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 of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The purpose of this study is to understand the environment for failure of the Perpetual Landslide. To achieve this objective a field investigation was carried out to evaluate the movement pattern of the slide and its geologic and hydrologic environments. This information, along with ground water flow and stability models was used to define major controls on stability. The following points are made: (1) The slide moves by rotation and translation of blocks with a transition to flow movement at the toe. (2) At depth, the failure surface lies within the Tertiary sedimentary rocks. Exposed gouge consists primarily of clay (most likely remolded claystone and some clay-rich till) with dispersed pebbles and rock fragments. (3) Discontinuities including inherent heterogeneity between and within geologic units, weathering, and jointing are significant to the unstable situation. (4) Changes in stress equilibrium, particularly those caused by removal of overburden and lateral support with downcutting in Trout Creek Canyon, are likely important in the origin of the slide. (5) The proposed mechanism of failure is: (a) The progressive reduction from peak to residual strength of the claystone. (b) In addition to the high water table, high pore-water pressures along the failure surface. At the present time, the slide's continuous movement is acting to establish a new stability in equilibrium with this ground water flow system and lower strength. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

During the late winter of 1855-56 or early spring of 1856 about 33,000,000 cubic yards of volcanic rock slid from the high cliff known as The Barrier, near Garibaldi, B.C. This debris travelled down a rather sinuous path along Rubble Creek valley to its confluence with Cheakamus River about A miles from the Barrier and about 3400 feet lower. The initial material appears to have travelled as a high velocity tongue of debris which swept from one side of the valley to the other as the debris stream rounded curves eventually to be deposited on Rubble Creek fan. Velocities calculated from the superelevation of the debris as it rounded three different curves indicate that the debris was moving between 88 and 110 feet per second. A minimum velocity of 80 feet per second was calculated using the principle of conservation of energy where the debris overtopped a small hill at the apex of the fan. All of the trees in the path of this slide were uprooted and carried away. The trees adjacent to the slide were scarred and bruised by moving debris. The initial high velocity tongue was apparently followed by mud flows which deposited large rounded boulders and poorly sorted, volcanic debris on an area of the fan which was not covered by the initial slide. This material was apparently slow moving, as it piled up high on the uphill side of some trees which later died and fell across the top of the debris. Some xenolithologic debris cones similar to those found at Sherman Slide in Alaska and elsewhere also occur in the area of mudflow material. The slide deposit is formed of angular poorly sorted volcanic clasts weighing up to about 250 tons. The slide debris can be distinguished from underlying fan deposits by the lack of fine gravel and silt sized particles in the fan material. Deposits of debris similar to the debris of the 1856 slide, beneath some of the fan deposits, show that an earlier slide may have occurred. The mechanism which triggered the landslide is not known, but blockage of a subsurface drainage system, which drains the area behind The Barrier and escapes as springs at its toe, could have raised groundwater pressures enough to trigger the slide. In addition, as the area is one of recent volcanic activity a local earthquake may have been the immediate cause. In any event the underlying cause for the landslide was that the excessively steep and high cliff face of lava was apparently deposited against glacial ice, and subsequently, lost support when the ice melted. Studies using a scale-model of the topography of the area and bentonite slurries were carried out to find out if the movement of the 1856 slide could be modelled and if so, could the movement of possible future slides be predicted. Although no mathematical basis was developed for the modelling it is thought that if a material could be found which modelled the complex movement of the 1856 slide,future slides could also be modelled. Although modelling of the 1856 slide was not entirely successful several insights were given into the movement and deformation of prototype slides of the same type as Rubble Creek Slide. There has been at least one destructive slide in the area of Rubble Creek fan in the recent past and because it cannot be demonstrated that conditions have changed substantially since the 1856 slide it is only prudent to accept the possibility of the occurrence of another slide in the near future. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

It is being increasingly realized that there exists an interrelationship between seismicity and rock slope failures. Possible chronological clustering of rock avalanches in the Fraser River corridor was investigated to determine if a common seismic trigger existed. It was determined that the events occurred throughout the Holocene indicating that either these slides were not seismically triggered or that seismic triggers were chronologically unrelated. Cascadia Subduction Zone earthquakes are believed to have occurred throughout the Holocene (Adams, 1989; Atwater, 1987; Hull, 1987). The ages of the earthquakes were compared to the ages of rock avalanches in the Fraser River corridor, but no distinct correlation could be made. The lack of distinct correlation between large rock avalanches in Fraser Corridor and paleoseismicity, and the absence of event clustering, indicated either seismicity was not a factor, or that these rock avalanches may not have been susceptible to seismic triggering. A stability study of Cheam Slide was performed to investigate the susceptibility of large rock avalanches to earthquake triggering. The results suggested that the seismic susceptibility of a slope is closely linked to the displacement the slope must undergo for failure to take place. A large critical displacement may render the slope relatively insensitive to seismic triggering, while a low critical displacement may result in high seismic susceptibility. A comparison was made between the effects of seismic and pore pressure related triggering. The results indicated that a high critical displacement slope, which is close to failure, may be more likely to fail by high pore pressures than by seismic loading. Low critical displacement slopes which are stable enough to surviving hydrodynamic loading may, because of their susceptibility to seismic triggering, pose the greatest hazard. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate