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Quantitative Risk Assessment in Geotechnical Engineering - A Comprehensive Overview

Explore the methodologies and tools for probabilistic geotechnical analysis to estimate failure probabilities, manage risks in design processes, and enhance reliability. Learn from event trees to advanced RFEM methods.

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Quantitative Risk Assessment in Geotechnical Engineering - A Comprehensive Overview

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  1. Quantitative Risk Assessment in Geotechnical Engineering D.V. Griffiths Colorado School of Mines, USA University of Colorado Boulder NREL Mini-Workshop Uncertainty and Risk Assessment in the Design Process for Wind National Wind Technology Center Boulder, Colorado, USA 12 - 13 July 2016. University of California Berkeley

  2. “……in earthwork engineering the designer has to deal with bodies of earth with a complex structure and the properties of the material may vary from point to point.” K. Terzaghi Inaugural edition of Géotechnique (1948) “Two specimens of soil taken at points a few feet apart, even if from a soil stratum which would be described as relatively homogeneous, may have properties differing many fold.” Donald W. Taylor Introduction to Fundamentals of Soil Mechanics Wiley, (1948) It is only quite recently however, that methodologies such as the RFEM have been developed to explicitly model the variability discussed by Terzaghi and Taylor.

  3. Bearing Capacity

  4. Geotechnical Engineers must increasingly be willing to deal with questions relating to the reliability of their designs. • Regulatory agencies (nuclear, offshore, mining, waste disposal) • Legal pressure (insurance companies, actuarial issues) • Financial decisions (whether to proceed, cost of actions) • Trend toward RBD (e.g. LRFD to achieve target reliability) • Making the most of limited site investigation data.

  5. WHAT IS THE GOAL OF A PROBABILISTIC GEOTECHNICAL ANALYSIS? To estimate the “probability of failure” as an alternative, (or complement to), the traditional “factor of safety” Some investigators prefer an alternative terminology, e.g. “the probability of inadequate performance” “the probability of design failure” “reliability”

  6. Key Questions • How could failure occur? • How likely is it? • What would happen if it did? Bureau of Reclamation definition of Risk What is “acceptable risk”?

  7. New Orleans Levees after Katrina…?

  8. PROBABILISTIC GEOTECHNICAL ANALYSIS • When are probabilistic approaches appropriate in geotechnical analysis? • Which method should we use and when? • How can we interpret the results once we have them? • Probabilistic analysis is no substitute for engineering judgement.

  9. THREE LEVEL OF PROBABILISTIC ANALYSIS • Expert Panel • Event Trees • First Order Methods • First Order Reliability Methods (FORM) • Monte-Carlo • Single Random Variable Approach (SRV) • Random Finite Element Method (RFEM)

  10. Level 1: Event Trees Probability of embankment breach due to foundation liquefaction

  11. Level 2: First Order Reliability Method (FORM) Probability of bearing capacity failure Units in kN and m

  12. Level 2: First Order Reliability Method (FORM) FS>1 FS<1

  13. Contours of the Reliability Index First Order Reliability Method NO FAILURE FS>1 “Design Point” FAILURE FS<1 pf is the volume under the hill on the failure side of the first order Limit State Function

  14. DataSolverSolve

  15. Unique relationship between the Probability of Failure and the Reliability Index for a Normal Distribution Probability of Failure: pf Reliability Index: b

  16. Level 3: Monte-Carlo, Single Random Variable Approach (SRV)

  17. Level 3: Monte-Carlo, The Random Finite Element Method (RFEM) • Developed in the 1990s for advanced • probabilistic geotechnical analysis. • Combines finite element and random field • methodologies in a Monte-Carlo framework. • Properly accounts for (anisotropic) spatial • correlation structures in soil deposits. • All programs are open-source. • Now a considerable bibliography on the method.

  18. Geotechnical Applications Settlement Mine pillar Stability Seepage Bearing Capacity

  19. Earth Pressures Slope Stability Laterally Loaded Piles Evidence of a “worst case” spatial correlation length leading to the highest probability of failure

  20. Slope stability • Bearing capacity • Foundation settlement • Steady seepage • Free-surface analysis • Consolidation • Mine pillar stability • Limiting earth pressure • Piled foundations (t-z, p-y) All RFEM source code covering a wide range of geotechnical applications has been made available by the authors at http://inside.mines.edu/~vgriffit/rfem/

  21. Concluding Remarks Risk assessment and management of geotechnical structures such as foundations and slopes ultimately involves quantitative estimates of failure probabilities. Engineers have an increasingly sophisticated toolbox of methods ranging from event trees to first-order method to state-of-the-art random finite element methods (RFEM). RFEM programs (source code) are freely disseminated, for maximum accessibility for practitioners. Research on RFEM is ongoing at CSM. Probabilistic tools are no substitute for adequate site characterization, engineering judgment and experience. You pay for a site investigation whether you have one or not!

  22. Thank you. www.georiskconference.org

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