Probabilities of Failure and Factors of Safety in Geotechnical Engineering

Professor D.V. Griffiths

There has been a rapid growth of interest in both academe and industry in the use of probabilistic methods in geotechnical engineering. In hindsight this is not too surprising, since soils and rocks are among the most variable of all engineering materials, and geotechnical engineers must often make do with materials they are dealt with at any particular site. This inherent variability makes geotechnical analysis highly amenable to a treatment based on statistically described input parameters. The outcome of a probabilistic analysis typically leads to a probability of failure as opposed to the traditional factor of safety, representing a fundamental change in the way geotechnical engineers need to think about the suitability of their designs. The presentation describes methods of stability analysis of highly variable soils by numerical methods, and the implications this may have for geotechnical design outcomes. The relationship between the probability of failure and the factor of safety is discussed, and some methods of probabilistic analysis are described, ranging from event trees, to first order reliability methods to computationally intensive research methodologies such as the Random Finite Element Method (RFEM).

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