The dam safety profession is moving toward risk-based decision making. The likelihood of failure (or unsatisfactory performance) is a key input to such evaluations. This paper focuses on simple methods for estimating the probability of sliding instability using standard spreadsheet programs and commercially available add-in macro software. The deterministic equations for factor of safety are programmed into a spreadsheet, but the input parameters are defined as distributions instead of single point values. The so called "Monte-Carlo" simulation process is used to automatically generate many values of factor of safety by sampling each of the input distributions. For the purposes of this paper, the probability of failure is determined as the area under the output factor of safety distribution for values less than 1.0 (although another value defining unsatisfactory performance could also be used). Examples are presented for sliding of a concrete gravity dam on a poorly bonded lift joint during flood loading, and for post-liquefaction slope instability of an embankment dam.
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