Performance of three-dimensional slope stability methods in practice

Study of several field case histories has shown that the difference between two- and three-dimensional factors of safety is most pronounced in cases that involve a translational failure. Two- and three-dimensional slope stability analyses of field case histories and a parametric study of a typical slope geometry revealed that commercially available three-dimensional slope stability programs have a number of limitations with respect to (1) accounting for the shear resistance along the sides of the sliding mass; (2) modeling the stress-dependent nature of failure envelopes of the materials involved; and (3) considering the internal forces in the slide mass. These limitations can significantly affect the calculated factor of safety for a translational failure mode. A technique is presented to overcome some of these limitations and provide a better estimation of the three-dimensional factor of safety. Field case histories are presented to show the importance of using a three-dimensional analysis in back-calculating the mobilized shear strength of the materials involved in a slope failure and in the design of slopes with complicated topography, shear strength conditions, and/or pore-water pressures.