Stability Analysis of Slopes with Spatially Variable Strength Properties

Natural variability of rock properties can significantly affect the strength of rock masses and factor of safety of slopes. The results of a comprehensive point load testing program showed that coefficient of variation of intact rock strength can reach unity in highly heterogeneous formations. Probabilistic numerical analysis was carried out to explore the effect of strength variability on uniaxial compressive strength of large heterogeneous samples. It was shown that mean large-scale strength decreases with increasing small-scale variability. The effect of spatial variability of strength properties on slope stability was examined using limit equilibrium and shear strength reduction methods. Both approaches gave similar results indicating that for stable slopes, increasing strength variability leads to a reduction in mean factor of safety and increase in the probability of failure. In addition, ignoring spatial variability in probabilistic slope analysis can lead to erroneous estimates of the probability of failure. Based on the results of probabilistic analyses on large heterogeneous samples and slopes, an equivalent uniaxial compressive strength can be obtained by reducing the mean strength by one-third of its standard deviation. This relationship was validated using a dataset of back-analyzed strength values in heterogeneous open pit slopes.