For analysis and design work involving the assessment of slope stability of spoil piles in strip coal mines, it is very useful to adopt an approach based on estimated risk of failure. Studies of several spoil pile failures in Queensland (Australia) have revealed a two-wedge failure mode and it is important that probability-based risk studies are made considering such a failure mechanism. Calculations for failure probability were first made using Monte Carlo simulations and assuming a potential two-wedge mechanism. An alternative procedure based on Rosenblueth's method of estimating moments was then used to compare the results with those based on simulation. Excellent agreement was found and it is recommended that, for practical purposes, the relatively quick Rosenblueth method should be used in estimating the probability of slope failures. The study also confirms the suitability of Normal and Beta distributions in this context in geomechanics.