Factor of safety calculation for unstable rock slopes using finite element methods

Factor of safety (FoS) procedures while using finite element methods (FEM) require stable calculation models. When FoS < 1 this condition is not verified, and the safety analysis cannot be completed. This paper describes an approach to obtain the FoS for unstable rock slopes, considering the generalized Hoek-Brown (HB) failure criterion and using Plaxis 2D software for the calculations. The proposed methodology defines a modified HB failure criterion (Material beta), with better properties than the original (Material alpha), by applying an upgrade factor (mu) in its yield function (f(HB)). Then, FOS beta >= 1 is determined by calculating with the modified material, and FoS(alpha) would be the result of dividing FOS beta by the corresponding upgrade factory mu. Various materials are required to calculate the same FoS to improve the accuracy of the results. This procedure has been tested with a rock slope example for different material conditions.