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Managing Geotechnical Risk Are we learning from the failures

Managing Geotechnical Risk Are we learning from the failures “ The Use of Instrumentation and Monitoring to Verify Design and Control Construction of Deep Excavations (modified)” Andy Pickles of GCG (Asia) Ltd. Overview of Presentation.

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Managing Geotechnical Risk Are we learning from the failures

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  1. Managing Geotechnical Risk Are we learning from the failures “The Use of Instrumentation and Monitoring to Verify Design and Control Construction of Deep Excavations (modified)” Andy Pickles of GCG (Asia) Ltd. Asia

  2. Overview of Presentation Presentation relates to deep excavation for major infrastructure projects • Risk in design and role of instrumentation • Typical scope and cost of instrumentation • Obtaining added value (reducing risk) from instrumentation • Interpretation of instrumentation Asia

  3. Risk in Design of Deep Excavations Instrumentation is associated with the management of risk (i.e. uncertainties). These include: • Soil stratigraphy • Geotechnical design parameters • Construction related factors Asia

  4. Uncertainty of Soil StratigraphyVariation of Continuity of Sand Layer Discontinuous Layer Continuous Layer Asia

  5. Wall horizontal disp. (m) 0.000 0.040 0.080 0.120 0.160 0.200 0.240 FILL 4 0 Soft CLAY -4 -8 _ -12 StiffCLAY 40 75 -16 Reduced level (m) Final exc -20 ClayeySAND -24 Hard CLAY -28 Strength_1 -32 Strength_2 -36 Uncertainty in Design Parameters - 1Deep Excavation in Clay Bend Mom 2800 kNm 5000 kNm Asia

  6. Uncertainties in Design Parameters – 2Variation of Rate of Softening Low Permeability High Permeability Asia

  7. Construction Related Uncertainties -1 Performance of Grout Cut-off CLAY Asia

  8. Construction Related Uncertainties - 2 • Ground settlement on wall installation • Effectiveness and extent of dewatering • Properties of jet grout layers • Effectiveness of ground treatment • Effects of previous construction work • Interaction with adjacent structures Asia

  9. Instrumentation to deal with these risks • All design approaches • Design parameters from previous experience • Determined from instrumentation results • Routine design approach • Offset risk by using conservative assumptions • Verify design using instrumentation results (AAA) • Observational approach (progressive modification) • Manage risk through engineering input • Less conservative assumptions • Continuous improvement of design based on instrumentation results • As knowledge improves design improves and risk remains constant. Asia

  10. Trigger Levels in Routine Design • Trigger levels are used as part of the AAA system • Important to distinguish between trigger levels set using the design predictions, on the basis of additional calculation and based on engineering judgement • Design predictions are unreliable and AAA value requires a design review, if appropriate AAA values can be updated • Additional calculation is based on actual serviceability and AAA values should not be exceeded • Engineering judgement are often guesses or best practice and more detailed analysis is then required and AAA values can be revised Asia

  11. Managing Risk and Obtaining Value • If the major role of instrumentation is to manage risk how can we make it more effective? • Alternatively, how can we get more value out of instrumentation? Asia

  12. The Role of Instrumentation 0% • Routine design and construction • To verify an already conservative design • Observational approach • Use data to continually improve design • Reduce construction costs • Save construction time • Long term perspective • Full interpretation published with all background information allows future designs to be improved • Benefits accrued by major clients INCREASING VALUE 10% Asia

  13. Typical Scope of Standard Instrumentation Asia

  14. Scope of Data Collection • Typically 1000 to 2000 separate instruments • Readings required daily during excavation and strut removal stages and weekly at other times • Typically of the order of 200 readings per instrument • 400,000 separate sets of readings • 1,000,000 or more data points Asia

  15. Typical cost of instrumentation Cost includes • Instrumentation • Monitoring teams (instrumentation and survey) • Engineering support to coordinate work • Development and maintenance of database • Daily, weekly and monthly reporting with associated meeting costs Asia

  16. Cost of Instrumentation Routine Design • Total cost for routine instrumentation of a deep excavation contract is S$ 5 to 10 million • Primary use is to verify the design of the excavation works • Cost is of similar order to the original design costs (i.e. pay as much to verify as to design) • Instrumentation is often seen as an onerous contract requirement • No added value obtained from instrumentation • Designer is often not closely involved with construction Asia

  17. How to Add Value to Instrumentation • Adding value is reducing risk and reducing cost • Involve designer in the construction work • Prepare good quality instrumentation interpretative reports (comprehensive rather than instrument specific) • Carry out back analyses to determine actual performance parameters • Encourage value engineering through observational approach • Publish results and improve standards and codes of practice Asia

  18. Cost of Adding Value to Instrumentation • Additional cost for proper interpretation of results and associated report is S$ 0.1 million • Additional cost for back analysis in order to obtain useful design data for future work is S$ 0.1 to 0.2 million • Large savings in Contract costs and times are achievable by making better use of monitoring data • Additional costs can usually be offset by better control of day to day monitoring (i.e. cut down frequency) Asia

  19. Jan 00 Nov 01 -5 -15 0 -120 Examples of Excessive MonitoringManual Settlement Readings Asia

  20. Examples of Excessive MonitoringManual Piezometer Readings 3 2 Groundwater Level mPD 1 0 Asia 18 Months with 500 Measurements

  21. Examples of Excessive MonitoringDatalog Piezometer Readings Tide Gauge 2 Groundwater Level mPD 1 1 Day Piezometer 0 Asia

  22. Jan 00 Nov 01 -5 -15 2.5 -1.5 Examples of Excessive MonitoringDatalogged Piezometer Readings Asia

  23. Monitoring Frequency, Value and Risk • My experience approximately 50% of monitoring is unnecessary. • Obtained because nobody is reviewing the data. • Extra value can be obtained without additional cost by better control of instrumentation • Better value is associated with reduced risk • Additional value can be obtained through Use of Observational Approach (Progressive Modification) Adoption of updated design codes (e.g. CIRIA C580) Feeding back information to future contracts Asia

  24. Proposal for Increasing Value of Instrumentation • Use of Observational Method can achieve substantial cost savings and more importantly time savings (e.g. Powderham, Nicolson) • Adoption of CIRIA C580 which makes more use of instrumentation and can reduce cost of Dwalls by approximately 5% (Sze and Chan 2005) • Scope for reducing factor of safety or load factor by 10% • Paper to ICDE 2006 proposes adoption of C580 with improved instrumentation as an interim measure. Better use of instrumentation, involvement of designer and more widespread adoption of Observational Approach. Asia

  25. Comment on Interpretation of InstrumentationThe 4 Basic Steps • Baseline readings and background trends must be established (daily rainfall, seasonal rainfall, tide, temperature etc.) • Construction records are critical to data interpretation • Instrumentation results must be correlated against baseline trends and construction history • Analysis and presentation of data must consider cause and effect Asia

  26. Establish Baseline Trend “X” Movement of Viaduct Structure Maximum Allowable Water Drawdown Is 1m “Y” Alert = 10 mm Action = 15 mm Alarm = 20 mm 40 m Asia

  27. 50 X-dir Jun 00 Oct 01 -30 20 Y-dir 0 Establish Baseline Trend – 1Movement of Bypass Pier Head Feb 01 40 mm July 00 18 mm Asia

  28. 50 X-dir Jun 00 Oct 01 -30 20 Y-dir 0 Establish Baseline Trend – 1Movement of Bypass 40 mm July 00 July 01 18 mm Asia

  29. Establish Baseline Trend – 2Variation of Groundwater Level 3 2 Groundwater Level mPD 1 Jan 00 July 00 Jan 01 July 01 0 Asia

  30. Construction Records – Tabulated Data Asia

  31. Construction Records - Environmental Asia

  32. Construction Records – Progress Summaries Asia

  33. Construction Records - Photographs 3 March 2002 Asia

  34. Jan 00 Nov 01 -5 Excavation level (mPD) -15 0 Settlement (mm) -120 50 Inclinometer (mm) +2.5 Piezometer (mPD) -1.5 Correlate Response to Construction ActivitiesCause and Effect Plots Asia

  35. Correlate Response to Construction Activitiesand Baseline Trends Tidal Response Rainfall Control +2 Start Dewatering Stop Dewatering +1 Groundwater Level mPD 0 Complete Dwall -1 Jan 00 July 00 Jan 01 July 01 -2 Asia

  36. Summary • The primary purpose of instrumentation is to manage risk • Risk arises due to uncertainties in ground engineering • Designer is aware of uncertainties so should be involved in construction (i.e. continuity) but is often excluded • Much instrumentation is wasted, not enough engineering input • Reduced costs and reduced risk (i.e. added value) can be obtained by better use of instrumentation • Requires good quality interpretation of data and reporting to manage the risks Asia

  37. End of PresentationThank you Asia

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