Evidence for excess pore-water pressure generated in subglacial sediment: Implications for clast ploughing

Pore water pressure in front of an object pulled through subglacial sediment and water pressure at the base of a borehole were simultaneously recorded at Unteraargletscher, Switzerland, to investigate the generation of excess pore pressures downglacier from ploughing clasts. Analysis of the strongly correlated pressure records revealed that the amplitude of the pore-pressure fluctuations was larger than that of the corresponding borehole pressure fluctuations. Guided by high temporal resolution measurements of glacier surface motion from a different year, this finding suggests that temporal changes in glacier speed lead to changes in the degree of excess pore-pressure in front of the ploughing object. The generation of excess pore water pressure has the potential to weaken the sediment downglacier from ploughing clasts. Instead of these clasts providing the roughness at the ice - bed interface to resist glacier motion, the ploughing process may in fact help to decouple the ice from the bed reducing basal resistance.