2d and 3d consolidation - biot theory (oct2011)
TRANSCRIPT
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
1/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
CE 5101 Lecture 7 2D and
OCT 2011
1
Prof Harry Tan
Outline Biot Theory (2D and 3D Coupled
Consolidation)
FEM compare with Schiffman et. al. 1967(Mandel-Cryer Effect in 2D)
Undrained, Consolidation and DrainedResponse
2
Compare CRISP with Plaxis
Case 2 Barcelona Breakwater
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
2/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Biot Theory (2D and 3D Coupled
Consolidation)
Equilibrium Equations
Compatibility Equations
Strain/Displacements
Constitutive Equations
Continuity Equations
3
Boundary Conditions
Assume infinitesimal strain conditions
Equilibrium Equations
4
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
3/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
5
6
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
4/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
7
8
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
5/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
9
10
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
6/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Cryer-Mandel Problem
11
Comments on True 3D
Consolidation
True 3D consolidation couples total stresses and
-
Psuedo 3D uncouples these two phenomena
When total stress distribution is constant at all
time, the rate of change of excess PP = rate of
change of volume at all points in the soil
12
s s rue on y or conso a on w erethere is a direct relationship between excess PP
and volume change
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
7/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Schiffman Strip Footing 1967
Plane-strain consolidation in 2D
resses rom e as c eory areindependent of elastic constants
Total stresses are same at start and endof consolidation
However they vary with time; they exceed
13
initial value during consolidation
So excess PP first increases before itstarts to dissipate with consolidation
FEM compare with Schiffman et. al.
1967 (Mandel-Cryer Effect in 2D)
Mean total st ress
Excess pore pressures
14
Mean effective stress
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
8/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Plaxis Biot ConsolidationSchiffman, Chen and Jordan 1967
ExcPP at x/a=1
15Strip Footing on Elastic Half-space
All round closed boundary
condition
0.6
Stress [kN/m2]
Stress at x/a=1
Mean Tot a lSt re ss
Mean Effective Stress
Plaxis Biot ConsolidationSchiffman, Chen and Jordan 1967
Mean total stress
0.2
0.3
0.4
0.5
Excess Pore Pressure
Mean excess PP
Input:
k=0.001 m/day
Eoed=10000 kPa
= *
16
1e -3 1e -2 0.1 1 10 100 1e30
0.1
Time [day]
Strip Footing on Elastic Half-space
Mean effective stress
w
cv=1 m2/day
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
9/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Plaxis Biot ConsolidationRamp Loading Schiffman 1960
1
Excess PP[kN/m2]
Chart 1
Ver 8.2
0.2
0.4
0.6
0.8Ver 7.2
17
1e-5 1e-4 1e-3 1e-2 0.1 1 10
0
Time [day]
ExcPP at Base Closed consolidation boundary
Ramp Loading with To=0.1
Plaxis Biot ConsolidationRamp Loading Schiffman 1960
0
Displacement [m]
Chart 2
Ver 8.2
-6e-3
-4e-3
-2e-3er .
18
Settlement at Top Closed consolidation boundary
Ramp Loading with To=0.1
1e-5 1e-4 1e-3 1e-2 0.1 1 10
-0.01
- e-
Time [day]
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
10/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Plaxis Biot ConsolidationRamp Loading Schiffman 1960
1
Excess PP[kN/m2]
At Base
Point BTo
0.2
0.4
0.6
0.8
To=0.1 To=0.5
19
Excess PP at Base Closed consolidation boundary
Ramp Loading with To=0.1 and 0.5
1e-5 1e-4 1e-3 1e-2 0.1 1 10
0
Time [day]
TYPES OF ANALYSIS Drained
Loadin /Construction/ excavation: ver slow in relation to the
soil permeability)
Undrained
Loading/Construction/ excavation: very fast (in relation to the
soil permeability)
Intermediate cases: consolidation analysis
20
o mec an ca an y rau c ow pro ems n erac More complex computations: coupled analysis
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
11/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Undrained, Consolidation and
Drained Response Defini tion drained / undrained
In terms of effective stresses with drained strengthparameters
In terms of effective stresses with undrained strengthparameters
In terms of total stresses with undrained strengthparameters
21
Example of Clay Embankment
Summary
DRAINED / UNDRAINED Drained analysis appropriate when
permeability is high
rate of loading is low
short term behaviour is not of interest for problem considered
Undrained analysis appropriate when
permeability is low and rate of loading is high
short term behaviour has to be assessed
Suggestion by Vermeer & Meier (1998) for deep excavations:
T < 0.1 (U=35%) use undrained condi tions
T > 0.40 (U=70%) use drained condit ions
22
tD
EkT
2
w
oed
k = permeability
Eoed = oedometer modulus
w = unit weight of water
D = drainage length
t = construction time
Tv = dimensionless time factor
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
12/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
UNDRAINED BEHAVIOUR WITH PLAXIS
1G2EK
PLAXIS automatically adds stiffness of water when undrainedmaterial type is chosen using the following approximation
uu
total213213n
'1213
1'EK
u
utotal
assuming u = 0.495
23
- this procedure gives reasonable B-values only for < 0.35 !
- real value of Kw
/n ~ 1.106 kPa (for = 0.5)
- NB: in Version 8, B-value can be entered explicitly
UNDRAINED BEHAVIOUR WITH
PLAXISExample 1:
E = 3 000 kPa, = 0.3, u = 0.495
K = 2 500 kPa, Ktotal = 115 000 kPa Kw/n = 112 500 kPa
with = 0.978 is reasonable value for saturated so il
Example 2:
wK
nKB
'1
1
24
E = 3 000 kPa, = 0.45, u = 0.495
K = 10 000 kPa, K total = 103 103 kPa Kw/n = 93 103 kPa
B = 0.903 is poor value for saturated soil
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
13/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
UNDRAINED BEHAVIOUR WITH
PLAXIS
Method A (analysis in terms of effective stresses):type of material behaviour: undrained
effective strength parameters c, ,
e ec ve s ness parame ers 50 ,
Method B (analysis in terms of effective stresses):type of material behaviour: undrained
undrained strength parameters c = cu, = 0, = 0
effective stiffness parameters E50,
25
Method C (analysis in terms of total stresses):type of material behaviour: drained
total strength parameters c = cu, = 0, = 0
undrained stiffness parameters Eu, u = 0.495
Notes on dif ferent methods:
Method A:
recommended
soil behaviour is always governed by effective stresses
increase of shear strength during consolidation included
essential for exploiting features of advanced models such as the
Hardening Soil model, the Soft Soil model and the Soft Soil
Creep model
Method B:
only when no information on effective strength parameters is
available
cannot be used with the Soft Soil model and the Soft Soil Creep
26
mo e
Method C:
NOT recommended
no information on excess pore pressure distribution (total stress
analysis)
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
14/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Consider fully undrained isotrop ic elastic behaviour
(Mohr Coulomb in elastic range)
pw = p > p = 0
'cos'c'sin2
1c
o'
y
o'
xu
27
Fig.6 Mohr Circle for evaluating undrained shear strength (plane strain)
28
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
15/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Undrained strengths of MC vs Real
NC Soils
29
Factor of Safety of Cuts/ExcavationsCritical FS is Long-
term unloading
condition,
For permanent cuts
drained strength is
key parameter for
safe design
For temporary cuts,
need to consider if
30
undrained or partiallydrained condition
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
16/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Factor of Safety of Embankments
Critical FS is Short-term loading condition,
key parameter for safe
design
31
Example of Underwater CUT Slope
(Unloading Problem)LIMIT EQUILIBRIUM ANALYSIS OF CUT SLOPES
The figures below show the results of SLOPE/W calculations
of FS for a underwater cut slope in the undrained and
', .
Drained and Undrained Parameters
The drained parameters are c'=2 kPa, '=240, =16 kN/m3
The equivalent undrained parameters are obtained from:
kPa83.124cos2cclay;oftopAt
'sincosoc'c
0u
,mu
32
kPa/m1.9424sin4.77'sin
kPa/m4.770.5916/2K12
.s n-s n
0,m
0
,v,
m
0
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
17/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Bishops FS for Drained CUTCut Slope in Clay (Drained)
1.403
Description: Clay Water
Water Level
tion(m)
14
16
18
20
22
24
26
33
o o e : o r- ou om
Unit Weight: 16
Cohesion: 2
Phi: 24
1:2 Cut
Distance (m)
0 5 10 15 20 25 30 35 40 45 50 55 60
Elev
0
24
6
8
10
Bishops FS for UnDrained CUTCut Slope in Clay (UnDrained)
2.085
Water
Water Level
ion(m)
14
16
18
20
22
24
26
34
Description: ClaySoil Model: S=f(datum)
Unit Weight: 16
C - Datum: 1.83
Rate of Increase: 1.94
Datum (elevation): 20
1:2 Cut
Distance (m)
0 5 10 15 20 25 30 35 40 45 50 55 60
Eleva
0
2
4
6
8
10
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
18/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
PLAXIS Analysis Cases
Drained Analysis with c=2 kPa and =24o
Method A (analysis in terms of effective stresses):
effective strength parameters c, ,
effective stiffness parameters E50,
Method B (analysis in terms of effective stresses):
type of material behaviour: undrained
undrained strength parameters c = cu, = 0, = 0
effective stiffness arameters E50
35
Method C (analysis in terms of total stresses):
type of material behaviour: drainedtotal strength parameters c = cu, = 0, = 0
undrained stiffness parameters Eu, u = 0.495
Drained CUT, Plaxis FS=1.39 cf LE=1.40Drained Analysis with
Effective strength parameters c=2 kPa, =24, =0
Effective stiffness arameters E50=15000 kPa, =0.2
36
ONLY ONE POSSIBLE METHOD IN DRAINED
ANALYSIS ie Drained Strength and Stiffness parameters
Solutions by FEM and LE will agree very well
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
19/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Method A - UnDrained CUT plus Full Consolidation
Plaxis c/phi FS=1.37 cf LEM=1.40
Method A (undrained)
Effective strength parameters c=2 kPa, =24o, =0o
50 , .
Slip circle same as Drained Case
37
Method A - UnDrained CUT,
Plaxis FS=2.26 cf LE=2.09
Method A (in terms of effective stresses, undrained)
Effective strength parameters c=2 kPa, =24o, =0o
, .
38
Deeper slip surface than Drained Case
FEM (A) and LE not identical because undrained strength
profile in the two cases are slightly different
But slip surface of FEM (A) and LE are nearly identical
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
20/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Method B - UnDrained CUT,
Plaxis FS=2.13 cf LE=2.09
Method B (in terms of effective stresses, undrained)
Undrained strength parameters c=1.83 kPa, c=1.94 kPa,
=0 =0,
Effective stiffness parameters E50=15000 kPa, =0.2
39
Deeper slip surface than Drained Case FEM (A and B) and LE not identical because undrained
strength profile in all three cases are slightly different
But slip surface of FEM (A and B) and LE are nearly identical
Method C - UnDrained CUT,
Plaxis FS=2.14 cf LE=2.09
Method C (in terms of total stresses, Drained)
Total strength parameters c=1.83 kPa, c=1.94 kPa,
= =,
Undrained stiffness parameters E50=18625 kPa, =0.49
40
Deeper slip surface than Drained Case
FEM (A,B,C) and LE not identical because undrained strength
profile in all 4 cases are slightly different; but B and C are
nearly identical
But slip surface of FEM (A,B,C) and LE are nearly identical
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
21/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
SUMMARY OF FS FOR CUT SLOPES
Analysis Condition PLAXIS SLOPE/W
. .
A+Consolidation 1.37 1.40
Undrained (A) 2.26 2.09
Undrained (B) 2.13 2.09
Undrained C 2.14 2.09
41
Only Drained analysis is FEM and LE identical
In Undrained analysis there are differences in strength profiles
Undrained plus Consolidation is close to Drained Case
Embankment Undrained Analysis
(Loading Problem)Embankment on Clay(Total Undrained Condition) Slope/W FS=1.029
1.029
Description: fill
Soil Model: Mohr-Coulomb
Unit Weight: 20
Cohesion: 0
Phi: 33
Piezometric Line #: 1
Description: Clay
Soil Model: S=f(datum)
Unit Weight: 16
C - Datum: 1.83
Rate of Increase: 1.94
Water Table
Height(m)
-8
-6
-4
-2
0
2
4
6
8
10
12
e o
FS=1.019
42
Piezometric Line #: 1
Distance (m)
0 10 20 30 40-10
-
For the same Undrained Strength profile,
Slip surface in FEM and LE are nearly identical
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
22/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Embankment Drained Analysis
2.592
Embankment on Clay(Drained Condition) Drained FS = 2.52
Description: Clay
Soil Model: Mohr-Coulomb
Unit Weight: 16
Cohesion: 2
Phi: 24
Piezometric Line #: 1
Description: fill
Soil Model: Mohr-Coulomb
Unit Weight: 20
Cohesion: 0
Phi: 33
Piezometric Line #: 1 Water Level
0 10 20 30 40-10
-8
-6
-4
-2
0
2
4
6
8
10
Undrained Method A+
Consolidation FS=2.11
43
Drained Analysis: FEM and LE nearly
identical results
Consolidation Analysis is not the
exactly same as Drained response
SUMMARY Undrained analysis shou ld be performed in effective stresses
and with effective stiffness and s trength parameters
Undrained Analysis wi th Full Consolidation may not agree with
Drained Analysis due to di fferent end state stress states
Note that for NC-soils in general
factor of safety against failure is lower for short term
(undrained) conditions for loading problems (e.g.
embankments)
44
factor of safety against failure is lower for long term (drained)
conditi ons for un loading prob lems (e.g. excavations)
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
23/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Case 1 - WCRS Excavation
Example of Deep Excavation
45
1D Closed Box Swelling
46
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
24/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Effects of Permeability
Heave at A (0.5m below Fmn Level)
0.03
Heave at A [m]
Uy-A(0.5...
k=1e-9
0.01
0.015
0.02
0.025UND
DRN
k=1e-7
k=1e-8
47
0 50 100 150 200
-5e-3
0
5e-3
Time [day]
Effects of Permeability
Exc PP at D (1.5m below Fmn Level)
250
Excess PP at D(1.5m) [kN/m2]
EPP-D(1...
=
100
150
200
-
UND
DRN
k=1e-7
k=1e-8
48
0 50 100 150 200
0
50
Time [day]
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
25/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Effects of Permeability
Exc PP at E (6.1m below Fmn Level)
250
Excess PP at E(6.1m) [kN/m2]
EPP-E(6...
100
150
200
k=1e-9
UND
DRN
k=1e-7
k=1e-8
49
0 50 100 150 200
0
50
Time [day]
Excavate to Formation Levelk=1e-7 m/s k=1e-8 m/s k=1e-9 m/s
50
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
26/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
WCRS CPG Design Mesh
Compare Drained and Undrained Case
Cases at k=1e-7m/s, 1e-8m/s, 1e-9m/s
51
WCS Soil Undrained Strength Profile in Plaxis
95
100
105
Cu=Method B or C
Cu-Method A
70
75
80
85
90
Depth(m)
52
60
65
0 50 100 150 200 250 300 350
Cu (kPa)
'sin)1(2
1'cos' '
vKocCu Method A, Cu is:
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
27/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Drained and Undrained (Method A)
Displacements at Formation Level
Undrained Drained
53
Drained and Undrained
BMs at Formation Level
Undrained Drained
54
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
28/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Consolidation Analysis
assume: k=1e-7, 1e-8 and 1e-9 m/s
55
Cases of k=1e-7 to 1e-9 m/s
Displacements at Formation Level
56k = 1e-7 m/s k = 1e-8 m/s k = 1e-9 m/s
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
29/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
2
Cases of k=1e-7 to 1e-9 m/s
BMs at Formation Level
57k = 1e-7 m/s k = 1e-8 m/s k = 1e-9 m/s
Cases of k=1e-7 to 1e-9 m/s
Excess PP at Formation Level
58k = 1e-7 m/s k = 1e-8 m/s k = 1e-9 m/s
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
30/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
Wall Deflection at B (15/83.85 1.65m above FL)
0.2
Ux at B [m]
Ux at B
DRN
0.1
0.15
UND
k=1e-7
k=1e-8
k=1e-9
59
0 50 100 150 200 250
0
0.05
Time [day]
Heave at C(0/78.7 3.5m below FL)
0.03
0.035
Heave at C [m]
Uy at C
DRN
5e-3
0.01
0.015
0.02
0.025
.
UND
k=1e-7
k=1e-8
k=1e-9
60
0 30 60 90 120
-5e-3
0
Time [day]
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
31/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
One North Station
xcava on
next to INSEAD
Is it Drained or Undrained?
61
Depth Vs. Log (Permeability, m/s) (One North and Fusionpolis Site)
0
- 1. 00 E+ 01 - 9. 00 E+ 00 - 8. 00 E+0 0 - 7. 00 E+0 0 - 6. 00 E+0 0 - 5. 00 E+ 00 - 4. 00 E+0 0 - 3. 00 E+0 0 - 2. 00 E+0 0 - 1. 00 E+ 00 0 .0 0E+0 0
Log (Permeability, m/s)
Field Permeability Tests Data from One North/Fusionpolis Site
(Jurong Formation Residual Soils)
5
10
15
20Depth,m
62
25
30
35
Single Packer Test
Falling Head Test
Variable HeadPermeability Test
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
32/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
WT-7 next to INSEAD D1.8m CBP Wall with 30m deep cut,
20m soils, 10m rock excavation
Sandy Clay G/Sa/SC=0/25/75
Sandy Silt G/Sa/SC=0/32/68
Sandy Clayey Silt G/Sa/SC=1/17/82
63
One North - WT7 I19
after cast base slab and remove lowest anchor
0.00
5.00
0 20 40 60 80 100 120
Wall Deflection (mm) Wall Response is much closer toDrained Behavior at One North
Wall Type 7
10.00
15.00
20.00
25.00
Depth(m)
Drained
64
30.00
35.00
40.00
Undrained
I19
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
33/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
CONCLUSIONSFrom the above study, the following conclusions can be made:
The site at WCS showed fairly consistent thick layers of sandy soils withGSD consisting of more than 60% sands and gravels.
Limited field permeability tests showed k-values in these soils ranging from1e-5 to 1e-7 m/s. .
These soils are likely to have k-values > 1e-6 m/s, therefore, they should bemodeled as drained materials.
The consolidation parametric studies showed that with soils of stiffnessesgreater than 20,000 kPa, k=1e-7 m/s will result in drained response overany reasonable construction period of 1.5 to 2 years.
Experience from the recent One North excavation supports this observation.
Undrained analysis cannot apply to this site.
Consolidation analysis must be done very carefully to reflect the true
65
stiffnesses and permeabilities of the site soils, and this will show resultsvery close to fully drained behavior.
It is more prudent to design the excavation system using fully drainedanalysis for this site
Compare CRISP with Plaxis 1D
swelling box experiment
Excavate 2m in 30 days
Excavate 3.5m in 30 days
Excavate 4m in 30 days
Excavate 3.5m in 30 days
Excavate 3.5m in 30 days
Excavate 3.5m in 30 days
Set PP=0 at
each top level
66
Track PP at 1m and 6mbelow last excavated
level
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
34/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
Swell at 1m below FML
67
Swell at 6m below FML
68
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
35/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
PP in CRISP and Plaxis
CRISP and Abacus are formulated in terms ofactive total ore ressures i.e.
U = Uss + Uexcess
Plaxis is formulated in terms of Excess PorePressures: U = Uexcess
Uexcess is produced by Undrained loading orunloading of soil clusters specified asUndrained type
69
Steady PP is obtained from phreatic GWT orSeepage computation by Plaxis or Plaxflowprogram
Active PP = Uss + Uexcess
PP in CRISP and Plaxis In Sage Crisp, the EXCESS pore pressure always refer to the
original in-situ definition of PWP, regardless of changes of. ,
EXCESS pore pressure in typ ical soil mechanics sense.Rather, it is a reflection of the incremental variation of PWP inreference to the original in-situ PWP.
While in Plaxis, the EXCESS is exactly the same definition ofconventional definition in typ ical soil mechanics sense with the EXCESS PWP referring to the PWP in excess of the phreaticline.
Thus, there is some subtle dif ference between the two
70
compared directly. As such, the total PWP at a same point wascompared instead which expected to give roughly the samevalues, and it is a ind icator of variation o f PWP during variousstages of constructions and accompanying consolidations.
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
36/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
Active (Total) PP at 1m below FML
71
Active (Total) PP at 6m below FML
72
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
37/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
Case 2 - Barcelona Breakwater
73
Barcelona breakwater
y
Caisson
Rubble
x
A
A
0 1
23 45 6
78
9 10
11
12 13
Soft silty clay
20m
50m
74
Gravel
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
38/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
Barcelona breakwater: stages (1)
re g ng
Bench
construction
75
Consolidation
Barcelona breakwater stages (2)
Caisson
Consolidation
76
Storm loading
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
39/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
3
Initial pore pressures
Active pore pressures
77Groundwater head
Pore pressures after placing the bench
Active pore pressures
78Excess pore pressures
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
40/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
4
Excess pore pressures during consolidation
Initial
After 30 days
79
Final
Displacements during construction and consolidation
Bench construction
80
Consolidation
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
41/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
4
Caisson construction
Displacements
81Incremental shear strains
Excess pore pressures during consolidation (caisson)
Initial
After 30 days
82
Final
-
7/26/2019 2D and 3D Consolidation - Biot Theory (OCT2011)
42/42
CE5101 Seepage and Consolidation
Lecture 7- 2D and 3D Consolidation
Prof Harry Ta
OCT 20
Failure (factor of safety)
Incremental displacements
Factors of safety
After construction
83Incremental shear strains
FS=1.06
After 30 days
FS=1.60
End of consolidation
F=1.74