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Singapore Building and Construction Authority (Building Engineering Division)

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. Optimum Cost of Site Investigation for Major Projects Asia

  8. 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

  9. Examples of Representative Cost/Value of Site Investigations for Major Tunnnelling Projects (Westland et al, 1998) Lower risk of cost over-run Asia

  10. 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

  11. 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

  12. Jubilee Line Extension - London Outcrop of Lambeth Group Asia

  13. General Sequence of Lambeth Group in Central London Up to 25m Asia

  14. 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

  15. 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

  16. 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

  17. 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)

  18. 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

  19. Reporting/Interpretation of site investigation test data Asia

  20. Results of engineering study on Lambeth Group (Hight, personal com 2004) Results of engineering study on variable stiff clays Asia

  21. 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

  22. 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

  23. Thank you Asia