The failure of a slope cut into the weathering profile developed over a porphyritic biotite granite

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At about 1.00 a.m. on 24 September, 1981, at km 24.5 of the Kuala Lumpur–Karak highway, there occurred a failure of a 26 m high slope cut into the weathering profile developed over a porphyritic biotite granite. The failure, which is best classified a slump-flow, occurred some 5.7 years after the end of excavation and involved the downslope movement of about 2000 m 3 of weathered bedrock material (mainly gravelly silty sands) and a few coreboulders. Continued movement of failed material through minor slumping and gulleying led to eventual exposure of the failure surface which was mainly located along a relict fault plane striking parallel to, and dipping 48° towards, the highway. Failure occurred as a result of several converging factors; the triggering factor being ground vibrations set up at the time of passing of three large trucks. Conditions of instability, however, were already present in the slope materials through infiltration of rainwater that led to saturation and the loss of apparent or effective cohesion. Infiltration had been encouraged by clearing of vegetation at the top of the cut and the creation of a track along its western edge some 6 months prior to failure. Infiltration was also encouraged by rainfall patterns with the failure occurring after some 29 days with almost continuous daily rainfall; this wet period being preceded by an unusual 27 days of no rain. It is concluded that stability assessments of slope cuts in weathering profiles over granitic bedrock need to take into consideration not only shear strength parameters, but also local climatic and other environmental factors, including likely triggering factors.