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Slope stability of Nemo and Wee Sandy Creek basins near Slocan Lake, British Columbia Pack, Robert Taylor


In order to determine the possible impacts of forest engineering on landslide occurrence in four eastward-draining basins near Slocan Lake, southeastern British Columbia, slopes are evaluated according to a landslide hazard classification scheme based on natural terrain subdivisions, a stochastic geotechnical model, and past engineering experience. Mass wasting processes currently at work in the area include shallow debris avalanches, debris flows, rockslides and rockfalls, and involve complex glacial and colluvial deposits overlying coarse grained plutonic and high grade metamorphic bedrock. Primary factors known to influence landslide occurrence in the region include slope angle, soil shear strength, tree root strength, groundwater, and shear plane geometry. A stochastic geotechnical model can only be applied to uniform slopes mantled with surficial material because of inherent assumptions and requires quantitative estimates of parameters for broad slope units. Ranges are estimated for values of angle of internal soil friction, soil cohesion, root cohesion, piezometric head, depth to shear plane, soil bulk density, tree surcharge weight, and slope angle. From the model it is possible to explain the observed distribution of many landslides in the study area and surrounding region in terms of expected factor of safety and probability of failure. However, the probabilities cannot actually predict the number of landslides likely to occur on a particular slope, nor the likelihood of a landslide occurring within a certain time period. Accurate, quantitative predictions of landslide occurrence can be made only where model variables are less subjectively determined, and where probabilities are calibrated and compared with observed events. These semi-quantitatively determined indices of stability are best used to compare the stability of slopes in the study area with slopes that have responded unfavorably to forest engineering in other areas. From such comparisons the indices can be grouped to form hazard classes of use to forest managers and engineers. In areas where the geotechnical model does not apply, hazards are assigned according to past engineering experience in natural terrain units similar to those in the study area. These units include colluvial fans and aprons, debris fans, and steep rocky terrain. Slopes classed in the 'very high hazard' group include those slopes which show signs of active landsliding as indicated by morphology or vegetation, and steep rocky terrain dominated by gravity processes. Slopes classed in the 'high hazard' group include colluvial fans, upper parts of debris fans, and slopes mantled with surficial material having probabilities of failure greater than 10%. Slopes classed in the 'moderate hazard' group include lower parts of debris fans and slopes mantled with surficial material having probabilities of failure less than 10% but expected factors of safety less than 1.6. 'Low hazard' slopes include gently sloping exposed bedrock and slopes mantled with surficial material having expected factors of safety greater than 1.6. The landslide hazard classification scheme has practical merits for use in planning road alignments and logging systems.

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