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.
A significant portion of the basal motion of soft-bedded glaciers can be attributed to 'ploughing'. This term designates the transitional state between sliding and bed deformation which occurs when clasts that protrude into the glacier sole are dragged through the upper layer of the sediment. This process may cause pore pressures in excess of the hydrostatic value that could weaken the sediment downglacier from ploughing clasts and thus affect the strength of the ice-bed coupling. A large laboratory apparatus was developed and constructed to study, systematically and under glacially relevant conditions, the influence of sediment properties, ploughing velocity and effective pressure on excess pore-pressure generation and sediment strength. In this device, an instrumented tip is dragged at different velocities through a water-saturated sediment bed subject to different effective normal stresses. The drag force on the tip and the pore-water pressure in the adjacent sediment are measured simultaneously. In preliminary experiments performed with subglacial sediment from Unteraargletscher, Switzerland, the sediment diffusivity was estimated from consolidation records. During ploughing, pore-pressure gradients developed rapidly around the tip. Excess pore pressures were due to sediment compression in front of the tip whereas pore pressures below the hydrostatic value resulted from dilatant shearing and a wake devoid of sediment that was left behind the tip. A zone of compressed sediment formed in front of the tip. The absolute magnitude of the pore pressure changes was small relative to the effective normal stress, so that the pore pressures did not significantly influence the resistance to ploughing. Rather, the drag force on the ploughing tip was influenced by the properties of the sediment in front of the compression zone, with a greater magnitude in a virgin sediment than in one that has been ploughed before.
A new laboratory device is used to investigate the resistance to clast ploughing at the base of glaciers. In experiments in which a ploughing tip is dragged at different velocities and effective normal stresses through water-saturated sediment from Unteraargletscher, Switzerland, pore pressures above and below the hydrostatic level develop around the tip. The absolute magnitude of these nonhydrostatic pore pressures increases with the ploughing velocity but remains small compared to the sediment yield strength, so that the pore pressures do not significantly weaken the sediment. The shear stress on the tip is independent of the velocity but scales with the applied effective normal stress, in agreement with a Coulomb-plastic behavior of the sediment. The results indicate that, depending upon position close to the object, both sediment compaction and dilation can influence the pore-pressure distribution and thus the sediment yield strength. Comparison with other studies of clast ploughing suggests that the significance of sediment weakening in front of ploughing clasts may depend on the relative magnitudes of the non-hydrostatic pore pressures. Therefore, depending on the dominant porepressure response of the deforming sediment, clast ploughing may have the potential to either trigger ice-flow instabilities or stabilize glacier motion.
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