Uncertainty in Drained Fully Softened and Residual Strength Correlations

Using previously proposed values of standard deviation (σ) for the drained fully softened and residual strength correlations, the probability of failure (Pf) can be calculated to supplement the drained factor of safety (FoS). Using the standard deviation (σ) of soil strength, e.g., fully softened (FS) and residual, the standard deviation of FoS (σFoS) and coefficient of variation of FoS (VFoS) can be calculated readily using slope stability software to estimate Pf. Being able to calculate Pf allows owners, regulators, and/or engineers to select a frequency of failure for a particular slope given the consequences of failure instead of a nebulous value of FoS. For example, it is easier for owners, regulators, and/or legal representatives to understand a frequency of failure, e.g., 1 failure in 1,000 years, than a drained or effective stress FoS of greater than 1.5 with some level of uncertainty. Even more confusing to owners, regulators, and/or legal representatives is the difference between various values of drained and undrained FoS, which is simplified via the selection of a suitable value of Pf.