Probabilistic Evaluation of Earthquake-Induced Slope Failure

When there is an estimate of the probability of static failure for a set of slopes, based on historical records, maps, and subjective judgment, it is often necessary to estimate the additional hazard presented by earthquakes. Combining the procedures of reliability analysis with conventional recurrence relations for horizontal earthquake accelerations leads to a simple formulation that is easily implemented on a spreadsheet. For simple failure modes the dynamic factor of safety is a simple function of the static factor of safety, the amplified acceleration, the slope of the failure surface, and the friction angle. Application of the technique to the slopes of the Montes de Caneja in Puerto Rico indicates that the additional risk due to earthquakes is about one tenth of the static probability of failure, but other sets of input parameters can give probabilities of failure during earthquakes that are comparable to the static values.