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The significance of progressive failure in slides of natural or cut slopes is reviewed and qualitative methods for taking it into account are discussed. Nevertheless, it is concluded that present procedures do not permit reliable assessment of the factor of safety of natural or cut slopes under most conditions encountered in practice. This conclusion is illustrated by the history of a landslide that destroyed the Lower Baker Power Plant near Seattle. The failure occurred while the sliding mass was thought to be temporarily under control although remedial measures were being planned. The reliability of predictions is enhanced in old landslide areas, provided the existence of the old slide material is recognized.
Ralph B. Peck (Sat,) studied this question.