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Singapore Building and Construction Authority (Building Engineering Division). Managing Geotechnical Risks – are we learning from our failures 29 th September 2006 Andy Pickles & Tom Henderson Geotechnical Consulting Group (Asia) Ltd. Contents.

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singapore building and construction authority building engineering division
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

contents
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

site investigation
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

site investigation4
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

site investigation5
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

phases of site investigation for management of geotechnical risk

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

costing site investigation
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

slide9
Examples of Representative Cost/Value of Site Investigations for Major Tunnnelling Projects (Westland et al, 1998)

Lower risk of cost over-run

Asia

planning site investigations for major projects
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

uniform spacing v clustering for si
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

jubilee line extension london
Jubilee Line Extension - London

Outcrop of Lambeth Group

Asia

jle issues associated with lambeth group identified at desk study
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

slide15
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

jle what was achieved in terms of addressing risk uncertainty associated with lambeth group
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

slide17

Example of the effects of sample disturbance – soft clay

30

15% reduction

20

Destructuring

(sa-sr)/2 (kPa)

Tube type

conventional

modified

10

500mm "ELE"

Japanese standard

Block

Sherbrooke

Reduction in p'

All advanced in borehole

i

0

5

0

10

0

10

20

30

40

(s’a+s’r)/2 (kPa)

Axial strain (%)

UU triaxial compression tests on Bothkennar Clay, 6.75-7.1m

(Hight, personal com 2006)

slide18

Example of the effects of sample disturbance – stiff clay

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

Rotary core samples

Driven tube samples

Asia

slide20

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

Results of engineering study on variable stiff clays

Asia

slide21

Inception/

Planning

Factual Report

Interpretation

and Review of

Data

Interpretative Report –

Detailed Ground Model,

Ground Parameters and

Engineering Implications

Desk Study Conceptual

Ground Model

Design

Recommendations

Ground Investigation,

Fieldwork and

Laboratory Testing

Design Report

Construction Monitoring,

Maintenance and

Decommissioning

Validation Report –

Geotechnical Structures

Manual

Ground Investigation,

Fieldwork and

Laboratory Testing & Factual Report

Form of Geotechnical Reporting

Continuous

Review of geotechnical uncertainty/

geotechnical risk register

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

Asia

site investigation reporting summary
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