Date post: | 04-Jun-2018 |
Category: |
Documents |
Upload: | melinda-gordon |
View: | 218 times |
Download: | 0 times |
of 49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
1/49
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
2/49
2
Outline
Introduction Climate and slope stability
Soil Strength as function of Infiltration Cases: Kranji Racecourse and NUS
Biz School
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
3/49
3
Introduction
Climate in the tropics
Singapore landslides and rainfall events empirical observations
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
4/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
5/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
6/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
7/49
7
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
8/49
8
Unsaturated soil strength
State Variables defined as:
=
=
)(
)(
wa
an
uu
u Net normal stress
Matric suction
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
9/49
9
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
10/49
10
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
11/49
11
Infiltration Seepage Analysis
Transient Seepage FEM Analysis
Soil Water Characteristic Function
Soil Permeability Function
Infiltration Input Results of Analysis
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
12/49
12
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
13/49
13
Water flow in unsaturated soilWater content and permeability in unsaturated zone is a
function of suction pressure
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
14/49
14
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
15/49
15
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
16/49
16
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
17/49
17
1D Infiltration Video
Click for Video
Input flux= 0.1mm/day
ClosedClosed
Closed
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
18/49
18
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
19/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
20/49
20
Develop tension crack below crest of slope, why?
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
21/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
22/49
22
Infiltration Analysis Rain 20 mm/dayEvolution of Saturation Zone
ConstantH2
Click for Video
ConstantH1
Closed No Flow
Input flux = 20mm/day
Input flux = 20mm/day
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
23/49
23
Shallow Slip of Saturated Zone (Head)
Click for Video
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
24/49
24
Shallow Slip of Saturated Zone (Flow)
Click for Video
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
25/49
25
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
26/49
26
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
Total Pore Pressures [kPa]
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)(' =
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
27/49
27
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
+=
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
28/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
29/49
29
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
30/49
30
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
Input flux = 20mm/day
Input flux = 20mm/day
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
31/49
31
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
32/49
32
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
33/49
33
Infiltration Analysis Rain 20 mm/dayTransient Flow for Slope with 8m Deep Internal Drains
8m-deep pipe drains
Click for Video
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
34/49
34
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
Total Pore Pressures [kPa]
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
35/49
35
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
36/49
36
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
37/49
37
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
38/49
38
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
39/49
39
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
40/49
40
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
41/49
41
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
42/49
42
GW discharge from internal geo-pipe drains has
performed well over last 10 years
C S S
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
43/49
43
Case 2 Failure at NUS Biz School
I filt ti A l i
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
44/49
44
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
Input flux = 20mm/day
S i f Sl b I fil i
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
45/49
45
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
46/49
46
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
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
47/49
47
Re-gradeslope
Soil Nails
Better drainsto minimize
infiltration
Long-term Repair Strategy
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
48/49
8/14/2019 Latsis - Rainfall Induced Landslides - Why They Occur and Some Mitigating Measures [Compatibility Mode]
49/49
49
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