Reliability-based approach for robust geotechnical design By Farrokh Nadim

Reliability-based approach for robust geotechnical design By Farrokh Nadim

1. RISK ASSESSMENT AND MANAGEMENT

   • The ISO Guide 73:2009 defines risk management as "coordinated activities to direct and control an organization with regard to risk". Its purpose is to reduce the risk to levels that are deemed tolerable or acceptable by the society. The management process is a systematic application of management policies, procedures and practices
   • Risk management integrates the recognition and assessment of risk with the development of appropriate treatment strategies
   • The ISO 31000 (2009) risk management process is an integrated process, with risk assessment, and risk treatment (or mitigation) in continuous communication and consultation, and under continuous monitoring and review. Due to the aleatory (inherent) and epistemic (lack of knowledge) uncertainties in hazard, vulnerability and exposure, risk management is effectively 'risk-informed' decision-making under uncertainty

2. CONCEPT OF ACCEPTABLE/TOLERABLE RISK

   • Risk acceptability depends on factors such as voluntary vs. involuntary exposure, controllability vs. uncontrollability, familiarity vs. unfamiliarity, short vs. long-term effects, existence of alternatives, type and nature of consequences, gained benefits, media coverage, information availability, personal involvement, memory, and level of trust in regulatory bodies

3. RELIABILITY-BASED GEOTECHNICAL DESIGN

   • Conventional geotechnical engineering design is based on the allowable stress design / working stress design approach, where the uncertainties in soil properties, loads and modelling are implicitly accounted for by a factor of safety.
   • Probability theory and reliability analyses provide a rational framework for dealing with uncertainties and making decisions under uncertainty. Depending on the level of sophistication, the analyses provide one or more of the following outputs: Probability of failure or probability of unsatisfactory performance, Reliability index, The most probable combination of parameters leading to failure, Sensitivity of result to any change in parameters
   • In reliability-based design, the uncertainties in load, resistance and modelling are quantified and the design objective is to ensure that the probability of failure of the component or system under consideration is less than a target value.

4. EXAMPLE APPLICATIONS

   • Several examples of application of the reliability approach to actual case studies are described to illustrate the added value of adopting the reliability approach. The cases studies included are: Settlement prediction of embankment on soft clay and settlement prediction for offshore structures Breach of a dam under operation Breach of a dam before construction Prioritising mitigation measures for a landslide dam Seismic stability of underwater slope Earthquake-triggered landslide management Partial safety factor for offshore foundation design Run-out of quick clay landslide

5. CHALLENGES AND EMERGING ISSUES

   • One of the basic challenges in the use of probability theory in geotechnics is that the classical frequentist statistics and sampling theory, which are the bases for derivation of probability distribution functions in most disciplines, are not of much use in geotechnical engineering.
   • A multi-risk approach is required for sound environment management and land use planning, as well as for a competent emergency preparedness and response in regions that are exposed to multiple hazards.
   • To reduce the hazard and risk, improved design, improved models, reduced uncertainty and improved risk management are required, in addition to improved risk assessment and management.

6. STRESS TESTING APPROACH

   • The QRA methods estimate the various components of risk, namely hazard, vulnerability and exposure of elements of risk, and utility or value of the elements at risk. The methods do this quantitatively and provide a value estimate of the expected loss.
   • In particular, the accident highlighted three areas of potential weakness in present safety approaches: Inadequacy of safety margins in the case of extreme external events (especially natural hazards). Lack of robustness with respect to events that exceed the design basis. Ineffectiveness of current emergency management under highly unfavourable conditions.