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In Southern California the conventional method for evaluating surficial stability has a bias towards cohesion. It suggests that surficial failures are more likely in slopes made of sandy and gravelly soils, than those composed of clays and silts. This contradicts actual observations. A study of the equations governing seepage and rainfall data in Southern California suggests that permeability plays an important role in the surficial stability of slopes. In homogeneous slopes this study finds that soils with a low permeability are more prone to develop the conditions leading to a surficial failure mechanism. Hence, clayey and silty soils slopes are more prone to develop surficial instability than slopes made of sandy or gravelly soils. This study also finds that soils with a permeability greater than a certain limiting value, klim, will not become saturated. It is found that the Unified Soil Classification System is useful in evaluating the likelihood that a surficial failure mechanism with seepage parallel to the face of the slope may occur.
Pradel et al. (1993) studied this question.