CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)

8/19/2019 CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)

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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

SEP 20

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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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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SEP 20

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10


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CE5101 Seepage FEM

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13

Why do we need a

permeability function?

Can the problem be

14

so ve w ou era ons


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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


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CE5101 Seepage FEM

SEP 20

Unconfined Flow in Sand

21

Equi-potential Plot of Groundwater

Head

22


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CE5101 Seepage FEM

SEP 20

PLAXIS Results

Dupuit’s Theory = 0.150 m3/day/m

23

Confined Flow Seepage

24


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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

28


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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


5 m Ht

10 m Ht

No Failure ?

31


32


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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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CE5101 Seepage FEM

SEP 20

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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AP Harry Tan

CE5101 Seepage FEM

SEP 20

SLOPE/W Analysis: FS After 2 Years

1.022

142

144

146

148

150

Description: Reddish Brown Clayey Silt


Unit Weight: 20

Cohesion: 8

Phi: 27

Unit Wt. above WT: 18

Description: Yellowish Brown Clayey Silt


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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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

SEP 20

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

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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

SEP 20

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

SEP 20

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

SEP 20

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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AP Harry Tan

CE5101 Seepage FEM

SEP 20

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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CE5101 Seepage FEM

SEP 20

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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CE5101 Seepage FEM

SEP 20

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



For SS case, GWT is not

74


Lower 5.6m

.Pattern of GW heads is

function of geometry only and

soil layer arrangements


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CE5101 Seepage FEM

SEP 20

GWT and Transient Seepage



Excavate 5m in 30 days.

75


Lower 0.3m

Sands, k=1e-5 m/s is like SS

case

Clays, k=1e-9 m/s very little

GWT lowered




Excavate next 5m in 30 days.

76


Lower 0.3m

Sands, k=1e-5 m/s is like SScase


GWT lowered


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CE5101 Seepage FEM

SEP 20




Excavate next 5m in 30 days.

77


Lower 0.3m

Sands, k=1e-5 m/s is like SS

case


GWT lowered

Science of Transient Seepage

• Governing Equations

• Hydraulic Material Models

• Boundary Conditions

78


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CE5101 Seepage FEM

SEP 20

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

SEP 20

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


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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CE5101 Seepage FEM

SEP 20


Van Genuchten Model

85


Van Genuchten Model

86


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CE5101 Seepage FEM

SEP 20


Van Genuchten Model

87


Van Genuchten Model

88


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AP Harry Tan

CE5101 Seepage FEM

SEP 20

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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AP Harry Tan

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

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Documents

CE5101 Lecture 4 - Seepage Analysis by FEM (SEP 2011)