latsis - rainfall induced landslides - why they occur and some mitigating measures [compatibility...

8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]

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Rainfall Induce Landslides Why they occur and some

mitigating measures

Tan S.A1, David Toll2, and K.K.Phoon11The National University of Singapore

2University of Durham, UK

Latsis Symposium by CCES at ETH-Z,Zurich, Switzerland, 17-19 Sep 2007


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Outline

Introduction Climate and slope stability

Soil Strength as function of Infiltration Cases: Kranji Racecourse and NUS

Biz School


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Introduction

Climate in the tropics

Singapore landslides and rainfall events empirical observations


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Inference from Measured Pore-Water Pressures

(pwp) at NTU-CSE

Wet period rainfall=86mm in Dec 1999; dry period rainfall = 1mm inMar 2000

Small rainfall in Mar 2000 produce large change pwp near surface at1-1.5m depth

After 24 hr equalization; pwp near surface drop back and pwp at 1-1.5m depth increased slightly, due to water infiltration

But at deeper depth of 2.5-3m no significant pwp change For large rainfall in Dec 1999; observe only small cahnges in pwp

near surface, with pwp approaching hydrostatic condition

Data suggests that there is still suction in slope even wettest time of

the year


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Unsaturated soil strength

State Variables defined as:

=

=

)(

)(

wa

an

uu

u Net normal stress

Matric suction


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Unsaturated soil strength

Shear Strength is the Extended Mohr-Coulomb failure criteria:

=

=

=

=

=

=

++=

)(

)(

'

'

:

tan)('tan)('

wa

an

b

bwaan

uu

u

c

where

uuuc

Shear strength

Effective cohesion for saturated soils

Effective friction angle for saturated soils

Friction angle with respect to matric suction in unsaturated soils

Net normal stress

Matric suction


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Saturated soil strength

Shear Strength is Mohr-Coulomb:

==

=

=

+=

=

)('

'

:

'tan)('

,'

wn

wn

b

u

c

where

uc

and

Shear strength

Effective cohesion for saturated soils

Effective friction angle for saturated soils

Normal effective stress


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Infiltration Seepage Analysis

Transient Seepage FEM Analysis

Soil Water Characteristic Function

Soil Permeability Function

Infiltration Input Results of Analysis


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Equation of Continuity

Constitutive Equation: Darcys Law

yx qq h

Q c

x y t

+ + =

Groundwater Flow Theory

x x hq kx

=

y y hq ky

=

q : the specific discharge (m/s)c : the storage coefficient (m-1)h: total head (m)Q: a source (m3/s) per unit vol.

k : the coefficient of permeability (m/s)

Flow in Flow out

Flow out

Flow in

Stored in


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Water flow in unsaturated soilWater content and permeability in unsaturated zone is a

function of suction pressure


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Water flow in unsaturated soil

() Soil water characteristic curve

K () permeability curve

( )s

CStorage Coefficient

=

( )k kSoil Permeability =

These curves are from experiments

Unsaturated soil has k severalorders less than saturated soils


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Precipitation:

max

rain max min

min

if Ponding

if and

if No infiltrationx x y y

h y h

q n q n q h y h h y h

h y h

= +

+ = < + > +

= +

Boundary Conditions for Rainfall

Flux boundary condition

ClosedClosed

Closed


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Result of 1-D Infiltration Analysis1D Infiltration

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

-5 0 5 10 15 20 25 30 35

Suction Pressure [kN/m2]

El

evation[m]

Initial SST=0.1 day

T=0.25day

T=0.5day

T=1.0day

T=5.0day

Gardner's Theory

Gardner's 1D Infiltration Problem

alpha 2

Sres= 0.23 ksat= 1 m/day

Ssat= 1

p S(p) K(p)

0 1.00 1.00

0.5 0.51 0.37

1 0.33 0.14

1.5 0.27 0.05

2 0.24 0.02

2.5 0.24 0.01

3 0.23 0.00

Gardner's Theory

q ksat y p [m] P[kN/m2]

2 0.1 1 0.00 0.00 0

2 0.1 1 0.50 0.42 4

2 0.1 1 1.00 0.75 8

2 0.1 1 1.50 0.97 10

2 0.1 1 2.00 1.08 11

2 0.1 1 2.50 1.12 11

2 0.1 1 3.00 1.14 11

+

=

sat

y

sat

pk

qe

k

q

.1ln

1

peSSSS ressatres

+= )(

pekksat

= .

Input rainfall = 0.1 m/day

SS solution

after 5 days

Infiltration caused significant loss of suction

Loss of suction result in reduction of shear strength, which may trigger slope failure


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1D Infiltration Video

Click for Video

Input flux= 0.1mm/day

ClosedClosed

Closed


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Case 1: Kranji Racecourse About Dec 1998, a 70m long slope with

gradient of 1(V):2(H) was cut in medium stiff

residual clayey soil

After period of intense rainfall, slip failure

landslip about 1 to 1.5m deep over slopeof 30m length

Slope repaired using dry cut fill soil obtainedfrom same site failed repeatedly without useof subsurface drains


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Develop tension crack below crest of slope, why?


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Infiltration Analysis Rain 20 mm/dayEvolution of Saturation Zone

ConstantH2

Click for Video

ConstantH1

Closed No Flow

Input flux = 20mm/day



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Shallow Slip of Saturated Zone (Head)

Click for Video


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Shallow Slip of Saturated Zone (Flow)

Click for Video


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Site investigation

Distance (m)

0 2 4 6 8 10 12Distance (m)

98

100

102

104

106

108

110

Eleva

tion(mRL)

Observed Slip Plane

Probable GroundWater TableP1

P2

P3

104.5

103.8

1V:2H

104.6

106.3

Waterstandpipes

After a few days

without rain, measured

GWT appears to

coincide with level ofobserved slip surface


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Influence of Infiltration on Suctions and Soil Strengths

Kranji Pore Pressure Distribution at Mid-Slope (3m up)

100

101

102

103

104

105

106

107

-70 -60 -50 -40 -30 -20 -10 0 10 20

Total Pore Pressures [kPa]

Elevation[m]

1-day

2-day

3-day

4-day

5-day

Kranji Pore Pressure Distribution at 4.5 m Up-Slope

102

103

104

105

106

107

108

-30 -20 -10 0 10 20 30 40


Elevation[m]

1-day

2-day

3-day

4-day

5-day

After 5 days of rain, nearly all suction is removed up to mid-slope height

Suction of < 15 kPa remains at 4.5m height above toe of slope; c < 3 kPa

Table 1. Variation of Cohesion with Matric Suction

Matric suction [kPa] 5 10 15 20 25 30

b[deg] kPa

10 0.88 1.76 2.64 3.53 4.41 5.29

15 1.34 2.68 4.02 5.36 6.70 8.0420 1.82 3.64 5.46 7.28 9.10 10.92

bwa uuc tan)(' =


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GWT

Parallel Seepage

Hh

slip plane

Failure analysis of Shallow Slip = slope angle (degrees)H = depth to slip surface (m)

h = height of GWT from slip surface (m)

tan

'tan))/()(1(

cossin

' Hh

H

cFS w

+=


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Design for permanent stable slope

Internal drainage design to maintain

significant unsaturated soil zone inshallow soils along slope

Transient seepage analysis to validate

design

Stability analysis to show that design

has adequate long-term FOS


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Transient Seepage with 8m deep drains

at toe of slope and 20 mm/day rain-15.000 -10 .000 -5 .000 0 .000 5 .000 10 .000 15.000 20.000 25 .000

85.000

90.000

95.000

100.000

105.000

110.000

115.000

Active groundwater hea d

Phase number: 1 Phase time: 5 day, Extreme groundwater head 109.10 m

m

99.600

100.000

100.400

100.800

101.200

101.600

102.000

102.400

102.800

103.200

103.600

104.000

104.400

104.800

105.200

105.600

106.000

106.400

106.800

107.200

107.600

108.000

108.400

108.800

109.200

Near Steady-State condition after 5 dayof rain at 20 mm/day

Unsaturated Zone

8m-deep drains

ConstantH2

ConstantH1

Closed No Flow




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Infiltration Analysis Rain 20 mm/daySaturation Zone for Slope with 8m Deep Internal Drains

8m-deep pipe drains

Click for Video

I fil i A l i R i /d


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Infiltration Analysis Rain 20 mm/dayGW Head for Slope with 8m Deep Internal Drains

8m-deep pipe drains

Click for Video

I filt ti A l i R i 20 /d


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Infiltration Analysis Rain 20 mm/dayTransient Flow for Slope with 8m Deep Internal Drains

8m-deep pipe drains

Click for Video


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Influence of Infiltration on Suction Pressures in

Shallow Soils with 8m Deep drainsKranji Pore Pressure Distribution at Mid-Slope (3m up)

100

101

102

103

104

105

106

107

-30 -20 -10 0 10 20 30Total Pore Pressures [kPa]

Elevation[m]

1-day

2-day

3-day

4-day

5-day

Kranji Pore Pressure Distribution at 4.5 m Up-Slope

102

103

104

105

106

107

108

-20 -10 0 10 20 30 40


Elevation

[m]

1-day

2-day

3-day

4-day

5-day

+ve suction

-ve pressure

Small changes of soil suction up to 1m depth at mid-slope (3m up slope)

No changes to soil suction at 4.5m up slope

Soil strength is little affected by Infiltration for rain at 20mm/day over 5 days


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Seepage with 4m deep drains

GEONET 4m Depth

Pond le vel a t 104 .6 m R L

GEONET

Co ncrete Liner

Recom pacted Res idual Soi l 0 .5 m Sand Track

15 0 mm /h Rainfa l l

GWT

2.1829e-012

5.2122e-008

5.2122e-008

1.4009e-011

3.0994e-004

Distance (m)

0 5 10 15 20 25 30 35 4090

92

94

96

98100

102

104

106

108

110

112

114


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FOS with 4m Deep Drains

1.8

1.8 2

2.2

2.4

1.269

D e s c r i p t io n : P o n d W a t e r

U n i t W e igh t : 9 . 8 0 7

D e s c r ip t io n : R e c o m p a c te d R e s id u a l S o il

U n it W e ig h t : 1 8C o he s io n : 3

P h i: 2 0

D e s c r ip t io n : In s it u R e s id u a l S o il

U n i t W e i g h t : 1 8

C o h e s io n : 1 0

Ph i : 2 7

G W T

E O N ET 4 m D e p th

G E O N E T

0 .5 m S a nd Tr ac k

1 5 0 m m /h R ain fa l l

Dis tance (m )

0 5 10 15 20 25 30 35 4090

92

94

96

98

100

102

104

106

108

110

112

114


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Seepage with 8m Deep Drains

GEON ET 8m Depth

Pon d le vel a t 104.6 m RL

GEONET

Concrete L iner

Recom pacted Res idua l So ilGW T

0.5m Sand track

150 m m /h R ain fa ll

7.0160e-008

7.0158e-008

7.6699e-012

3.1082e-004

Distance (m)

0 5 10 15 20 25 30 35 4090

92

94

96

98100

102

104

106

108

110

112

114


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FOS with 8m Deep Drains

1.7

1.8

1.9

2

2.2

2.2

1.617

D e s c r i p t io n : P o n d W a t e rU n it W e ig h t : 9 .8 07

D e s c r ip t io n : R ec o m p a c t e d R e s id u a l S o ilU n i t W eig h t: 1 8

C o h e s io n : 5

P h i : 2 1De s c ri p t io n : In s it u R e s id u a l S o il

U n i t W e ig h t : 1 8

C o h e s io n : 1 0

P h i : 2 7

G W T

0 .5 m S a n d T r a c k

G E O N E T 8 m D e p th

1 5 0 m m / h R a in f a ll

G E O N E T

D is tanc e (m)

0 5 10 15 20 25 30 35 4090

92

94

96

98

100

102104

106

108

110

112

114

Parametric Study of influence of Depth of Internal


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Parametric Study of influence of Depth of InternalDrains

Table 2: Influence of GEONET depth on GWT and FOS of repaired slope

GEONET Depth

(m)

0 1 2 4 8 12 15

GWT at Slope

Crest (m RL)

108.1 108.0 107.9 107.6 106.8 104.7 104.7

GWT at Mid-

Slope (m RL)

107.1 106.9 106.4 105.7 104.7 104.7 104.7

Seepage intoSlope (m

3/s /m)

1.89x 10

-8 1.72x 10

-9 9.80x 10

-12 2.18x 10

-12 < 1.0x 10

-12 < 1.0x 10

-12 < 1.0x 10

-12

Soil State in Slope Fully

Soak

Fully

Soak

Fully

Soak

Fully

Soak

Soften Comp-

acted

Comp-

acted

Drained cohesion

c (kPa)

3 3 3 3 5 10 10

Drained friction

angle, deg

20 20 20 20 21 22 22

Drained FS 0.923 0.968 1.137 1.269 1.617 1.780 1.808


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Installation of 8m Deep DrainsModified Mannings Eqn. for discharge of equivalent pipedrain in-place of GEONET drain

Q = 1.137A RH0.66 S0.5 (m3/s)A=flow cross-section area (sq-m)

RH=hydraulic radius (m)

=R/2 for full flowS=slope (m/m)

Actual Repair used 75mm diameter perforated drainswrapped with geotextile filters at 1.5m centers to achievesame discharge capacity as Geonet Sheet Drain


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Install geotextile-wrapped 15m long, 75-mm

diameter pipe drains at 1.5 m intervals

GW di h f i l i d i h


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GW discharge from internal geo-pipe drains has

performed well over last 10 years

C S S


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Case 2 Failure at NUS Biz School

I filt ti A l i


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Infiltration Analysis0.000 20.000 40.000 60.000 80.000

80.000

100.000

120.000

140.000

Active groundwater head

Phase number: 6 Phase time: 5 day, Extreme groundwater head 126.01 m

m

108.000

109.000

110.000

111.000

112.000

113.000

114.000

115.000

116.000

117.000

118.000

119.000

120.000

121.000

122.000

123.000

124.000

125.000

126.000

127.000

Building No influx

Unsaturated Zone

Near Steady-State condition after 30days of rain at 0.02 mm/day

Nature of Slip is fairly deep seated

Likely cause is combination of rain induced saturation leading to loss ofsuction and rising GWT

Raised GWT

Estimated Slip SurfaceConstantH2

Constant

H1

Closed No Flow


S i f Sl b I fil i


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Saturation of Slope by Infiltration

Takes more than 20 days of rain at 20 mm/day to raisedGWT sufficiently to cause deep seated failure

Initial GWT

Click for Video


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Transient Flow in Slope by Infiltration

Takes more than 20 days of rain at 20 mm/day to raisedGWT sufficiently to cause deep seated failure

Initial GWT

Click for Video

Long term Repair Strategy


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Re-gradeslope

Soil Nails

Better drainsto minimize

infiltration

Long-term Repair Strategy


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Conclusions Why shallow slip occurs is the result of unsaturated soilcreating a buffer with very low permeability thatprevents the saturation front from penetrating into

deeper soils; where GWT are greater than 10m deep Infiltration analysis of the Kranji slope failure illustrates

the saturation of a shallow slip mass for a small slope

With the inclusion of a horizontal deep drains at the toe

of the slope; a large unsaturated soil zone can bemaintained despite sustained heavy rainfall, providing apermanent safe slope over the last 10 years

For the case of NUS large slope failure; it is more likelythat sustained heavy rain during the period of Dec 05 toJan 06 caused the rise of GWT to saturate the slopemass and trigger the deeper seated failure

latsis - rainfall induced landslides - why they occur and some mitigating measures [compatibility...

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