevating the shear

stress, then why failure

????

Because, shear strength

decrease when the

wetting front arrived at A

due to suction loss !!!!!!

must be incorporated in slope

stability analysis !!!!!

M d l i filt ti i t l


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Volumetric moisture content (cm3/cm3)

Soildepth(cm)

0.00

t = 581.7 s

0.20 0.30 0.40

5

10

15

20

25

30

35

40

45

50

55

t = 701.7 s

t = 821.7 s

t = 941.7 s

Saturated volumetric moisture

content = 0.387 cm3/cm3

0.10

Model infiltration into slope

Richard equation for moisture

flow in unsaturated media

Gravity has

negligible

effect on

infiltration

1

zhhK

zthhK

xthhC

l f i i il h i


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A needle floating on water surface.

Its weight depresses the surface,and is balanced by the surface

tension forces on the sides.

Surface tension force ?

Role of suction in soil mechanics.

http://en.wikipedia.org/wiki/Image:SurftensionDiagram.gifhttp://en.wikipedia.org/wiki/Image:Wasserl%C3%A4ufer_bei_der_Paarung_crop.jpg


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Surface Tension and the Water Strider

Beading of rain water on awaxed surface.

Water does not adhere towax, surface tension

prevents water drops fromspreading out over the wax.


S f t i f ?
http://en.wikipedia.org/wiki/Image:Wasserl%C3%A4ufer_bei_der_Paarung_crop.jpghttp://en.wikipedia.org/wiki/Image:Dew_2.jpg


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Surfacetension

prevents thecoin from

sinking.

The coin isindisputablydenser than

water, so

cannot befloating due tobuoyancy

alone.

SUCTION IN SOIL MECHANICS
http://upload.wikimedia.org/wikipedia/commons/9/9c/2006-01-15_coin_on_water.jpghttp://upload.wikimedia.org/wikipedia/commons/9/9c/2006-01-15_coin_on_water.jpg


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SUCTION IN SOIL MECHANICS

Soil

particle

Bigger

suction force

Suction = (ua-uw) Agitation force due to

surface tension of water.

Water pressure is lower than air

pressure.

In the field air pressure is zero

atmospheric or zero kPa (gauge

pressure) and pore water pressure

is negative.

Cavitation occurs at -101.3 kPa or

-1 Atmosphere (gauge pressure).

Soil

particle

Suction Force

ua uw

Surfacetension

force

H d tl t d ?


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How sand castle stands ?

Mohr Coulomb failure envelope
http://www.shorebreeze.com/photos/photoalbum/jalbum/margate/slides/Margate%20Welcome%20Boat.html


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Mohr Coulomb failure envelope

Mohr-Coulomb failure criteria is concerned with

stress conditions on potential rupture planes

within the soil.

The shear stress at failure is defined by straight line

which is known as the Mohr-Coulomb failure

envelope, = c + tan .

If Mohrs circle of effective stress touches the line

envelope then failure of the soil will occur.

For sliding to occur on any plane, the shear stresshas to overcome:-

1. The frictional resistance tan which is

dependent on the effective normal stress,

acting on the plane and on the friction angle, .

2. The cohesion, c which is independent of thenormal stress.

t

sO

13

Mohr-Coulomb

failure envelope

3

1

BASIC CONCEPT OF SLOPE STABILITY ANALYSIS


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= tan

1. Determine effective stress at slice base

2. Determine internal friction angle,

3. Calculate shear stress

4. Calculate the shear force

5. Calculate disturbing moment = Multiply the force with the lever arm

Pulling force, P

W

N effective stress

F = N

In soil . = tan

N =

Law of

mechanicsW

W

EFFECTIVE STRESS CONCEPT IN


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EFFECTIVE STRESS CONCEPT IN

SLOPE STABILITY

u'

Stress sustained by the soil skeleton is known aseffective stress.

The hydrostatic stress from the water in the voids is

known as pore water pressure.

In unsaturated soils the pore water pressure isnegative w.r.t. the atmospheric pressure and this is

contributing to a higher effective stress and thus a

higher shear strength.

RESEARCH HISTORY ON THE BEHAVIOUR OF UNSATURATED SOILS


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waa uuu 'Bishop 1959

Expression for effective stress in an

attempt to link the deformation

behaviour of unsaturated soil with a

single-valued effective stress equation.Lecture in Oslo, Norway, in 1955

A parameter related to degreeof saturation

= 1 for saturated condition

= 0 for dry condition

'tan'' cTerzaghi 1936

Jenning and Burland (1962)

No unique relationship between, e and

.

question the validity of Bishop equation.

Suction , , Vol

Usually

, Vol

'tan'tan' waa uuuc Bishop, Alpan, Blight and Donald (1960)

No unique relationship between and Sr.

question the validity of Bishops effective equation

Donald, 1961

Blight, 1961

Cohesionless silt

RESEARCH HISTORY ON THE BEHAVIOUR OF UNSATURATED SOILS


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Po Shan Road Landslide 1972,

Hong Kong (Killed 67 people)

A slope failure that triggered the intensity of

research on the behaviour of unsaturated soil

Then in 1978 Fredlund, Morgenstern andWidger has introduced the shear

strength equation for unsaturated soils.

bwaa

uuuc tan'tan'

c

(ua-uw)

(-ua)

b

Model cannot explain the alternate wetting and drying

volume change behaviour in unsaturated soils.

Wheeler et al. (Geotechnique 2003)

Then Alonso et al. (1990) have introduced criticalstate model for unsaturated soils followed by

Wheeler and Sivakumar 1993 and 1995.

wawaa uuMupMq

MECHANISM OF SLOPE FAILURE


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DUE TO INFILTRATION

As rain water infiltrates into the unsaturated soil ;

The bulk unit weight of the soil will be increased.

As water filled up the void spaces, suction diminishes due to the

increase in the radius of curvature of the water meniscus between the

soil particles. The suction will completely vanish when the soil become

saturated.

When suction decreases shear strength will consequently decreases.

The decrease in shear strength will reduce the resisting factor.

The increase in the bulk unit weight and the presence of the seepage

force will increase the effective stress i.e. the disturbing factor.

As a result FOS will decrease and when the value goes lower than

unity failure will be triggered.

That is why slope failure in tropical countries

often occurs after a long period of rainfall.

SHEAR STRENGTH MODELS


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Cannot produce a good representative shear strength behaviourespecially at low stress levels (e.g. Terzaghi, 1936; Fredlund et al.,

1978; Vanapalli et al., 1996). Cannot be used to explain the collapse settlement due toalternate wetting and drying (noted by Wheeler, Sharma andBuisson, 2003).

LIMITATION OF EARLIER SHEAR STRENGTH MODELS

b

(-uw)

Mohr-Coulomb envelope(Terzaghi, 1936)

'tanwu

Open type critical state yield surface(Alonso et al., 1990 & Wheeler and

Sivakumar, 1993 and 1995)

wawaa uuMupMq

Closed type criticalstate yield surface(Tang and Graham

2002)

bwaa

uuuc tan'tan'

Extended Mohr-Coulombenvelope

(Fredlund et al., 1978)

c

(ua-uw)

(-ua)

SHEAR STRENGTH MODELS

DEFINING SHEAR STRENGTH BASE ON

S SS S C O


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a

u

wa uu

NET STRESS AND SUCTION IN EXTENDED

MOHR-COULOMB SPACE Equation for thesurface envelope

?

500

kPacs 30max

1. kPauu 152.

'min'

min tantan

ff twta uu SOIL

SHEAR STRENGTH


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0

100

200

300

400

500

0

20

40

60

80

0

100

200

300

400

500

Suction (kPa)

Net

stress (kPa)

Shear

strength

(kPa)

0' uwaauwa

uuuuu

Zone 2

Zone4

rwauwa

warwa

rwauwa

wauwa

suuuu

uuuu

uuuu

uuuuc 1max

t

tw

atw

tw

a

uN

uu

u

u

1

max1

s

rwa

warwa

rwa

wa cuu

uuuu

uu

uu

t

tw

atw

tw

a

uN

uu

u

u

1

kPauurwa

152.

kPautw 2003.

kPat 2304.5.

05.06.

kPauu uwa 500'

7.

34'min

rwauwa

warwa

rwauwa

wauwa

s

uuuu

uuuu

uuuu

uuuuc 1max

max1

s

rwa

warwa

rwa

wac

uu

uuuu

uu

uu

'min

'

min tantan

ff twta uu

4 equations &

7 parameters

t

twu

N

'

mintan

1

1

SHEAR STRENGTH

WARPED-SURFACE EXTENDED MOHR-COULOMB SHEAR

STRENGTH ENVELOPE OF THE TEST MATERIAL500


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STRENGTH ENVELOPE OF THE TEST MATERIAL

0

100

200

300

400

500

020

4060

80

0

100

200

300

400

500

Suction (kPa)

Net stress (kPa)

Shearstrength

(kPa

)

kPacs 30max

kPauu

rwa 15

5005.0 auwa uuu

kPat 230

kPau tw 200

34'min f

1

2

3

4 5

6

= 0 05 i e rate of increase of ultimate suction (u u ) w r t net stress

7

kPauuuwa 50

0'

Zone

4

Zone

3

Zone

1

Zone

2

This is a Mohr (1900)

failure criterion, which

is a non-linear shearstrength behaviour

w.r.t. effective stress.

Mohr-Coulomb failure

criterion is a linear shear

strength behaviour w.r.t.

effective stress proposedby Terzaghi (1936).

ecg503 week 4 lecture note chp2

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