The force on a ‘ploughmeter’ and subglacial water pressure have been measured in the same borehole at Unteraargletscher,\ud Switzerland, in order to investigate ice–sediment coupling and the motion at the base of a soft-bedded\ud glacier. A strong inverse correlation of the recorded pressure and force fluctuations, in conjunction with a significant\ud time lag between the two signals, suggests that pore-water pressures directly affect the strength of the subglacial\ud sediment. The lag is interpreted to reflect the time required for the water-pressure wave to propagate through the pores\ud of the sediment to the depth of the ploughmeter. Analysis of the propagation velocity of this pressure wave yielded\ud an estimate of the hydraulic diffusivity, a key parameter necessary to characterize transient pore-water flow. Furthermore,\ud the inferred inverse relationship between pore-water pressure and sediment strength implies that Coulomb-plastic\ud deformation is an appropriate rheological model for the sediment underlying Unteraargletscher. However, the sediment\ud strength as derived from the ploughmeter data was found to be one order of magnitude smaller than that calculated\ud for a Coulomb-frictional material using the water-pressure measurements. This significant discrepancy might result\ud from pore-water pressures in excess of hydrostatic down-glacier from the ploughmeter. As the ploughmeter is dragged\ud through the sediment, sediment is compressed. If the rate of this compression is large relative to the rate at which\ud pore water can drain away, excess pore-water pressures will develop that have the potential to weaken the sediment.\ud The same process could lead to highly fluid sediment down-glacier from clasts that protrude into the glacier sole and\ud thus would otherwise provide the roughness to couple the glacier to its bed (Iverson, 1999). Rapidly sliding glaciers\ud overlying sediments might therefore move predominantly by ‘ploughing’, which tends to focus basal motion near the\ud glacier sole rather than at depth in the bed
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