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