Transient granular model for the response of subglacial till

Antarctic glaciers slide over subglacial till, a wet mixture of grains and clay, and their dynamics depend strongly on the degree of basal sliding. Hence, understanding the dominant physical processes for till deformation are crucial to modelling ice sheet dynamics. In steady-state experiments, till has been shown to behave plastically with a pressure dependent yield stress.  Many ice streams show velocity variations that have been linked to daily fluctuations in subglacial water pressure. To understand the response of till to periodic forcing requires a model that extends beyond the steady state.

We construct a one-dimensional, two-phase model of coupled fluid and solid flows, using Darcy flow for the fluid phase and a wet granular μ(I_ν) rheological model for the solid part. After verifying our model against steady-state observations of till deformation, we force the model with a fluctuating effective pressure at the ice-till interface and infer the resulting relationships between basal traction, solid fraction, rate of deformation, and till flux. Shear dilation introduces internal pressure variations, leading to hysteretic behaviour in low-permeability materials, which may help explain the large-scale transient response of ice sheets to changes in the subglacial hydrology.