Laboratory simulation of periglacial solifluction: Significance of porewater pressures, moisture contents and undrained shear strengths during soil thawing

Laboratory simulation of periglacial solifluction in two natural soils, one a sandy silt, the other a gravelly silty sand, was undertaken on a 12 degrees slope in a thermally controlled 5 m square chamber. Soil freezing and thawing took place from the top downwards in an open hydraulic system. Frost heaving and surface downslope movements were much greater in the silt-rich soil than in the sandy soil. In both cases, solifluction took place during the thaw period. At any given depth, porewater pressures rose rapidly immediately following soil thaw, and were generally positive for several days, though not in excess of hydrostatic pressure. Soil moisture contents following thaw exceeded the liquid limit and measured in situ undrained shear strengths were low, ranging between 0.8 kPa near the surface and 9.0 kPa at depth. Conventional slope stability analyses based on total stress and effective stress conditions indicated that the thawing slope was not susceptible to rapid failure. It was concluded that the observed soil displacements represent post-thaw strain when soil moisture contents were high and rheological properties were intermediate between those of a viscous fluid and a plastic creeping solid.