SOME THOUGHTS ON SLOW, STEADY MOVEMENT OF LARGE LANDSLIDES

Large landslides of clay-rich materials have been observed to move for periods of days, months, or years at slow, relatively steady rates, without abrupt accelerations or catastrophic movements despite pore pressure fluctuations. Various mechanisms have been invoked to explain these mysterious observations. These include viscous or rate-dependent shear strength, plastic internal deformation, dilation, and forced circulation of pore water as the landslide deforms over asperities in the slip surface. Viscous models treat landslide movement as slow viscous flow, contrary to abundant observations that clay-rich landslides move mainly by sliding. The rate-dependent component of shear strength at slow rates of movement is weak. Plastic internal deformation of a landslide is not inherently rate dependent. Whether dilation attendant to internal deformation induces pore pressure gradients sufficient to regulate movement is unknown, as are the effects of clay intrusion.

Experiments by others have documented shear-induced dilation, with attendant suctions and temporary strengthening of deforming sediments. Stick-slip or start-stop movement has typically been observed in controlled-stress experiments. Sustained, persistent movement over days to years requires continuous regeneration (by reconsolidation) of the finite capacity of landslide material to dilate. In landslides that appear to move continuously, at least three possibilities exist: (1) stick-slip movement occurs at finer time scales (daily – subhourly) than typical measurements, (2) continual shifting of the shear zone occurs as small segments temporarily lock due to dilation and increased effective stress, (3) slip-surface asperities force alternating dilation and contraction. Stick-slip movement on daily to sub-hourly timescales is consistent with observed ranges of shear zone thickness and diffusivity. Shifting of shear zones occurs in many large slides, although the surface traces of lateral shear zones are commonly stationary through meters of displacement. Deformation and pore pressure gradients associated with slip-surface asperities might be difficult to observe instrumentally. Nevertheless, these possibilities suggest directions for future research on this topic.