Back Analysis of a Slope Failure in Permafrost in the Mackenzie Valley, Canada

A small landslide was observed next to an existing large slide in the Travaillant Lake area in summer 2005. The area was burned by forest fire in 1998. Information available from air photos, field observations and recent photographs indicated that the observed slope failure is relatively new (probably within the last year or two). This paper describes field investigations conducted in the area in late summer 2005, and slope stability studies carried out to back analyze the slope failure mechanisms. Test pits were excavated in the area to investigate subsurface soil conditions and soil samples were taken for geotechnical laboratory testing. Field vane shear tests were conducted on the active layer to measure the shear resistance. Laboratory tests showed that the moisture content in the active layer increased with depth. The field vane shear tests indicated very low undrained shear strength of the fine-grained active layer soils. Slope stability analyses revealed that the slope would be in a critical condition when the vane shear strength parameters were used. The studies provided insights into understanding initiation of landslides in the region. The analyses also indicated that the field vane could be a useful tool for slope stability studies in fine-grained permafrost soils.