Method AnB Plaxis

1Asia

Managing Geotechnical RiskManaging Geotechnical RiskLearning from the FailuresLearning from the Failures

““Issues related to the use of Numerical Issues related to the use of Numerical Modelling in Design of Deep Excavations in Modelling in Design of Deep Excavations in

Soft Clay”Soft Clay”

Andy PicklesAndy Picklesof of

GCG (Asia) Ltd.GCG (Asia) Ltd.

2Asia

Content of PresentationContent of Presentation

Describe the Method A/B ProblemDescribe the Method A/B Problem

Comment on Cam Clay model in routine designComment on Cam Clay model in routine design

Highlight Difficulty of modelling piles in 2D AnalysesHighlight Difficulty of modelling piles in 2D Analyses

Comments on modelling of JGPComments on modelling of JGP

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Simplified Soil BehaviourSimplified Soil Behaviour

Most engineers are familiar with E and Most engineers are familiar with E and υυ

Preferable to adopt Shear Modulus (G) and Bulk Modulus (K)Preferable to adopt Shear Modulus (G) and Bulk Modulus (K)

Shear strains due to changes in shear stress are proportional to 1/GShear strains due to changes in shear stress are proportional to 1/G

Volume strains due to changes in mean stress are proportional to 1/KVolume strains due to changes in mean stress are proportional to 1/K

Water has zero G and very high KWater has zero G and very high Kw

For drained and undrained conditions G is the sameFor drained and undrained conditions G is the same

For drained conditions K is K for soilFor drained conditions K is K for soil

For undrained conditions K becomes very high (i.e. is KFor undrained conditions K becomes very high (i.e. is Kw))

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Mohr Coulomb Model and Method A/BMohr Coulomb Model and Method A/B

Most analyses adopt simple Mohr Coulomb model with no dilationMost analyses adopt simple Mohr Coulomb model with no dilation

For undrained condition no volume changeFor undrained condition no volume change

Soil particles are only affected by changes in effective stressSoil particles are only affected by changes in effective stress

No volume change means no change in mean effective stress (p’) in soilNo volume change means no change in mean effective stress (p’) in soil

Soil is constrained to constant p’ stress pathSoil is constrained to constant p’ stress path

Soil will fail where constant p’ crosses failure lineSoil will fail where constant p’ crosses failure line

Method A/B refers only to choice of strength criteria in undrained analyses Method A/B refers only to choice of strength criteria in undrained analyses using Mohr Coulomb modelusing Mohr Coulomb model

Method A uses c Method A uses c φφand Method B uses Cuand Method B uses Cu

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Method AC, phi

Method BCu

Normally Consolidated Clay Undrained Loading

FE Model Constant p’Zero dilatancy

Cam ClaySoil is contractive

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Over-consolidated ClayKo Consolidated Clay

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Method A at Nicoll Highway M3 SectionMethod A at Nicoll Highway M3 Section

● Method A/B problem is not unique to PlaxisMethod A/B problem is not unique to Plaxis

● Method A was in widespread use in Singapore Method A was in widespread use in Singapore (and is widely adopted internationally)(and is widely adopted internationally)

● Method A was adopted for design of C824Method A was adopted for design of C824

● Method A (and other methods) should be Method A (and other methods) should be compared with design Cu profilecompared with design Cu profile

● Excavations at C824 were deepest ever in Excavations at C824 were deepest ever in SingaporeSingapore

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Nicoll Highway M3 Design SectionNicoll Highway M3 Design Section

MCLower

MCUpper

EC

So

ft Clay 40

m

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Effect of Method A on Cu ProfileEffect of Method A on Cu Profile

0

5

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15

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25

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40

45

50

0 20 40 60 80 100 120 140

Undrained Strength, cu (kN/m2)

Dep

th B

elow

Gro

und

Leve

l (m

) Method A, Ko = 1

Method A, Ko = 0.6

Design CuProfile

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Method A on Net Pressure Profile Excavation for 6Method A on Net Pressure Profile Excavation for 6thth Strut Strut

14

16

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30

-50 0 50 100 150 200 250

Net Pressure on Wall (kN/m2)

Dep

th B

elow

Gro

und

Leve

l (m

)

5th Strut Excavation Level

Upper JGP Layer

Design CuProfile

Method AKo = 0.6

Net Pressure +ve

Pa > Pp15mSpan

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Effect of Method A on Wall DisplacementEffect of Method A on Wall Displacement

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60

65

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105

-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Wall Disp. (m)

RL

(m)

55

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95

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105

-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Wall Disp. (m)

RL

(m)

Method A Method B

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Effect of Method A on Bending MomentsEffect of Method A on Bending Moments

55

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105

-3000

-2000

-1000 0

1000

2000

3000

4000

Bending Moment (kNm/m)

RL (

m)

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-300

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-200

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-100

0 0

1000

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Bending Moment (kNm/m)

RL

(m)

Method A Method B

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Effect of Method A on Strut LoadsEffect of Method A on Strut LoadsStrut Row Predicted Strut Load

Using Method BDesign Strut Load

Using Method ARatio 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 ModelsMohr Coulomb and Cam Clay Type Models

● For deep excavations Method A can under-estimate wall For deep excavations Method A can under-estimate wall displacement and BMdisplacement and BM

● For shallow excavations Method A will over-estimate wall For shallow excavations Method A will over-estimate wall displacement and BMdisplacement and BM

● Method B matches the design undrained strength profile Method B matches the design undrained strength profile and is preferableand is preferable

● Neither Method A or B model the real behaviour of soft Neither Method A or B model the real behaviour of soft clayclay

● Post collapse recommendation to use Cam Clay type Post collapse recommendation to use Cam Clay type models models

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FE Model Constant p’Cam Clayor real Soil

Idealised behaviour of soil using Cam Clay type models

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Actual behaviour of Singapore Marine ClayActual behaviour of Singapore Marine Clay

● Real behaviour of Marine Clay determined from Real behaviour of Marine Clay determined from high quality lab testshigh quality lab tests

● Sampling carried out using thin wall with 5 Sampling carried out using thin wall with 5 degree cutting angledegree cutting angle

● Samples anisotropically re-consolidated to in Samples anisotropically re-consolidated to in situ stresses prior to testingsitu stresses prior to testing

● Testing carried out undrained in extension and Testing carried out undrained in extension and compressioncompression

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Real Behaviour

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Parameters for Upper Marine ClayParameters for Upper Marine Clay

Cu Peak 68 kPaφ at Peak undrained

25º

Cu Large Strain

52 kPa φLarge Strain 34º

% Change25%

reduction% Change 35% Increase

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 ClayMohr Coulomb v Modified Cam Clay

Modified Cam Clay model includes features of soft clay Modified Cam Clay model includes features of soft clay behaviourbehaviour

Some natural soft clays differ from Modified Cam ClaySome natural soft clays differ from Modified Cam Clay

Physically unrealistic values may be required to match Physically unrealistic values may be required to match undrained strength profileundrained strength profile

For managing risk care must be taken to understand For managing risk care must be taken to understand implication of differencesimplication of differences

Possibly simpler to adopt Mohr Coulomb with Method BPossibly simpler to adopt Mohr Coulomb with Method B

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Modelling Piles in 2 D AnalysesModelling Piles in 2 D Analyses● Structures constructed in deep excavations in Singapore are often Structures constructed in deep excavations in Singapore are often

founded above soft clay on pilesfounded above soft clay on piles

● Piles are often constructed after installation of JGP layers but before Piles are often constructed after installation of JGP layers but before commencement of excavationcommencement of excavation

● Piles will be bonded to the JGPPiles will be bonded to the JGP

● Heave of ground during excavation results in tension in pilesHeave of ground during excavation results in tension in piles

● Presence of piles will restrain heave and also restrict wall Presence of piles will restrain heave and also restrict wall movementsmovements

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Comments on modelling of PilesComments on modelling of Piles

● Modelling piles in 2D analyses as walls connected to the Modelling piles in 2D analyses as walls connected to the ground can severely restrict the predicted wall movementground can severely restrict the predicted wall movement

● Wall displacements will be under-predicted and wall Wall displacements will be under-predicted and wall bending moments also under-predictedbending moments also under-predicted

● If 3D modelling is not available then it may be preferable to If 3D modelling is not available then it may be preferable to carry out sensitivity studies without piles and with piles carry out sensitivity studies without piles and with piles modelled as “anchors” not connected to the soil meshmodelled as “anchors” not connected to the soil mesh

● For managing risk you must understand the limitations For managing risk you must understand the limitations implicit in simple 2D models – sensitivity analysesimplicit in simple 2D models – sensitivity analyses

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Modelling JGPModelling JGP Numerical models for design typically adopt Mohr Numerical models for design typically adopt Mohr

Coulomb type modelCoulomb type model

E = 150MPa, CE = 150MPa, Cuu = 300kPa (minimum UCS is 900kPa) = 300kPa (minimum UCS is 900kPa)

JGP strength is a factored value used in analyses where JGP strength is a factored value used in analyses where soil strength is unfactoredsoil strength is unfactored

How are design values justified?How are design values justified?

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0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4U nconfined com pression strength (M Pa)

0

1

2

3

4N

umbe

r of

res

ults

Min

imum

com

plia

nt v

alue

0 100 200 300 400 500 600 700 800 900 1000 1100 1200E v (M Pa)

0

1

2

Nu

mbe

r of

res

ults

Da ta from cores

B ack-analysedE h=65M P a

D erived from shear w avesE h=81M Pa

Design Value 150 MPa

USC Results

Design900kPa

Average2000kPa

E50 from UCS Tests

Average500 MPa

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0 0 .2 0.4 0.6 0.8 1 1.2 1.4 1.6 1 .8 2A xia l st ra in a t fa ilu re, af (% )

0

2

4

6

8

10

Nu

mbe

r o

f res

ults

Axial strain at failure in UCS tests on JGPAverage 0.8%

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Summary of JGP PropertiesSummary of JGP Properties

ModelModel CCuu EE Fail Strain Fail Strain %%

LaboratLaboratoryory

UCSUCS >1000>1000 500500 0.80.8

DesignDesign M-CM-C 300300 150150

Back Back AnalyzeAnalyzedd

RealReal 500500 7070*1*1 >2>2

AdvanceAdvanced d AnalysisAnalysis

Brittle?Brittle? 500500*2*2/ 200/ 200 8080 22*2*2

*1 – Non linear response*2 – Peak to residual at 20% plastic strain

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Modelling of JGPModelling of JGP

Actual mass characteristics of JGP not well understoodActual mass characteristics of JGP not well understood

No direct relationship between lab and field performanceNo direct relationship between lab and field performance

Parameters and model presently used for design are probably Parameters and model presently used for design are probably incorrect and may be unsafeincorrect and may be unsafe

JGP is probably a brittle material whereas Mohr Coulomb is JGP is probably a brittle material whereas Mohr Coulomb is elastic/perfect plasticelastic/perfect plastic

Sensitivity analyses with high and low strength and stiffness values Sensitivity analyses with high and low strength and stiffness values are essentialare essential

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Concluding RemarksConcluding Remarks

Numerical modelling has an important role in designNumerical modelling has an important role in design

Numerical modelling requires specialist knowledgeNumerical modelling requires specialist knowledge

For managing risk make sure that the limitations of the For managing risk make sure that the limitations of the model are well understood (investigated)model are well understood (investigated)

Do not rely on preciseness of resultsDo not rely on preciseness of results

Sensitivity/ trends in behaviour more importantSensitivity/ trends in behaviour more important

Always perform sanity checks by alternative meansAlways perform sanity checks by alternative means

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End of PresentationEnd of Presentation