Overview of Geotechnical Physical Modelling ... - Geo-Engineering

15 th ICSMGE: Istanbul 2001
TC2 Workshop - Session W7
Overview of Geotechnical Physical
Modelling
Professor Mark Randolph
Special Research Centre for Offshore Foundation Systems 1
The University of Western Australia
1
Established and supported under the Australian Research Council’s Research Centres Program

Outline
• Modelling in Design
− formulation of conceptual models
• Why Physical Modelling
• Soil Characterisation
• Examples
15th ICSMG
Istanbul, August 2001

Contribution of Modelling to Design
Rigorous
numerical
analysis
Data from
full-scale
events
Validation of
modelling
techniques
Design
Calibration of
conceptual
model
Physical
model
Demonstration of
applicability
Conceptual
model
15th ICSMG
Istanbul, August 2001

Verification 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
15th ICSMG
Istanbul, August 2001

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 interactions
15th ICSMG
Istanbul, August 2001

Soil Characterisation in Model Testing
• For conceptual model development, essential to
plan 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), G o measurements
15th ICSMG
Istanbul, August 2001

Examples
• Micro-structural response of sands during pile
installation
− 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
15th ICSMG
Istanbul, August 2001

0
1
Local shear stress, τ s (kPa)
0 10 20 30 40 50 60
Friction Fatigue: Data
Measured in the field
Observed in the lab
2
3
4
5
6
h/D= 25
h/D= 14
h/D= 4
Instrumented steel pile
Diameter, D = 102 mm
h/D= 25
Instrument cluster
h/D= 14
h/D= 4
Distance from pile tip, h
Lehane, 1992
From Lehane et al (1993)
UNIVERSITY OF
CAMBRIDGE
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, CUEL

Friction Fatigue: Challenge
How 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%
Unbroken soil
Shear zone
Pile
Stiffness
10 mm Grain size =
Roughness height =
Shear zone: D 50 reduced by
factor of 2 within 3 mm of pile.
Zone of fines able to migrate
into voids in far field.
UNIVERSITY OF
CAMBRIDGE
Courtesy of David White, CUEL

Foundations on Soft Clay
Objective
Courtesy Dr Hackmet Joer, UWA
• Evaluate relative merits of alternative piling schemes and
conditioning of soft (n.c.) clay by drawdown of water table
Unpiled raft
Piled raft
sand
surcharge
soft
clay
PPT
PPT
PPT
PPT
PPT
PPT
27 mm33 mm
25 mm
30 mm
127
mm
192 mm
sand
layer
PPT
PPT
wick
drains
162.5
mm
162.5
mm
650 mm
162.5
mm
15th ICSMG
Istanbul, August 2001

Site Investigation: T-bar Tests
Prototype penetration (m)
T-Bar Resistance (MPa)
-2 0 2 4 6 8
0
Test 1-1
Test 1-2
5
Test 2-1
Test 2-2
Upper Clay Test 3-1
10
Test 3-2
Linear fit
15
Middle sand layer
20
Six tests in 3 separate samples:
Upper clay:
Resistance profile of 25 kPa/m
ds u /dz = 2.4 kPa/m (N t = 10.5)
Middle sand:
Peak resistance of 5 to 6 MPa
N q of about 100 (6 T-bar Ø)
25
30
Lower Clay
Lower clay:
High resistance due to sand
carried into clay by T-bar
15th ICSMG
Istanbul, August 2001

Installation of Wick Drains
15th ICSMG
Istanbul, August 2001

Finished Wick Drains
15th ICSMG
Istanbul, August 2001

Loading Device - Schematic
• Independent loading
of each foundation
• 4 loading increments
(max of 100 kPa)
• Separate weights
linked by sliding rods
• Central and edge
settlement measured
Displacement
transducer
Weights
Raft
Strongbox
Loading frame
Actuator
Displacement
transducer
162.5 mm
325 mm
162.5 mm
15th ICSMG
Istanbul, August 2001

Loading Device - Photographs
15th ICSMG
Istanbul, August 2001

Settlement Response During Loading
• Wick drains functioned well, leading to much
faster consolidation
• Drawdown of water table was shown to be
effective in reducing settlements to 20 % of
those without drawdown
• Detailed modelling of different pile geometries
and drawdown strategies was effective in
assessing an appropriate design approach
15th ICSMG
Istanbul, August 2001

Discussion 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 design
models
• Complex Problems
− Are we kidding ourselves that our (scaled) results are
a good guide to prototype performance
15th ICSMG
Istanbul, August 2001