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8/19/2019 CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)
1/51
AP Harry Tan
CE5101 Seepage FEM
SEP 20
CE5101 Lecture 4
Seepage and FEM
by
Prof Harr Tan
1
SEP 2011
Outline
• Seepage and 1D Slope Stability
• Seepage in FEM (Steady State Analysis)
• Case History of SICC Slope Failure
• FEM Seepage in Excavations
• Case History of One North Excavation with
2
GWT lowering• Transient Seepage in Excavations
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AP Harry Tan
CE5101 Seepage FEM
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Seepage Analysis
• Simple Flow nets
• ap ace qua on
p
xk q x x
Darcy’s Law
3
02
2
xk
x
q x
w
Steady State Laplace Eqn
Seepage in Drained Slope Failure
4
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8/19/2019 CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
(c) GWT below Slip Plane with suction
(d) Waterlogged Slope with Steady Parallel Seepage
7
8
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
13
Why do we need a
permeability function?
Can the problem be
14
so ve w ou era ons
8/19/2019 CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
TRANSITION SATURATED/UNSATURATED
r
x xq K k x
r y y y
4
4
1 saturated zone
10 unsaturated zone
4k
r
r
h hr r
K
K
h
15
0.7m (PLAXIS)
k
k
h
h
TYPES OF FLOW PROBLEMS
Confined flow Unconfined flow
16
Domain defined Domain undefined
8/19/2019 CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
Unconfined Flow in Sand
21
Equi-potential Plot of Groundwater
Head
22
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
PLAXIS Results
Dupuit’s Theory = 0.150 m3/day/m
23
Confined Flow Seepage
24
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
Confined Flow Seepage
H=15m H=13m
25
Closed flow boundary
Groundwater
Head
26
= m
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CE5101 Seepage FEM
SEP 20
27
Case History of Slope Failure in Residual Soil
Cut at SICC
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
CIU or CID Test Should Give Same Strength
Parameters
29
Slip in Cut Soil After 2 Years
30
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CE5101 Seepage FEM
SEP 20
Slip in Cut Soil After 2 Years
5 m Ht
10 m Ht
No Failure ?
31
Slip in Cut Soil After 2 Years
32
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
Soil Profile of Cut Slope
33
Stress History of Cut Slope
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CE5101 Seepage FEM
SEP 20
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37Summary of Lab Test Results
SLOPE/W Analysis: FS After CUT
1.714
142
144
146
148
150
Description: Reddish Brown Clayey Silt
Soil Model: Undrained (Phi=0)
Unit Weight: 19
Cohesion: 35
Description: Yellowish Brown Clayey Silt
Soil Model: Mohr-Coulomb
Unit Weight: 20
:
E l e v a
t i o n
( m )
120
122
124
126
128
130
132
134
136
138
38
:
Phi: 34Unit Wt. above WT: 18
Distance (m)
0 10 20 30 40 50 60110
112
114
116118
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CE5101 Seepage FEM
SEP 20
SLOPE/W Analysis: FS After 2 Years
1.022
142
144
146
148
150
Description: Reddish Brown Clayey Silt
Soil Model: Mohr-Coulomb
Unit Weight: 20
Cohesion: 8
Phi: 27
Unit Wt. above WT: 18
Description: Yellowish Brown Clayey Silt
Soil Model: Mohr-Coulomb
i i
E l e v a
t i o n
( m )
122
124
126
128
130
132
134
136
138
39
ni e ig :
Cohesion: 20
Phi: 34
Unit Wt. above WT: 18
Distance (m)
0 10 20 30 40 50 60110
112
114
116
118
120
PLAXIS UnDrained Analysis: FS=1.51
Incremental Displacements Pattern
Soil Unloaded – no sign of failure mechanism
40
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AP Harry Tan
CE5101 Seepage FEM
SEP 20
PLAXIS Drained Analysis: FS=1.02
43
PLAXIS Drained Analysis: FS=1.02
GWT Heads showed seepage front
exiting on slope face; this is bad
situation for slope Phreatic s urface
44
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CE5101 Seepage FEM
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1.5
1.6
FS
Chart 1
5m CUT Draine...
5m CUT Undra...
PLAXIS c/phi method FS Estimation
1.1
1.2
1.3
1.4
m ran...
45
0 1 2 3 4 5
1
Displacement [m]
5m Cut Undrained, FS=1.02
5m CUT Drained, FS=1.51
10m CUT Drained, FS=1.34
PLAXIS Drained 10m CUT
Incremental Displacements Pattern indicate
stable slope – no failure mechanism
46
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CE5101 Seepage FEM
SEP 20
Drained 5m CUT with Internal Drains
GWT drawndown to below slope face, stable
situation
47
1.5
1.6
FS
Chart 1
5mCUT Draine...
5mCUT Undra...
Drained 5m CUT with Internal Drains
1.1
1.2
1.3
1.4
10mCUT Drain...
5mCUT with In...
48
0 1 2 3 4 5
Displacement [m]
GWT drawn down to below slope face, stable
situation, and FS increased to 1.5 cf to 1.02
without internal drains
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CE5101 Seepage FEM
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Ground Water in Excavation
Analysis
49
Effects of GWT on Excavation Analysis
For PLAXIS FEM Program:
• Steady State GWT Calculation i s a separate program from
Excess Pore Pressure and Consolidation Calculation
• This can lead to many different ways to include Effects of GWT
on Excavation Analysis
• The GWT or Phreatic Surface can be determined by either
• Method A – Steady State Flow calculation (Prefered
50
• Method B – User Defined Phreatic Surface, ie head is
constant on a vertical section (to model hydros tatic
pressure on both sides of excavation)
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CE5101 Seepage FEM
SEP 20
Possible GWT
Conditions in
Excavations
wC ab
bau
2
2
51
wG acb
ac
u
2
PLAXIS Model of Full GWTh=Ha (const)
h=Hb(const)
Hb
Ha
52
CLOSED FLOW Boundary
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CE5101 Seepage FEM
SEP 20
One North Excavation in
30m Depth of Jurong Formation•By: A/Prof Harry Tan, National University of
Singapore
•At: ER2010 2‐4 Aug 2010 (Seattle USA)
55
Use of Sub-soil Drains to Lower GWT
for Deep Excavation
56
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CE5101 Seepage FEM
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Full Anchors not possible due to
site access
57
Seepage of GWT through wall
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CE5101 Seepage FEM
SEP 20
GW Seepage by WSP data-
Drained/Undrained Conditions
GW(S)17, 18 & 19
• GWT drawdown lags behind excavation and drains installation by 1-2 weeks
• Steady-state seepage appears to be reached in about 2 weeks
•
105.000
110.000
115.000
120.000
n d W a t e r L e v e l ( m )
GW(S)18
GW(S)19
GW(S)17
8m
16m
90.000
95.000
100.000
1 0 - O c t - 0 3
1 0 - N o v - 0 3
1 0 - D e c - 0 3
1 0 - J a n - 0 4
1 0 - F e b - 0 4
1 0 - M a r - 0 4
1 0 - A p r - 0 4
1 0 - M a y - 0 4
1 0 - J u n - 0 4
1 0 - J u l - 0 4
1 0 - A u g - 0 4
1 0 - S e p - 0 4
1 0 - O c t - 0 4
1 0 - N o v - 0 4
1 0 - D e c - 0 4
1 0 - J a n - 0 5
1 0 - F e b - 0 5
1 0 - M a r - 0 5
1 0 - A p r - 0 5
1 0 - M a y - 0 5
1 0 - J u n - 0 5
1 0 - J u l - 0 5
1 0 - A u g - 0 5
1 0 - S e p - 0 5
1 0 - O c t - 0 5
1 0 - N o v - 0 5
1 0 - D e c - 0 5
1 0 - J a n - 0 6
1 0 - F e b - 0 6
Date
G r o u
• 16-Feb-04 Excavate to RL110.5m and Instal l 1st row Drains atRL112.5m
• 29-Mar-04 Excavate to RL102.5m and Install Drains at RL108.5, 106.5
and 104.5m• 12-Jul-04 Excavate to RL98.0m and Install Drains at RL100.5m, then
Excavate to berm top level at RL96.0m
59
Drained / Undrained Conditions
• undrained analysis
One North - WT7 I19
after cast base slab and remove lowest anchor
0.00
0 20 40 60 80 100 120Wall Deflection (mm)
Section 2 - Stage 8
• – mm
• drained analysis
• – 97 mm
• actual – 85 mm
5.00
10.00
15.00
20.00
D e p t h ( m )
• Drained Analysis
25.00
30.00
35.00
40.00
Drained
Undrained
I19
60
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CE5101 Seepage FEM
SEP 20
2b. Influence of Horizontal
Drainage Systemno drains 4 drains
117.5m
6 drains2 drains
m
108m
100m
65
2c. Influence of HorizontalDrainage System
Influence of Hori zontal Drainage System• Wall deformation
87.5
92.5
97.5
102.5
107.5
112.5
117.5
R e d u c e d L e v e l ( m )
no drains
6 drains
4 drains
2 drains
drains which determine
height of water level
behind the wall
• When no drains
77.5
.
0 100 200 300 400
Deflection (mm)
installed, max. walldeflection is greater
300mm
Collapse of wall66
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CE5101 Seepage FEM
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3a. NO Drains (swi tch off ) - Wall Collapsed
Drains in Active Zone
NOT Activated
um - tage <
Anchor Force = 180 Ton >150 Ton (design)
GWT
Wall deflect > 300 mm
67
3b. WITH Drains (switch on ) – Wall OK
Drains in Active Zone
Act ivated
GWT
M-Stage =1
Anchor Force = 110 Ton
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CE5101 Seepage FEM
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4a. Global FOS by c/phi Reduction
as c,
Failure with no
Plastic Hinge,
FOS=1.75
as o-
Plastic Failure
with Plastic
Hinge, FOS=1.40
• Elastic w all excludes possibility of w all plastic hinge; and over-estimateFOS=1.75
• Allow ing for wall p last ic h inge (Elasto -plast ic w all) gave lower FOS=1.40 and
smaller soi l yielded zone behind the wall 69
4b. Wall is Stable with GWT lowered; but FOS by
c/phi reduction must account for wall plastic moments
70
as c a = .
Plastic DWall FOS=1.40
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Section 1- Stage 3& 4
(after installing / preloading of raker anchor)
102.500
107.500
112.500
117.500
v e l ( m )
Section 1 - Stage 1 & 2
(after installation of CBPwall)
102.500
107.500
112.500
117.500
e l ( m )
r
Section 1- Stage 5
(after excavate to RL102.5m and installation of
first 2 drains)
102.500
107.500
112.500
117.500
( m )
5. Wall Deflection Predictions
77.500
82.500
87.500
92.500
97.500
- 50. 00 0 .0 0 50 .0 0 1 00 .0 0 1 50 .0 0 2 00 .0 0
Deflection (mm)
R e d u c e d L e v
Measured
Calculated
77.500
82.500
87.500
92.500
97.500
- 5 0. 00 0 .0 0 5 0. 00 1 0 0. 00 1 5 0. 00 2 00 . 00
Deflection (mm)
R e d u c e d L e v easure
Calculated
77.500
82.500
87.500
92.500
97.500
0. 00 50 .00 100.00 150.00 200.00
Deflection (mm)
R e d u c e d L e v e l
Measured
Calculated
Section 1- Stage 6
(after excavate to bermtop and installing of last
2drains and anchors)
112.5
117.5
Sec t ion 1 - Stage 7
(af t er c ut berm)
112 .5
117 .5
Section 1- Stage 13 & 14
(after removal of contingencyand raker anchor)
112.5
117.5
77.5
82.5
87.5
92.5
97.5
102.5
107.5
0.00 50.00 100.00 150.00 200.00
Deflection (mm)
R e d u c
e d L e v e l ( m )
Measured
Calculated
77 .5
82 .5
87 .5
92 .5
97 .5
102 .5
107 .5
0.00 50.00 100.00 150 .00 200.00
D ef lect ion (mm)
R e d u c e d L e v e l ( m )
Measured
Calculated
77.5
82.5
87.5
92.5
97.5
102.5
107.5
0 50 100 150 200
Deflection (mm)
R e d
u c e d L e v e l ( m )
Measured
Calculated
71
Seepage and Excavations
• GWT lowering by Steady State Seepage
• GWT lowering by Transient Seepage
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AP Harry Tan
CE5101 Seepage FEM
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GWT lowering SS Seepage
Excavate 5m, k=1e-5 m/s Excavate 10m, k=1e-5 m/s
Lower 1.3mLower 3.0m
73
Excavate 15m, k=1e-5 m/s
Lower 5.6m
s near y
proportional to
excavation depth
GWT lowering SS Seepage
Excavate 15m, k=1e-5 m/s Excavate 15m, k=1e-7 m/s
Lower 5.6mLower 5.6m
For SS case, GWT is not
74
Excavate 15m, k=1e-9 m/s
Lower 5.6m
.Pattern of GW heads is
function of geometry only and
soil layer arrangements
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GWT and Transient Seepage
Excavate 5m, k=1e-5 m/s Excavate 5m, k=1e-7 m/s
Lower 1.3mLower 0.8m
Excavate 5m in 30 days.
75
Excavate 5m, k=1e-9 m/s
Lower 0.3m
Sands, k=1e-5 m/s is like SS
case
Clays, k=1e-9 m/s very little
GWT lowered
GWT and Transient Seepage
Excavate 10m, k=1e-5 m/s Excavate 10m, k=1e-7 m/s
Lower 3.0mLower 1.8m
Excavate next 5m in 30 days.
76
Excavate 10m, k=1e-9 m/s
Lower 0.3m
Sands, k=1e-5 m/s is like SScase
Clays, k=1e-9 m/s very little
GWT lowered
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CE5101 Seepage FEM
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GWT and Transient Seepage
Excavate 5m, k=1e-5 m/s Excavate 5m, k=1e-7 m/s
Lower 5.6mLower 3.6m
Excavate next 5m in 30 days.
77
Excavate 15m, k=1e-9 m/s
Lower 0.3m
Sands, k=1e-5 m/s is like SS
case
Clays, k=1e-9 m/s very little
GWT lowered
Science of Transient Seepage
• Governing Equations
• Hydraulic Material Models
• Boundary Conditions
78
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Governing Equations
Steady-state
continuity condition
79
Governing Equations
80
• Need to define two soil properties functions:
• K as f(S) and Ksat - k function
• c as f(csat, n, S(p)) - SWCC
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CE5101 Seepage FEM
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Governing Equations (FEM)
at element by element level
81
Governing Equations (FEM)
82
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CE5101 Seepage FEM
SEP 20
Hydraulic Material Model-
Van Genuchten Model
83
Hydraulic Material Model-
Van Genuchten Model
84
• AEV defines the suction value that must be exceeded before air enters the soil pore
• Clays have very high AEV compared to Sands
• ga is inversely related to AEV
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SEP 20
Hydraulic Material Model-
Van Genuchten Model
85
Hydraulic Material Model-
Van Genuchten Model
86
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Hydraulic Material Model-
Van Genuchten Model
87
Hydraulic Material Model-
Van Genuchten Model
88
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1. Water Table
P h
Boundary Conditions
1
1
w
w
Ph y
w p w
2. Inflow
external x x y yq n q n q 4. Close boundary
2 3
4
89
3. Outflow
external x x y yq n q n q
0 x x y yq n q n
5. Prescribed heads
1 2,h h h h
Boundary Conditions
6. Well/Drain
7. Free Seepage
Q Q
5
6
78
90
8. Screen
0 x x y yq n q n
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CE5101 Seepage FEM
SEP 20
Rapid Drawdown Example – Time Dependent Boundary Conditions
rom = m o = m n ays
H=25m
H=5m
95
Rapid Drawdown Example – Time Dependent Boundary Conditions
rom = m o = m n ays
H=25m
H=5m
96
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CE5101 Seepage FEM
SEP 20
99
100
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