Extension of Hassan and Wolff method for system reliability analysis of soil slopes

• The characteristics of slope system reliability are revealed.
• The accuracy of the Hassan and Wolff method is assessed.
• The Hassan and Wolff method is extended for system reliability analysis.
• A procedure is suggested to judge if the system effect is obvious.
• Equations are presented to estimate the bounds of system failure probability.

While a soil slope may have a large number of potential slip surfaces, its system failure probability is usually governed by a few representative slip surfaces due to the high correlation between factors of safety (FOS) of different slip surfaces. This paper shows that, despite its embedded empiricism, the Hassan and Wolff method can identify with reasonable accuracy not only the most critical slip surface, but also representative slip surface for system reliability analysis. In the Hassan and Wolff method, the assumed distribution of FOS has important effect on the calculated failure probability and it is not appropriate to always assume FOS is lognormally distributed. Whether the system effect in a slope reliability analysis is obvious can be judged easily by comparing the values of reliability index of different representative slip surfaces. Combined with the response surface method, the Hassan and Wolff method can be extended into a practical tool for system reliability analysis utilizing existing deterministic slope stability analysis programs. A simple equation is provided to estimate the bounds of system failure probability based on the most critical slip surface.