Selecting Minimum Factors of Safety for 3D Slope Stability Analyses

Geotechnical engineers understand there is uncertainty and risk in the input parameters for slope stability analyses and within the analysis methodologies themselves. Decades of research and inverse analyses of slope failures have resulted in widespread acceptance of certain factors of safety (FS) in typical situations, e.g., a static two-dimensional (2D) factor of safety of 1.3 is often used for temporary or low risk slopes and 1.5 for permanent slopes. However, these FSs are not appropriate for use with three-dimensional (3D) analyses because 3D analyses account for additional shear resistance that is generated along the sides of the slide mass. The contribution of the additional shear resistance can be significant in shallow slide masses or for translational slide masses with a width to height ratio less than six, resulting in calculated values of 3D FS that are greater than the calculated 2D FS. To achieve the same level of safety or risk as a static 2D FS of 1.3 or 1.5, the user must use a greater minimum FS for 3D analyses. This paper presents methods for calculating a suitable minimum 3D FS to achieve a similar level of safety or risk as a minimum 2D FS, such as 1.3 or 1.5, would afford.