Using a software package that can be used for either three or two-dimensional (2D) slope stability analyses of the same model we analyse the stability of columns (3D) or slices (2D) overlying a shear failure surface. The failure surface can be any shape, such as that formed by interconnecting fault surfaces in an open pit slope failure that we analyse. We show that there is a significant difference in the factors of safety calculated using 3D analyses, compared with 2D sections through the same model. This means that different shear strength parameters will apply when the values are varied to obtain a Factor of Safety of 1.0. Systematic back-calculations of open pit slope failures using appropriate 3D analysis methods can give a better understanding of appropriate shear strengths to use in forward 3D analyses. This should lead to more efficient pit slope designs.