On the retrogression of landslides in sensitive muddy sediments: Discussion

Abstract

Introduction It was with interest that I read Carson's (1977) novel theoretical discussion of the parameters controlling the occurrence of retrogressive landslides in fine-grained sensitive deposits. I t would be extremely useful if we could predict, using only estimates of undrained shear strength and Carson's subsidence ratio, the susceptibility of landslides to retrogression, although it is not immediately apparent how this ratio might be estimated for a slope that has not failed. Although the theory Carson develops may have validity, he makes an implicit assumption about the mode of failure and retrogression that may well be invalid for at least four of the landslides he uses to test his theory. Applicability of his concept of retrogression is conditional upon the absence of buildup of pore-water pressure in permeable strata beneath the clay, a condition he neglects to state until the last page of his presentation. He also waits until the end of his paper to mention that beneath his landslides 1-4 (Table 1, p. 595) in the MaskinongC Valley there are indeed such sandy strata beneath the clay but that their absence in the other two flow slides he studied in the valley provides conclusive that underlying sand strata are not a prerequisite for retrogressive earthflows. Such conclusive evidence at two sites, however, hardly seems to justify ignoring or not investigating geologic data in the other four landslides beneath which buildup of such piezometric pressure in confined sandy aquifers is a very real possibility. Stratigraphic, sedimentological, geophysical, and geochemical investigations in this part of the upper MaskinongC Valley (Donovan 1977) have produced geological data that may well bear on the geotechnical problem but were not presented and only obliquely recognized in the Carson paper. It is the purpose of this note to present some of this geological evidence, which does not invalidate Carson's model but precludes