fundações-piled raft- randolph

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Professor Mark Randolph

Special Research Centre for Offshore Foundation Systems1

The University of Western Australia

1 Established and supported under the Australian Research Council’s Research Centres Program

TC2 Workshop - Session W7TC2 Workshop - Session W7Overview of Geotechnical PhysicalOverview of Geotechnical Physical

ModellingModelling

1515thth ICSMGE: Istanbul 2001 ICSMGE: Istanbul 2001



15th ICSMG

Istanbul, August 2001

OutlineOutline

• Modelling in Design

− formulation of conceptual models

• Why Physical Modelling ?

• Soil Characterisation

• Examples



15th ICSMG


Contribution of Modelling to Design

Calibration of

conceptual

model

Validation of

modelling

techniques

Rigorous

numerical

analysis

Physical

model

Conceptual

modelDemonstration of

applicability

Data fromfull-scale

events

Design



15th ICSMG


Verification of Conceptual ModelsVerification of Conceptual Models

Two different categories:

• Simplified Analytical Models

−e.g. approximate elastic solutions; plastic limit analyses

− generally, must validate through rigorous numerical

analysis, NOT through physical modelling

• Correlations with Soil Parameters

− e.g. pile shaft friction; limiting vane torque; bearing

'capacity' in compressible granular materials

− essential to mimic typical (prototype) measurement of

key soil properties for correlations



Why Physical Modelling ?Why Physical Modelling ?

• Complexity of Soil Response− particle crushing, destructuring (e.g. cemented soils)

− (strength) anisotropy, strain softening (shear bands)

− cyclic loading, creep

• Complexity of Geometry or Construction

− pile installation: jacked, driven, bored

− large penetration (e.g. drag anchors, caissons)

− tunnelling (?), deep excavation/retaining walls (?)

• New Phenomena

− e.g. fundamental soil response; thermomechanical or

chemical interactions15th ICSMG




15th ICSMG


SoilSoil CharacterisationCharacterisation in Model Testingin Model Testing

•For conceptual model development, essential toplan soil characterisation studies as integrated

part of model testing

− spatial variation of strength (3-D)

− temporal variation in strength

• Clays

− penetrometer testing (cone, T-bar etc), vane (?)

• Sands

− cone, buried samplers (void ratio), Go measurements



15th ICSMG


ExamplesExamples

• Micro-structural response of sands during pileinstallation

− courtesy of White/Bolton experimental studies

− particle crushing, consequential stress changes

• Complex piled raft foundation study

− consolidation of soft clay using wick drains

− conditioning of clay by drawing down water table

− incremental loading of piled and unpiled foundations



0

1

2

3

4

5

6

0 10 20 30 40 50 60Local shear stress, τs (kPa)

h/D= 4

h/D= 25

h/D= 14

Instrument cluster

Distance from pile tip, h

Instrumented steel pile

Diameter, D = 102 mm

h/D= 4

h/D= 25

h/D= 14

Friction Fatigue: DataFriction Fatigue: Data

Measured in the field

Observed in the lab

Lehane, 1992

Mechanism: Horizontal compression around base creates

high horizontal stress on pile shaft. Subsequent shearing

leads to unloading and reduced horizontal stress.

Courtesy of David White, CUELUNIVERSITY OF

CAMBRIDGE

From Lehane et al (1993)



Friction Fatigue: ChallengeFriction Fatigue: ChallengeHow can this behaviour be modelled (and predicted)?

Initial conditions:

Stress path: Up to 2 MPa and

back to a few kPa

Strain path: shear strain > 100%

volumetric strain > 30%

Shear zone: D50 reduced by

factor of 2 within 3 mm of pile.

Zone of fines able to migrate

into voids in far field.

PileUnbroken soil Shear zone

10 mmGrain size = ?

Roughness height = ?

Stiffness ?

UNIVERSITY OF

CAMBRIDGE Courtesy of David White, CUEL



15th ICSMG


Objective

• Evaluate relative merits of alternative piling schemes and

conditioning of soft (n.c.) clay by drawdown of water table

Foundations on Soft ClayFoundations on Soft Clay

127

mm

30 mm

192 mm

PPT

PPT

PPT

PPT

25 mm

162.5mm 162.5mm 162.5mm650 mm

33 mm

PPT

PPT

PPT

PPT

27 mm

Unpiled raft Piled raft

Courtesy Dr Hackmet Joer, UWA

sand

surcharge

soft

clay

sand

layer

wick

drains



15th ICSMG


Site Investigation: T-bar TestsSite Investigation: T-bar Tests

0

5

10

15

20

25

30

-2 0 2 4 6 8

T-Bar Resistance (MPa)

P

r o t o t y p e p e n e t r a t i o n ( m )

Test 1-1

Test 1-2

Test 2-1

Test 2-2

Test 3-1

Test 3-2

Linear fit

Upper Clay

Middle sand layer

Lower Clay

Upper clay:

Resistance profile of 25 kPa/mdsu/dz = 2.4 kPa/m (Nt = 10.5)

Middle sand:Peak resistance of 5 to 6 MPa

Nq of about 100 (6 T-bar Ø)

Lower clay:

High resistance due to sand

carried into clay by T-bar

Six tests in 3 separate samples:



15th ICSMG


Installation of Wick DrainsInstallation of Wick Drains



15th ICSMG


Finished Wick DrainsFinished Wick Drains

i i S iL di D i S h i



15th ICSMG


Loading Device - SchematicLoading Device - Schematic

162.5 mm325 mm162.5 mm

Displacement

transducer

Loading frame Actuator

Displacement

transducer

Weights

Raft

Strongbox

• Independent loading

of each foundation

• 4 loading increments(max of 100 kPa)

• Separate weights

linked by sliding rods• Central and edge

settlement measured

L di D i Ph hL di D i Ph t h



15th ICSMG


Loading Device - PhotographsLoading Device - Photographs

S ttl t R D i L diS ttl t R D i L di



15th ICSMG


Settlement Response During LoadingSettlement Response During Loading

• Wick drains functioned well, leading to much

faster consolidation

• Drawdown of water table was shown to beeffective in reducing settlements to 20 % of

those without drawdown

• Detailed modelling of different pile geometriesand drawdown strategies was effective in

assessing an appropriate design approach



15th ICSMG


Discussion IssuesDiscussion Issues

•Physical Modelling− What rôle does it have in development of conceptual

models for design?

• Micro-structural Response− New techniques to quantify micro-structural response

during construction processes

− Can we make quantitative links for use in designmodels?

• Complex Problems

− Are we kidding ourselves that our (scaled) results are

a good guide to prototype performance?

fundações-piled raft- randolph

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