Managing Geotechnical Risk
Learning from the Failures
“Issues related to the use of Numerical
Modelling in Design of Deep Excavations in
Soft Clay”
Andy Pickles
of
GCG (Asia) Ltd.
Asia
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Content of Presentation

Describe the Method A/B Problem

Comment on Cam Clay model in routine design

Highlight Difficulty of modelling piles in 2D Analyses

Comments on modelling of JGP
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2
Simplified Soil Behaviour

Most engineers are familiar with E and υ

Preferable to adopt Shear Modulus (G) and Bulk Modulus (K)

Shear strains due to changes in shear stress are proportional to 1/G

Volume strains due to changes in mean stress are proportional to 1/K

Water has zero G and very high Kw

For drained and undrained conditions G is the same

For drained conditions K is K for soil

For undrained conditions K becomes very high (i.e. is Kw)
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Mohr Coulomb Model and Method A/B

Most analyses adopt simple Mohr Coulomb model with no dilation

For undrained condition no volume change

Soil particles are only affected by changes in effective stress

No volume change means no change in mean effective stress (p’) in soil

Soil is constrained to constant p’ stress path

Soil will fail where constant p’ crosses failure line

Method A/B refers only to choice of strength criteria in undrained analyses
using Mohr Coulomb model

Method A uses c φand Method B uses Cu
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Normally Consolidated
Clay Undrained Loading
Method A
C, phi
Method B
Cu
Cam Clay
Soil is contractive
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FE Model Constant p’
Zero dilatancy
5
Over-consolidated Clay
Ko Consolidated Clay
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Method A at Nicoll Highway M3 Section
●
Method A/B problem is not unique to Plaxis
●
Method A was in widespread use in Singapore
(and is widely adopted internationally)
●
Method A was adopted for design of C824
●
Method A (and other methods) should be
compared with design Cu profile
●
Excavations at C824 were deepest ever in
Singapore
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Nicoll Highway M3 Design Section
Soft Clay 40 m
MC
Upper
MC
Lower
EC
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Effect of Method A on Cu Profile
Undrained Strength, c u (kN/m2)
0
20
40
60
80
100
120
140
0
5
Depth Below Ground Level (m)
10
Method A, Ko = 1
15
20
Method A, Ko = 0.6
25
30
35
40
Design Cu
Profile
45
50
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Method A on Net Pressure Profile Excavation for 6th Strut
Net Pressure on Wall (kN/m2)
-50
0
50
14
5th
Strut
100
150
200
250
Excavation Level
16
Depth Below Ground Level (m)
18
20
Method A
Ko = 0.6
Net Pressure
+ve
Pa > Pp
15m
Span
22
24
Design Cu
Profile
26
28
30
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Upper JGP Layer
10
Effect of Method A on Wall Displacement
Method B
105
105
100
100
95
95
90
90
85
85
RL (m)
RL (m)
Method A
80
80
75
75
70
70
65
65
60
60
55
-0.05
55
-0.05 0.00
0.00
0.05
0.10
0.15
Wall Disp. (m)
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0.20
0.25
0.30
0.05
0.10
0.15
0.20
0.25
0.30
Wall Disp. (m)
11
Effect of Method A on Bending Moments
Method B
105
100
100
95
95
90
90
85
85
RL (m)
105
80
80
75
75
70
70
65
65
60
60
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4000
3000
2000
1000
0
-1000
-2000
4000
3000
2000
1000
0
-1000
-2000
Bending Moment (kNm/m)
-3000
55
55
-3000
RL (m)
Method A
Bending Moment (kNm/m)
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Effect of Method A on Strut Loads
Strut Row
Predicted Strut Load
Using Method B
Design Strut Load
Using Method A
Ratio Method B to
Design Strut Load
1
379
568
67%
2
991
1018
97%
3
1615
1816
89%
4
1606
1635
98%
5
1446
1458
99%
6
1418
1322
107%
7
1581
2130
74%
8
1578
2632
60%
9
2383
2173
110%
Design Strut Load may be controlled by backfilling process
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Mohr Coulomb and Cam Clay Type Models
●
For deep excavations Method A can under-estimate wall
displacement and BM
●
For shallow excavations Method A will over-estimate wall
displacement and BM
●
Method B matches the design undrained strength profile
and is preferable
●
Neither Method A or B model the real behaviour of soft
clay
●
Post collapse recommendation to use Cam Clay type
models
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Idealised behaviour of soil using Cam Clay type models
Cam Clay
or real Soil
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FE Model Constant p’
15
Actual behaviour of Singapore Marine Clay
●
Real behaviour of Marine Clay determined from
high quality lab tests
●
Sampling carried out using thin wall with 5
degree cutting angle
●
Samples anisotropically re-consolidated to in
situ stresses prior to testing
●
Testing carried out undrained in extension and
compression
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Real Behaviour
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Parameters for Upper Marine Clay
Cu Peak
68 kPa
φ at Peak
undrained
Cu Large
Strain
52 kPa
φLarge Strain
34º
% Change
25%
reduction
% Change
35% Increase
25º
Design φ adopted in Singapore is 22º (NSF calcs?)
To obtain correct design Cu profile with modified Cam Clay
model, φ = 17º is required
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Mohr Coulomb v Modified Cam Clay

Modified Cam Clay model includes features of soft clay
behaviour

Some natural soft clays differ from Modified Cam Clay

Physically unrealistic values may be required to match
undrained strength profile

For managing risk care must be taken to understand
implication of differences

Possibly simpler to adopt Mohr Coulomb with Method B
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Modelling Piles in 2 D Analyses
●
Structures constructed in deep excavations in Singapore are often
founded above soft clay on piles
●
Piles are often constructed after installation of JGP layers but before
commencement of excavation
●
Piles will be bonded to the JGP
●
Heave of ground during excavation results in tension in piles
●
Presence of piles will restrain heave and also restrict wall
movements
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Comments on modelling of Piles
●
Modelling piles in 2D analyses as walls connected to the
ground can severely restrict the predicted wall movement
●
Wall displacements will be under-predicted and wall
bending moments also under-predicted
●
If 3D modelling is not available then it may be preferable
to carry out sensitivity studies without piles and with
piles modelled as “anchors” not connected to the soil
mesh
●
For managing risk you must understand the limitations
implicit in simple 2D models – sensitivity analyses
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Modelling JGP

Numerical models for design typically adopt Mohr
Coulomb type model

E = 150MPa, Cu = 300kPa (minimum UCS is 900kPa)

JGP strength is a factored value used in analyses where
soil strength is unfactored

How are design values justified?
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4
Design
3
900kPa
Minimum compliant value
USC Results
E50 from UCS Tests
2
Back-analysed
Eh=65MPa
Average
500 MPa
Average
2000kPa
Derived from
shear waves
Eh=81MPa
Number of results
Number of results
Design Value 150 MPa
2
Data from cores
1
1
0
0
0
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0.4
0.8 1.2 1.6
2
2.4 2.8 3.2
Unconfined compression strength (MPa)
3.6
4
0
100 200 300 400 500 600 700 800 900 1000 11001200
Ev (MPa)
23
Axial strain at failure in UCS tests on JGP
Average 0.8%
10
Number of results
8
6
4
2
0
0
Asia
0.2
0.4
0.6
0.8
1
1.2
1.4
Axial strain at failure, af (%)
1.6
1.8
2
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Summary of JGP Properties
Model
Cu
E
Fail Strain
%
Laboratory
UCS
>1000
500
0.8
Design
M-C
300
150
Back
Analyzed
Real
500
70*1
>2
Advanced
Analysis
Brittle?
500*2/ 200
80
2*2
*1 – Non linear response
*2 – Peak to residual at 20% plastic strain
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Modelling of JGP

Actual mass characteristics of JGP not well understood

No direct relationship between lab and field performance

Parameters and model presently used for design are
probably incorrect and may be unsafe

JGP is probably a brittle material whereas Mohr Coulomb
is elastic/perfect plastic

Sensitivity analyses with high and low strength and
stiffness values are essential
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Concluding Remarks

Numerical modelling has an important role in design

Numerical modelling requires specialist knowledge

For managing risk make sure that the limitations of the
model are well understood (investigated)

Do not rely on preciseness of results

Sensitivity/ trends in behaviour more important

Always perform sanity checks by alternative means
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End of Presentation
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