New data and re-evaluation of the 1965 Hope Slide, British Columbia

The 1965 Hope Slide is one of the largest rock avalanche to have occurred in recent historic time. Although this landslide is very well known, virtually no comprehensive investigation was undertaken. This study represents a first, but essential, step to begin a detailed evaluation of the mass movement; it is also part of a research program investigating the landslide hazards along strategic transportation corridors in southwestern British Columbia (Savigny, 1990, in prep.). The geology at the slide site was confirmed to consist of greenstone and felsite, however, two varieties of each of the rock type were found: the greenstone occurs in a massive and a slightly schistose form, and the felsite occurs in a buff coloured and a greyish-white coloured variety. Discontinuities at the study site include two steeply dipping faults, three dominant sets (Jl, J2 and J3) and a shallower dipping set of joints, the orientations of the latter set closely relate to those of Jl, and a number of gouge filled shear zones along the buff felsite and greenstone contacts. The 1965 failure surface was probably controlled by two mechanisms, in which the steeper upper portion of the slope was largely controlled by pervasive step-like discontinuities (Jl and the shallower joints); the shallower lower part of the slope was controlled by gouge filled buff felsite-greenstone contacts. These two mechanisms also support the two slide events hypothesis put forward by Weichert et al. (1990), who suggested that the seismic signals recorded on the day of the landslide were the results, rather than the cause, of the mass movement(s). Based on the evidence found in this study, it is proposed that the lower slope (below the upper northeast trending fault) failed first along the gouge filled lithologic contacts, due to the debuttressing effects of the lower slope and the existing weakness along the joints, the upper slope subsequently failed. Slope stability analyses indicate that-the slope was in critical conditions prior to the 1965 slide. The results also demonstrate that the inherent weakness withinjthe rock mass was sufficient to explain the occurrence of failure without external influences.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat