Experimental study of glacier slip over a deformable bed

Abstract:

Constitutive rules exist in various forms for glacier slip over a deformable substrate. Although experiments have generally shown that till behaves as a Coulomb material, in models deformable beds are often treated as viscous or viscoplastic fluids. Processes at the ice-bed interface, which cannot be studied with experiments on till alone, may result in rate-dependent slip resistance that deviates from Coulomb models.

We have conducted a series of experiments using a ring-shear apparatus designed to study glacier sliding. Ice is slid over a 6 cm thick layer of deformable till containing either no clasts larger than 6 mm, or six evenly spaced larger clasts (~5 cm in length) placed at the deformable bed's surface. The device rotates a ring of ice (20 cm wide, 20 cm thick, with outer diameter of 90 cm) across the till bed. Sliding speed or shear stress is set, while the other of these parameters is allowed to attain a steady state. Data are collected at sliding speeds of 29-465 m/yr and shear stresses of 36-91 kPa, while a vertical (normal) stress of 150 kPa is applied to the ice ring. The temperature of the ice is held at the pressure melting point. Water is allowed to drain from the bed, so effective stress is equal to the total vertical stress.

Results indicate that a bed with clasts exhibits rate dependence over a small range of shear stress values less than the yield strength of the till. At shear stresses equal to the yield strength, the bed shears in a thin layer near the ice-bed interface with resistance independent of slip velocity. Clasts are displaced down-flow from their original location but less than the ice displacement. At stresses below the yield strength, the rate dependence is likely the result of classical sliding mechanisms allowing flow of ice around clasts partially lodged in the bed. These results provide the first experimental guidance for flow models that seek to include the combined effects of ice and till mechanics in a constitutive rule for basal slip.