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Surficial failures are quite common and are caused by a reduction in shear strength due to saturation and swell coupled with a condition of seepage parallel to the slope face. An equation relating factor of safety to depth of seepage flow is used to investigate the surficial instability of compacted clays. The major unknowns in this equation are the drained strength parameters (ϕ′, C′) and depth of seepage flow. It is concluded that surficial stability evaluations, which are commonly performed in a fashion similar to that prescribed by Los Angeles County, may result in an overestimation of the factor of safety. The reason is that, for the tested clays, the effective cohesion intercept for gross stability is not appropriate for surficial stability analysis since it is an extrapolated value which is much greater than actual cohesion at low effective stresses.
Day et al. (1989) studied this question.