Subglacial hydrological and mechanical processes play a critical role in determining the flow characteristics and stability of glaciers and ice sheets, but our understanding of these processes remains incomplete. Instrument and modelling studies of conditions beneath Trapridge Glacier, a small surge-type glacier in the St. Elias mountains, Yukon, yield additional insight into subglacial hydromechanical behaviour. High-pressure pulses in the subglacial drainage system are indicated by sudden offsets in measured pressure and result from damage to the pressure sensor measurement diaphragm. Laboratory and modelling studies confirm that pressures significantly above the transducer rating produce offsets comparable to those observed in field records. Instrument records suggest that high-pressure pulses are generated by abrupt glacier motion that compresses or dilates the subglacial hydraulic system. Analysis of instrument records taken during summer 1995 reveals that a series of hydromechanical events occurred following the establishment of a subglacial drainage system. Pressure fluctuations in this drainage system weakened a basal region that was acting as a pinning point, resulting in three episodes of strong basal motion. A simple hydromechanical model of basal processes acting beneath a softbedded alpine glacier was developed. In this model, the glacier bed is divided into three regions: softbedded and hydraulically-connected to the subglacial drainage system, soft-bedded and unconnected, and hard-bedded. Each basal region is modelled as a one-dimensional column. The time evolution of pore-water pressure, till dilatancy, sediment deformation and glacier sliding is calculated in soft-bedded regions; hard-bedded regions are considered rigid and impermeable. The regions are coupled by a simple ice-dynamics model. Sediment deformation is calculated for four till flow laws: linear-viscous, nonlinear-viscous, nonlinear- Bingham and Coulomb-plastic. Modelled pore-water pressure profiles, deformation profiles, basal shear stress and instrument responses are presented for the four flow laws. Comparison of synthetic and field instrument responses suggests that till behaviour is best represented as Coulomb-plastic.
