ABSTRACT
The prevention of Rapid Drawdown (RDD) slope failures controls the upstream slope design of many earth dams. The factor of safety against rapid drawdown failure is typically evaluated deterministically using undrained limit equilibrium or finite element approaches. Application of probabilistic methods for rapid drawdown is illustrated in this paper using data from Cobbs Creek Dam in Virginia. The variability of shear strength parameters is based on data from 57 isotropically consolidated undrained triaxial compression tests. Finite element RDD analyses are used to develop an explicit performance function. Three methods are used to evaluate probability of failure following rapid drawdown—Taylor series, Hasofer-Lind, and Monte Carlo. Assuming that rapid drawdown occurs, the different methods result in a probability of failure between 35 and 45% for a design with a deterministic factor of safety of 1.1. Consideration of the effects of spatial averaging decreases the probability of failure to about 32%. The probability that RDD occurs should also be considered when assessing the likelihood of failure.
