Form and internal structure of active-layer detachment slides, Fosheim Peninsula, Ellesmere Island, Northwest Territories, Canada
Three recent shallow landslides over permafrost are described. Slides occur in low- to medium-plasticity clays containing some bands of silts and fine sands. Slope failure results from rapid thaw at the base of the active layer of soil that is ice-rich due to antecedent two-sided freezing. Displaced slide blocks retain their integrity because of hardening of the active layer by cryodesiccation and summer evaporation. Blocks move over a soft basal shear zone a few millimetres to several centimetres thick. Compression in the toe zone of slides is low at sites where runout is possible, but in other locations causes emergent shears and complex folding. Failure histories are varied and range from simple unitary slides to complex sequential failures in which active-layer segments are mobilized progressively higher up the slope. This study demonstrates the importance of active-layer thermal and hydrological regimes, in addition to material properties, in determining the mode of slope failure.