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Deterministic design approach Physical capacity Traditional safety margin Probabilistic design approach Physical capacity Refined safety margin Refined calculated capacity Calculated capacity Calibrated factor of safety Factor of safety Refined design capacity Unnecessary safety margin Design capacity Figure 9.1: Capacities of deterministic and probabilistic design approaches (after Lacasse and Nadim, 1996) Phoon, et al. (2006) state that: âIn principle, no category of approaches is preferable over another. The quality of geotechnical characterisation or design is not conceptually bound to the level of explicitness of soil variability modellingâ. Whitman (2000) discusses how good communication with the client is essential in determining the type of analysis required. If the client (or regulator) is not interested in quantifying risk as part of their decision-making then traditional deterministic methods will continue to be used. When the risks are large and the cost of absolute safety is large, then clients may be interested in discussing risks and a probabilistic analysis may then be required. If there are no guidelines or regulated standards on acceptable probabilities of failure then deterministic approaches will continue to be used as there are accepted, published, factors of safety (e.g. ANCOLD recommends a minimum factor of safety of 1.5). The published values are generally recommended minima; setting exact factors of safety or probabilities of failure for a design is unsuitable in geotechnical engineering as each structure is inherently different. Table 9.1 shows the results of case 1 (undrained and drained conditions) and the case study results from the deterministic and probabilistic analyses (where F is factor of safety, P(f) is the probability of failure and Î² is the reliability index). 117