The influence of the unsaturated soil zone on 2-D and 3-D slope stability analyses

It is commonly understood that a 2-D slope stability analysis will provide a lower factor of safety than a 3-D slope stability analysis. The difference in the calculated factors of safety between a 2-D and a 3-D analysis is generally less than 15% for simple slope geometries. Most past comparative studies between 2-D and 3-D stability analyses have ignored the effect of negative pore-water pressures (i.e., matric suctions) in the soil zone above the groundwater table. In this paper, a comparison is made between 2-D and 3-D slope stability analyses on soil slopes where a portion of the soil profile has matric suctions. The factors of safety on simple geometry slopes and complex geometry slopes (i.e., slopes which have two intersecting slope surfaces), are investigated for a range of shear strength parameters and groundwater conditions. For simple slopes with a low slope angle, the difference in factor of safety between a 2-D and a 3-D slope stability analysis, (i.e., ΔFs/Fs2 -D), generally ranges from 9% to 16% when ϕb is equal to 15°. The value of ΔFs/Fs2 -D for a steep, simple slope is generally larger than for a low angle, simple slope. When ϕb is 15°, the values of ΔFs/Fs2 -D for the simple, steep slope generally range from 12 to 18%. The difference between a 2-D and a 3-D stability analysis was most pronounced for concave geometries where a portion of the soil profile contained unsaturated soils. The values of ΔFs/Fs2 -D for corner angle concave slopes with angles ranging between 180 to 270° can be as large as 20 to 59% when ϕb is equal to 15°. Two case histories, (i.e., the highwall stability failure at the Poplar River coal mine and the Kettleman Hills landfill slope failure), were used to illustrate the effect of the unsaturated zone on changes in the factors of safety.