Clast Plowing as a Potential Flow Instability for Soft-bedded Glaciers

Fast flow of most glaciers is controlled by conditions at the ice-bed interface. When glaciers move over deformable till, clasts at the ice-bed contact provide the roughness that resists glacier motion. These clasts, however, plow through the bed surface when basal water pressure is high. Factors that control plowing resistance, including plowing speed, are therefore important to study. A ring-shear device was used to study the resistive forces on plowing clasts as a function of plowing speed. Hemispheres of different sizes were dragged through water-saturated till at speeds ranging from 15 to 380 m/year. Resistive forces on the hemispheres and pore-water pressure in adjacent till were measured. Tests were conducted with two tills that differed in hydraulic diffusivity by two orders of magnitude. Results indicated that shear resistance decreased dramatically with increased plowing speed. At high plowing speeds, excess pore-water pressures were generated in front of the hemispheres because the rate of till compaction exceeded the rate at which pore pressure could diffuse. The excess pore-water pressures weakened the till in front of plowing clasts and decreased resistive forces. Using measured pore pressures, resistive forces on a plowing hemisphere were estimated successfully with a geotechnical theory of cone penetration. These laboratory results indicate how plowing speed controls resistive forces on plowing clasts. Increases in glacier sliding speed will reduce plowing resistance promoting even faster glacier flow: a potential flow instability for glaciers on soft beds that has not been noted previously.