In dam projects, the analysis of the rapid drawdown in the reservoir is commonly neglected, although it is capable of compromising stability. This is partly due to the complexity of the problem, that requires data related to transient flow in unsaturated soil: the water retention and permeability curves. The safety factor can be calculated as a direct measure of stability, and the introduction of a probabilistic approach on the analysis informs how likely the failure of the dam is under rapid drawdown conditions. This work presents results for the probabilistic analysis of the rapid drawdown in the reservoir of a rockfill dam with clay core. Two different velocities for the drawdown were simulated, and for each velocity two probabilistic methods were used to calculate the variance of the factor of safety and the probability of failure: first-order-second-moment (FOSM) and point estimation method (PEM). Both methods presented very similar results. The fastest drawdown had a calculated probability of failure of 20%, while for the other velocity it was around 5%. Although these are high values, these probabilities are conditioned by the occurrence of a rapid drawdown with the modelled features.
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