Abstract
AbstractIf basal-water discharge and pressure are sufficiently high, a soft-bedded glacier will slip over its bed by ploughing, the process in which particles that span the ice–bed interface are dragged across the bed surface. Results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity (velocity weakening). During ploughing at various velocities (15–400 m a−1), till was compacted in front of idealized particles, causing pore pressures there that were orders of magnitude higher than the ambient value. This excess pore pressure locally weakened the till in shear, thereby decreasing ploughing resistance by a factor of 3.0–6.6 with a six-fold increase in ploughing velocity. Characteristic timescales of pore-pressure diffusion and compaction down-glacier from ploughing particles depend on till diffusivity, ploughing velocity and sizes of ploughing particles. These timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred. Existing ploughing models do not account for velocity weakening. A new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38% but requires that excess pore pressures be measured. Velocity weakening by this mechanism may affect fast glacier flow, sediment transport by bed deformation and basal seismicity.
Highlights
Resistance to basal movement of glaciers is usually thought to increase with slip velocity
An exception occurred in experiments at high ploughing speeds with the Engabreen till, in which excess pore pressures were decaying toward hydrostatic values when experiments were terminated (Fig. 4c); at lower ploughing speeds with this till, pore pressures became nearly hydrostatic with sufficient displacement (Fig. 4b)
These results demonstrate the pore-pressure feedback that can potentially cause velocity weakening at the surface of a soft bed and the dependence of this feedback on till properties
Summary
Resistance to basal movement of glaciers is usually thought to increase with slip velocity. Minor velocity weakening was exhibited in the study of Iverson and others (1998), in which steady-state shear strength of two basal tills decreased by $1% per 100 m a–1 increase in shear velocity. This is a small effect, and the observation that some tills, rather than weakening, strengthen by small amounts with increasing deformation rate (e.g. Tika and others, 1996; Tulaczyk, 2006) indicates that velocity weakening is not an intrinsic property of till
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