Slope instability in a profile of weathered norite

Abstract

Abstract On Christmas Day 1982, a combination of heavy rain and a broken water supply pipe resulted in significant mass movement on a hillside in the Bukit Gombak area of Singapore. Subsequent investigations revealed a sliding mass of about 50000 m 3 , approximately 100 m from crest to toe in plan with a height of 22 m. Average slope angle was about 12°. Ground conditions consisted of fill overlying residual soil derived from the underlying norite bedrock. This resulted in a situation where heavy rainfall caused rapid rises in ground water levels followed by very slow dissipation. Mass movements occurred when water levels in the slope rose above a certain value. From observations and local knowledge of the area, it was concluded that the mass movements had been taking place over many years previously and were still continuing. The geometry of the actual failure surface was not established, however analyses of a number of possible slip surfaces indicated an average ø’ of less than 18°. Two ring shear tests on samples of residual soil from the slope gave values of ø r ’ a little below 15°. Thus, although the data measured are limited, these results suggest that the stability of the slope was controlled by residual strength. Furthermore, the case history demonstrates how permeability distribution within a slope can have a major influence on its stability.