A laboratory study of ploughing

Abstract

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.

A laboratory study of particle ploughing and pore-pressure feedback: a velocity-weakening mechanism for soft glacier beds

“…6a). Although ploughing velocities in the study of Rousselot (2006) (see also Rousselot and Fischer, 2007) were about an order of magnitude higher than in this study, the much higher diffusivity of the sediment in that study (more than two orders of magnitude) results in t D / t C <1.0, which accounts for the low excess pore pressures (Fig. 13) and lack of velocity weakening observed in that study.…”

“…The only laboratory ploughing experiments conducted with till, apart from those of this study, were those of Rousselot (2006) (see also Rousselot and Fischer, 2007), who used a customized apparatus to push a vertical rod horizontally through water-saturated till in a large chamber. These experiments approximated the action of a plough-meter, an instrumented rod that can be driven into a soft bed through a borehole and used to study soft-bed mechanical properties (Fischer and Clarke, 1994).…”

“…These ranges are within those relevant to many soft-bedded, wetbased glaciers, although basal tills of sufficiently high hydraulic permeability, and hence diffusivity, will not be subject to this effect. Small excess pore pressures without velocity weakening in laboratory ploughmeter experiments (Rousselot and Fischer, 2007) resulted from the high diffusivity of the sediments used.…”

“…The only laboratory ploughing experiments conducted with till, apart from those of this study, were those of Rousselot (2006) (see also Rousselot and Fischer, 2007), who Fig. 11.…”

Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology, hydro-mechanical interactions and till formation

“…The porosity reduction during compaction increases till pore-water pressures. In contrast, Rousselot and Fischer (2007) argue that the degree of sediment weakening in front of ploughing clasts depends on the relative magnitudes of excess pore pressure generated by sediment compression, as opposed to strengthening by dilatant shearing. They discuss how, according to critical state theory, a sediment may increase or reduce its volume during shearing depending on its initial porosity (Lambe and Whitman, 1979).…”