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On October 1, 1980, a moderately intense storm at Trap Bay, Alaska, triggered a small debris slide in a forested midslope depression that was instrumented to record the maximum piezometric rise. Although the return period for the storm was <2 years, 54% (3.68 cm) of the total rain fell during the final 3 h causing the soil mantle to become nearly saturated. This resulted in an average maximum pore-water pressure of 2.2 kPa at the sliding surface, causing failure. Analysis of the mechanics of the failure showed that an apparent cohesion of 2.0 kPa, probably due to rooting strength, was acting on the 43° hillslope. Calculated factors of safety for such thin, partially saturated soils are very sensitive to small changes in apparent cohesion and should be used with caution on such sites.
Sidle et al. (Sat,) studied this question.