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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: “In 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 modelling”. 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 β is the reliability index).
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