A New Laboratory Apparatus for Investigating Clast Ploughing

Abstract

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 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.…”

“…No velocity weakening was observed. In the following section we show that the high diffusivity of the sediment used by Rousselot (2006) accounts for the low excess pore pressure measured in that study and the lack of velocity weakening.…”

“…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).…”

“…Figure 13 shows excess pore pressures measured in the two tills of this study and those measured in the glacial sediment used by Rousselot (2006), plotted as a function of t D / t C . Excess pore pressures are clearly correlated to conditions for which this ratio exceeds 1.0.…”

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…Concepts linking ice rheology and nature of the substrate have been essentially developed based on modelling (Rousselot et al, 2007;Pattyn, 2010), the observation of present-day glaciers (Peters et al, 2006), and from the Quaternary record (Boulton and Hindmarsh, 1987;Lee and Phillips, 2008;Phillips et al, 2008;Denis et al, 2009;Clerc et al, 2012). However, the most impressive resulting sedimentary structures, in particular extensive striated surfaces -either former ice-bed interfaces or intraformational, décollement planes -have been best illustrated from older glacial records preserved in Permian-Carboniferous (Aitchison et al, 1988;Visser, 1990; Lang Table 1 Criteria to recognise ancient ice-marginal wedges, based on the inventory of deformation and depositional structures of the end-Ordovician record of Libya.…”