8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 1/17
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 2/17
15th ICSMG
Istanbul, August 2001
OutlineOutline
• Modelling in Design
− formulation of conceptual models
• Why Physical Modelling ?
• Soil Characterisation
• Examples
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 3/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 4/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 5/17
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
Istanbul, August 2001
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 6/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 7/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 8/17
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)
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 9/17
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 10/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 11/17
15th ICSMG
Istanbul, August 2001
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:
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 12/17
15th ICSMG
Istanbul, August 2001
Installation of Wick DrainsInstallation of Wick Drains
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 13/17
15th ICSMG
Istanbul, August 2001
Finished Wick DrainsFinished Wick Drains
i i S iL di D i S h i
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 14/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 15/17
15th ICSMG
Istanbul, August 2001
Loading Device - PhotographsLoading Device - Photographs
S ttl t R D i L diS ttl t R D i L di
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 16/17
15th ICSMG
Istanbul, August 2001
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
8/6/2019 Fundações-Piled raft- Randolph
http://slidepdf.com/reader/full/fundacoes-piled-raft-randolph 17/17
15th ICSMG
Istanbul, August 2001
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?
Top Related