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Investigation of the stability of the steepland forest soils in the coast mountains, southwest British Columbia O'Loughlin, Colin Lockhart

Abstract

In British Columbia's Coast Mountains a large area of forested steepland of probably more than 100 km² , is clearfelled annually by high-lead logging methods. Although the deforestation of steep slopes has been shown to seriously accelerate mass wasting in Alaska, Oregon and elsewhere in the United States, the effects of clearfelling on slope stability are largely unknown for coastal British Columbia. The objective of this study was to determine the extent and seriousness of lands!iding on undisturbed, forested slopes and on clearfelled slopes and elucidate some of the natural and human-caused factors which are detrimental to slope stability in the Coast Range. Within a study area of 640 km², 77 large landslides of the debris avalanche or debris slide type were discovered. Landslides not associated with roads were predominantly confined to long, uniform slopes of over 30° underlain by poorly-drained podsolic soils. Slopes underlain by shallow, regosolic soils were relatively resistant to mass wasting. Large landslides were more frequent on clearfelled areas than on undisturbed slopes. Road construction, which was responsible for 14 large landslides and more than 100 smaller failures, appeared to be more detrimental to the stability of the Coast Range slopes' than other activities carried on by man. A network of simple piezometers established in steep drainage depressions revealed that the piezometric surface within the soil mantle approached the ground surface during rain storm or snow melt periods to cause marked increases in pore water pressures. Curvilinear relationships between the piezometric head and the daily (24 hr.) rainfall indicated that daily rainfalls which exceeded approximately 120 mm caused complete saturation of most drainage depression soils. Pore water pressures larger than 800 kg/m² were recorded at the base of the soil mantle. Such pressures significantly reduce the effective normal stress acting on potential failure planes at the soil-basal till or soil-bedrock interface and decrease the stability of the slope. General observations of the growth habits of tree root systems in the steepland soils suggested that roots help bind the cohesionless soils into a coherent mantle and anchor the mantle to the substratum. Direct shear tests performed in the field indicated that the soil strength was linearly related to the bulk weight of roots in the soil. Where root networks were dense the soil's shear strength may be increased by several hundred kg/m² compared to soils with few or no roots. Under saturated conditions the forest soils' shear strengths are largely derived from the apparent cohesion provided by the tree root network. Laboratory strength tests of tree roots showed that Douglas fir and cedar tree roots deteriorate rapidly after death of the parent tree. Within 3 to 5 years after cutting of the parent tree, small roots may lose over half of their original tensile strength. Five small landslides were investigated with simple, approximate, stability analyses encompassing a range of possible shear strength and soil water conditions. The results confirmed that the stability of road-fill slopes as well as natural, forest soil slopes is very sensitive to changes in the ground water conditions and that the stability of the forest soil mantle on steeper slopes is very dependent on the reinforcement provided by roots. In the light of these findings it is apparent that current forest cutting and road building practices on the steep slopes of British Columbia's Coast Mountains are not compatible with sensible mountainland management which fosters protection of the soil resource.

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