Gelifluction: Observations from large-scale laboratory simulations

Despite extensive field studies, progress in understanding gelifluction pro cesses has been limited. Controlled laboratory simulation experiments offer an alternative and potentially extremely effective approach. Such an exper iment is described here. It was conducted on a 12 degrees slope formed of t wo natural soils, one a fine sandy silt derived from slate bedrock, the sec ond a gravelly silty sand derived from mudstone bedrock. Continuous measure ments were made of soil temperatures, porewater pressures, frost heave, tha w settlement, and downslope displacements of the soil surface over seven fr eeze/thaw cycles. Two-dimensional vectors of soil surface movements togethe r with evidence from excavated displacement columns suggest that geliflucti on occurred only during thaw consolidation of the upper parts of the soil p rofile; thawing of the deeper layers caused thaw consolidation but little d ownslope displacement. Cryogenic processes are shown to cause progressive d ecreases with depth in void ratio and moisture content and increases in und rained shear strength within the continuous soil matrix that separates ice lenses. Since self-weight stress levels are low, thawing leads to significa nt shear strain only in the softer, wetter near-surface soil layers.