Safety Factors for Probabilistic Slope Design

by Robert A. D'Andrea, (M.ASCE), Assoc. Prof. of Civ. Engrg.; Worcester Polytechnic Inst., Worcester, Mass.,
Dwight A. Sangrey, (M.ASCE), Prof. and Head; Dept. of Civ. Engrg., Carnegie-Mellon Univ., Pittsburgh, Pa.,

Serial Information: Journal of the Geotechnical Engineering Division, 1982, Vol. 108, Issue 9, Pg. 1101-1118

Document Type: Journal Paper


Inconsistencies in present solution methods for slope stability problems under undrained conditions are noted and a first-order, second-moment, solution technique with a probabilistic base is suggested. The proposed procedure examines these problems with regard to the separate variables involved and utilizes partial safety factors which are proportional to the coefficient of variation of the pertinent parameters. For simplicity, a circular arc failure mechanism is assumed, and design acceptability is based on the derived value of a reliability measure, the safety index, which reflects the probability of occurrence of the assumed failure mechanism. Statistical data for the required load, resistance, and bias variables are presented. Using these, the safety index associated with current design techniques is determined, and the implications of its magnitude are examined. Sensitivity studies, performed to determine which variables have the greatest effect on design results are also described. Finally, partial safety factors are proposed for design corresponding to a desired failure probability.

Subject Headings: Failure analysis | Safety | Probability | Slopes | Slope stability | Parameters (statistics) | Load and resistance factor design | Sensitivity analysis

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