Moving from 2D to a 3D Unsaturated Slope Stability Analysis

Two-dimensional (2D) limit equilibrium analyses remain the most common method of analysis in slope engineering practice. It is commonly perceived that 2D slope stability analysis always provides a more conservative estimate of the 3D slope stability problem. Most previous studies comparing 2D and 3D stability analysis also ignore the effect of negative pore-water pressures (i.e., matric suctions) in the soil zone above the groundwater table. In this paper, a comparison study is reported between 2D and 3D slope stability analysis for soil slopes with a portion of the soil profile having matric suctions. The differences between a 2D and a 3D factor of safety are found to be heightened when unsaturated conditions are considered. The paper also presents a framework for the inclusion of soil suctions in the calculated factor of safety for geotechnical engineering practice. A discussion on reducing a 3-D numerical model to give answers similar to a 2-D slope stability analysis will be presented. A basis for transitioning to 3-D analysis will be developed.