Why Consider Reliability Analysis for Geotechnical Limit State Design

There is a need to draw a clear distinction between accepting reliability analysis as a necessary theoretical basis for geotechnical design and downstream calibration of simplified multiple- factor design formats, with emphasis on the former. Reliability analysis provides a consistent method for propagation of uncertainties and a unifying framework for risk assessment across disciplines (structural and geotechnical design) and national boundaries. Simplified reliability-based design (RBD) equations are probably required for routine design at present, but their limitations have no bearing on the generality of reliability theory. If reliability analysis is accepted as the basis for developing multiple-factor formats, then it is necessary to define the characteristic values in an unambiguous way with reference to the probability distribution function. The key consideration is that the engineer should not be allowed to introduce additional conservatism into the design by using some lower bound value, when the RBD equations are calibrated using, say, mean parameters. The implementation of reliability-based LRFD equations, along with potential advantages and pitfalls, is discussed using a specific example. If the goal of LRFD is to maintain uniform reliability, the example shows that a single resistance factor is not adequate. In practice, it is probably sufficient to partition the parameter space (spanning typical ranges of deterministic and statistical parameters) into smaller domains and calibrate a single resistance factor for each domain. Deviations from the target reliability index can be controlled to an acceptable level by adjusting the sizes of the domains.