37 void collapse. To overcome such difficulties, simplified stability charts may be combined with reliability concepts to characterize the risk of collapse of a void in the soil overlying the rock surface. Simplified Charts for Soil Stability Stability charts are widely used for the evaluation of soil slopes (Taylor, 1937; Bishop and Morgenstern, 1960) where the charts were developed in terms of the slope height and inclination, and the soil shear strength is expressed in terms of the soil cohesion intercept, c, and friction angle φ. These stability charts are typically presented in terms of a dimensionless stability number, N, which is often defined by Equation 1. (1) where N is a dimensionless stability number, γ is the unit weight of the soil, H is the height of the slope, and c is the cohesion component of the soil shear strength. Typically, the charts allow the potential for failure to be expressed in terms of a factor-of-safety (FS) or the ratio of the available soil strength to the strength required to maintain stability. (2) where the parameters c d and φ d are the corresponding values of the cohesion intercept and friction angle required to maintain equilibrium. Using some of the concepts originally applied to soil slopes, Drumm et al. (2009) prepared simplified charts for the evaluation of the stability of a void in the soil overlying the rock surface.