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Singapore Building and Construction AuthoritySingapore Building and Construction Authority(Building Engineering Division)(Building Engineering Division)

Managing Geotechnical Risks – are we learning Managing Geotechnical Risks – are we learning from our failuresfrom our failures

2929thth September 2006 September 2006

Andy Pickles & Tom HendersonAndy Pickles & Tom Henderson

Geotechnical Consulting Group (Asia) Ltd.Geotechnical Consulting Group (Asia) Ltd.

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ContentsContents

Overview of the management of geotechnical risk Overview of the management of geotechnical risk (Tom Henderson)(Tom Henderson)

Overview of deep excavation practice in Hong Kong Overview of deep excavation practice in Hong Kong (Tom Henderson)(Tom Henderson)

Comment on the scope and reporting of site investigations Comment on the scope and reporting of site investigations (Tom Henderson)(Tom Henderson)

Design aspects for deep excavations and in particular the use of numerical Design aspects for deep excavations and in particular the use of numerical

modelling modelling (Andy Pickles)(Andy Pickles)

Instrumentation to verify design and control during construction of deep Instrumentation to verify design and control during construction of deep

excavations excavations (Andy Pickles)(Andy Pickles)

Miscellaneous lesson learnt from the investigation into Nicoll Highway Miscellaneous lesson learnt from the investigation into Nicoll Highway

collapse collapse (Andy Pickles)(Andy Pickles)

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Site InvestigationSite Investigation

Site Investigation is a continuous process - Site Investigation is a continuous process - particularly for major projectsparticularly for major projects

must have multiple phases

must be planned and structured

for complete duration of project

must have adequate budget

must be justifiable

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Site InvestigationSite Investigation

Onus is on geotechnical engineers to demonstrate the Onus is on geotechnical engineers to demonstrate the value of continuity of site investigation to project value of continuity of site investigation to project managersmanagers

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”

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Site InvestigationSite Investigation

Principal ObjectivesPrincipal 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

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Phases of Site Investigation for Phases of Site Investigation for Management of Geotechnical RiskManagement of Geotechnical Risk

Preliminary ground investigation to compliment feasibility studiesPreliminary ground investigation to compliment feasibility studies

Main ground investigation to provide input to engineering design – and Main ground investigation to provide input to engineering design – and envisaged method of constructionenvisaged method of construction

Allowance for supplementary investigation prior to construction that Allowance for supplementary investigation prior to construction that addresses anomalies/uncertainties arising from the design processaddresses anomalies/uncertainties arising from the design process

Additional investigation during construction (eg instrument installation - Additional investigation during construction (eg instrument installation - the the best site investigation availablebest site investigation available))

Collation and interpretation of as-built conditions (pile installation records, Collation and interpretation of as-built conditions (pile installation records, slope mapping etc)slope mapping etc)

Essential

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Optimum Cost of Site Investigation for Optimum Cost of Site Investigation for Major ProjectsMajor Projects

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Costing Site InvestigationCosting Site Investigation

Precedent suggests overall SI costs in Asia rarely exceed 1% of civil construction costs Precedent suggests overall SI costs in Asia rarely exceed 1% of civil construction costs

USNCTT (1984) recommends 3% for tunnel projectsUSNCTT (1984) recommends 3% for tunnel projects

JLE Costs possibly >4% (not directly comparable)JLE Costs possibly >4% (not directly comparable)

LTA NEL Costs recorded as <0.5% (Shirlaw et al, 2002)LTA NEL Costs recorded as <0.5% (Shirlaw et al, 2002)

Hong Kong – major projects – around 1%Hong Kong – major projects – around 1%

Is this total cost – Is this total cost –

does it include contractor’s SI during construction?does it include contractor’s SI during construction?

Does it include for all interpretation?Does it include for all interpretation?

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Examples of Representative Cost/Value of Site Investigations for Major Examples of Representative Cost/Value of Site Investigations for Major Tunnnelling Projects (Tunnnelling Projects (Westland et al, 1998)Westland et al, 1998)

Lower risk of cost over-run

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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

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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

BenefitsExtrapolate 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

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Outcrop of Lambeth Group

Jubilee Line Extension - London

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General Sequence of Lambeth Group in Central London

Up to 25m

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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

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JLE - Actions taken to address risk/uncertainty on Lambeth Group during SI prior to design & construction

Selective high quality site investigationSelective high quality site investigation

Triple tube rotary coring full depth in Lambeth GroupTriple tube rotary coring full depth in Lambeth Group

Large number of samples split and loggedLarge number of samples split and logged

In situ testing involving piezocones/self boring pressuremetersIn situ testing involving piezocones/self boring pressuremeters

Definitive classification of sub-unitsDefinitive classification of sub-unitsDetailed logging of recovered full depth cores Detailed logging of recovered full depth cores

BGS logging of full sequenceBGS logging of full sequence

Logging teach-in for all SI contractorsLogging teach-in for all SI contractors

Laboratory Testing ProgrammeLaboratory Testing ProgrammeLarge number of basic tests for material classification Large number of basic tests for material classification

High quality undisturbed samples for advanced laboratory testingHigh quality undisturbed samples for advanced laboratory testing

Laboratory testing regime to shadow insitu testingLaboratory testing regime to shadow insitu testing

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JLE - What was achieved in terms of addressing JLE - What was achieved in terms of addressing risk/uncertainty associated with Lambeth Grouprisk/uncertainty associated with Lambeth Group

Information available to Designers and ContractorsInformation available to Designers and Contractors Boreholes logs containing detailed and consistent Boreholes logs containing detailed and consistent descriptions of principal lithologydescriptions of principal lithology

Large volume of measurements of engineering properties Large volume of measurements of engineering properties for each of the sub-unitsfor each of the sub-units

Interpretative Report for permanent and temporary works Interpretative Report for permanent and temporary works designdesign

Engineering InterpretationEngineering InterpretationInfluence of structure and fabric on engineering behaviour Influence of structure and fabric on engineering behaviour highlightedhighlighted

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Axial strain (%)

0 5 10

UU triaxial compression tests on Bothkennar Clay, 6.75-7.1mUU triaxial compression tests on Bothkennar Clay, 6.75-7.1m(Hight, personal com 2006)(Hight, personal com 2006)

0 10 20 30 40

0

10

20

30

Destructuring

conventionalmodified

Tube type

500mm "ELE"

All advanced in borehole

Japanese standardBlock

Sherbrooke

Reduction in p'i

(a-

r)/2

(kP

a)

(’a+’r)/2 (kPa)

15% reduction

Example of the effects of sample disturbance – soft clay

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Effects of Effects of sampling sampling method in UU method in UU triaxial triaxial compression compression teststestson Upper on Upper Mottled Clay, Mottled Clay, Lambeth Group Lambeth Group (Hight, personal com (Hight, personal com 2004)2004)

Rotary core samples

Driven tube samples

Example of the effects of sample disturbance – stiff clay

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Reporting/Interpretation of site investigation test dataReporting/Interpretation of site investigation test data

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

s' (kPa)

t (k

Pa

)

Triaxial tests

Best-fit

Design line

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(-

)/2

ar

( ’+ ’)/2 a r

0

(-

)/2

ar

( ’+ ’)/2 a r

0

(-

)/2

ar

( ’+ ’)/2 a r

0

Stiff Sandy ClaysC = f(w )u

Stiff F issuredplastic C laysC = f(p ’)u 0

Stiff Mediumplastic C laysC = f(w , p ’)u 0

Results of engineering Results of engineering study on Lambeth Group study on Lambeth Group (Hight, personal com 2004)(Hight, personal com 2004)

Results of engineering study on variable stiff clays

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Inception/Planning

Factual Report

Interpretationand Review of

Data

Interpretative Report – Detailed Ground Model,Ground Parameters andEngineering Implications

Design Recommendations

Design Report

Construction Monitoring,Maintenance and Decommissioning

Validation Report –Geotechnical Structures

Manual

Ground Investigation,Fieldwork and

Laboratory Testing

Desk Study ConceptualGround Model

Guidelines for the preparation of the Ground Report (AGS, 2003)

Form of Geotechnical Reporting

Continuous Review of geotechnical uncertainty/

geotechnical risk register

Ground Investigation,Fieldwork and

Laboratory Testing & Factual Report

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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

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Thank youThank you