Transcript Geotechnical HKIE Seminar 2003

Singapore Building and Construction Authority
(Building Engineering Division)
Managing Geotechnical Risks – are we learning
from our failures
29th September 2006
Andy Pickles & Tom Henderson
Geotechnical Consulting Group (Asia) Ltd.
Asia
1
Contents

Overview of the management of geotechnical risk (Tom Henderson)

Overview of deep excavation practice in Hong Kong (Tom Henderson)

Comment on the scope and reporting of site investigations (Tom Henderson)

Design aspects for deep excavations and in particular the use of numerical
modelling (Andy Pickles)

Instrumentation to verify design and control during construction of deep
excavations (Andy Pickles)

Miscellaneous lesson learnt from the investigation into Nicoll Highway
collapse (Andy Pickles)
Asia
2
Site Investigation

Site Investigation is a continuous process particularly for major projects
must have multiple phases
must be planned and structured
for complete duration of project
must have adequate budget
must be justifiable
Asia
3
Site Investigation

Onus is on geotechnical engineers to demonstrate the
value of continuity of site investigation to project
managers
Requires stand alone document introduced at
inception of project that is updated as project evolves
- relates ground condition uncertainties to elements of
design and construction that are particular to project
needs - and how these uncertainties will be addressed
Should be driven by ground conditions “Risk
Register”
Asia
4
Site Investigation

Principal Objectives
provide adequate information for a safe economic
engineering design
provide information to plan most appropriate
method of construction and how to deal with
difficulties anticipated during construction
Asia
5
Essential Phases of Site Investigation for
Management of Geotechnical Risk





Preliminary ground investigation to compliment feasibility studies
Main ground investigation to provide input to engineering design – and
envisaged method of construction
Allowance for supplementary investigation prior to construction that
addresses anomalies/uncertainties arising from the design process
Additional investigation during construction (eg instrument installation - the
best site investigation available)
Collation and interpretation of as-built conditions (pile installation records,
slope mapping etc)
Asia
6
Optimum Cost of Site Investigation for Major Projects
Asia
7
Costing Site Investigation

Precedent suggests overall SI costs in Asia rarely exceed 1% of civil construction
costs
USNCTT (1984) recommends 3% for tunnel projects
JLE Costs possibly >4% (not directly comparable)
LTA NEL Costs recorded as <0.5% (Shirlaw et al, 2002)
Hong Kong – major projects – around 1%
Is this total cost –
does it include contractor’s SI during construction?
Does it include for all interpretation?
Asia
8
Examples of Representative Cost/Value of Site Investigations for Major
Tunnnelling Projects (Westland et al, 1998)
Lower risk of
cost over-run
Asia
9
Planning Site Investigations for Major Projects



Balance spatial constraints with engineering requirements
avoid over application of uniform spacing
Consider “clustering” approach
Be realistic with laboratory testing schedules
match the quality of the test with the quality of the
sample
ensure there is sufficient information on basic
properties to allow adequate interpretation of
sophisticated tests
Asia
10
Uniform Spacing v “Clustering” for SI

Use closely spaced “clusters” of SI to
obtain continuous samples fully logged profile of ground
calibrate in-situ tests (eg CPTs) with fully logged borehole
obtain high quality samples for full range of laboratory testing
(ie basic indices combined with sophisticated strength tests

Benefits
Extrapolate results to wider spaced in-situ testing and basic lab
testing using reliable site specific correlations
This can often be cost effective on large projects
Asia
11
Jubilee Line Extension - London
Outcrop of Lambeth Group
Asia
12
General Sequence of Lambeth Group in Central
London
Up to 25m
Asia
13
JLE - Issues Associated with Lambeth Group Identified at Desk Study




Engineering interpretation was interebedded sands, silts and clays
15 to 25m thick -variable properties
oversimplified and too inconsistent for major tunnelling works
Previous experience indicated a problematic, high risk material for
tunnelling and deep excavations
low OCR material and variable K0
significant lateral and vertical lithological variation
localised deposits of cemented material and/or pebble beds
evidence of perched water tables
Geologists had the basis for a well-developed model based on
depositional history but no engineering parameters
Project site investigation had to correlate geological model with
engineering interpretation to establish reference ground conditions for
contract and provide representative parameters for engineering analyses
Asia
14
JLE - Actions taken to address risk/uncertainty on Lambeth
Group during SI prior to design & construction

Selective high quality site investigation
Triple tube rotary coring full depth in Lambeth Group
Large number of samples split and logged
In situ testing involving piezocones/self boring pressuremeters

Definitive classification of sub-units
Detailed logging of recovered full depth cores
BGS logging of full sequence
Logging teach-in for all SI contractors

Laboratory Testing Programme
Large number of basic tests for material classification
High quality undisturbed samples for advanced laboratory testing
Laboratory testing regime to shadow insitu testing
Asia
15
JLE - What was achieved in terms of addressing
risk/uncertainty associated with Lambeth Group

Information available to Designers and Contractors
Boreholes logs containing detailed and consistent
descriptions of principal lithology
Large volume of measurements of engineering properties
for each of the sub-units
Interpretative Report for permanent and temporary works
design

Engineering Interpretation
Influence of structure and fabric on engineering behaviour
highlighted
Asia
16
Example of the effects of sample disturbance – soft clay
30
(sa-sr)/2 (kPa)
15% reduction
20
Destructuring
Tube type
conventional
modified
500mm "ELE"
Japanese standard
Block
Sherbrooke
10
Reduction in p'i
0
0
10
20
30
All advanced in borehole
40 0
(s’a+s’r)/2 (kPa)
5
10
Axial strain (%)
UU triaxial compression tests on Bothkennar Clay, 6.75-7.1m
(Hight, personal com 2006)
Example of the effects of sample disturbance – stiff clay
Rotary core
samples
Effects of sampling
method in UU
triaxial compression
tests
on Upper Mottled
Clay, Lambeth Group
(Hight, personal com 2004)
Asia
Driven tube
samples
18
Reporting/Interpretation of site investigation test data
800
700
600
t (kPa)
500
400
300
200
Triaxial tests
Best-fit
100
Design line
0
0
Asia
100
200
300
400
500
600
s' (kPa)
700
800
900
1000
1100
1200
19
( s a - s r )/2
Results
Resultsofofengineering
engineeringstudy
studyon
on
variable
clays
Lambethstiff
Group
0
Stiff S a n d y C la y s
C u = f(w )
( s a ’+ s r ’)/2
( s a - s r )/2
(Hight, personal com 2004)
( s a - s r )/2
0
0
Asia
Stiff F is s u re d
p la s tic C la y s
C u = f(p 0 ’)
( s a ’+ s r ’)/2
Stiff M e d iu m
p la s tic C la y s
C u = f(w , p 0 ’)
( s a ’+ s r ’)/2
20
Ground Investigation,
Fieldwork
and
Factual
Report
Laboratory Testing & Factual Report
Form of Geotechnical Reporting
Inception/
Planning
Interpretation
and Review of
Data
Desk Study Conceptual
Ground Model
Interpretative Report –
Detailed Ground Model,
Ground Parameters and
Engineering Implications
Design
Recommendations
Ground
Investigation,
Continuous
Fieldwork and
Review of geotechnical
uncertainty/
Laboratory
geotechnical
riskTesting
register
Design Report
Construction Monitoring,
Maintenance and
Decommissioning
Asia
Guidelines for the preparation of the
Ground Report (AGS, 2003)
Validation Report –
Geotechnical Structures
Manual
21
Site Investigation & Reporting - Summary



Site Investigation is not a one-off exercise on major
projects – continuous iterative process
Must be driven by “uncertainty” associated with the
ground conditions and how this impacts on design
and construction – Risk Register
Should have flexible approach – avoid blanket
application
• Reporting should reflect increasing level of
knowledge and interpretation
Asia
22
Thank you
Asia
23