Gelifluction: viscous flow or plastic creep?

This paper reports results from two scaled centrifuge modelling experiments, designed to simulate thaw‐related gelifluction. A planar 12° prototype slope was modelled in each experiment, using the same natural fine sandy silt soil. However two different scales were used. In Experiment 1, the model scale was 1/10, tested in the centrifuge at 10 gravities (g) and in Experiment 2, the scale was 1/30, tested at 30 g. Centrifuge scaling laws indicate that the time scaling factor for thaw consolidation between model and prototype is N2, where N is the number of gravities under which the model was tested. However, the equivalent time scaling for viscous flow is 1/1. If gelifluction is a viscosity‐controlled flow process, scaling conflicts will therefore arise during centrifuge modelling of thawing slopes, and rates of displacement will not scale accurately to the prototype. If, however, no such scaling conflicts are observed, we may conclude that gelifluction is not controlled by viscosity, but rather by elasto‐plastic soil deformation in which frictional shear strength depends on effective stress, itself a function of the thaw consolidation process. Models were saturated, consolidated and frozen from the surface downwards on the laboratory floor. The frozen models were then placed in the geotechnical centrifuge and thawed from the surface down. Each model was subjected to four freeze–thaw cycles. Soil temperatures and pore water pressures were monitored, and frost heave, thaw settlement and downslope displacements measured. Pore water pressures, displacement rates and displacement profiles reflecting accumulated shear strain, were all similar at the two model scales and volumetric soil transport per freeze–thaw cycle, when scaled to prototype, were virtually identical. Displacement rates and profiles were also similar to those observed in earlier full‐scale laboratory floor experiments. It is concluded therefore that the modelled gelifluction was not a time‐dependent viscosity‐controlled flow phenomenon, but rather elasto‐plastic in nature. A first approximation ‘flow’ law is proposed, based on the ‘Cam Clay’ constitutive model for soils. Copyright © 2003 John Wiley & Sons, Ltd.