Abstract
AbstractThe relation between the local effective pressure and shear stress on till beneath Storglaciären, Sweden, discussed in Iverson and others (1999), provides an empirical basis for studying the processes that control the strength of the ice/bed coupling. Particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms. Model calculations, based on studies of cone penetration through fine-grained sediment and sliding theory, agree with the observed relation between shear stress and effective pressure if the water layer at the ice/bed interface is assumed to thicken rapidly as the effective pressure approaches zero. Studies of the hydraulics of linked cavities provide support for this assumption, if the mean thickness of the water layer reflects the extent of microcavity development at the interface. Comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath Storglaciären only at intermediate effective pressures; at very low effective pressures, like those inferred at the site of the tiltmeter discussed in Iverson and others (1999), and at sufficiently high effective pressures, ploughing and sliding should focus motion near the glacier sole. A calculation using parameter values appropriate for Ice Stream B, West Antarctica, suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth. This conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down-glacier from ploughing particles, thereby weakening the bed near the glacier sole. However, given the apparent sensitivity of the ice/bed coupling to basal conditions that may be highly variable, any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism.
Highlights
Th ere have been two theo retical analyses of th e coupling betwee n a glacier and a soft bed that have reached quite different conclusions
Alley calculated that ploughing might occ ur over a na rrow ra nge of basal shear stress, he a rg ued th at till is best idealized as a viscous fluid and that its effecti ve viscosity is too large to a llow a la rge pl oughing vcloeity without bed d eform ati on
The p a rticle will plo ug h, a nd the shear stress on thc p a rticle will be limited by plo ug hing, rather th a n by sliding
Summary
Th ere have been two theo retical analyses of th e coupling betwee n a glacier and a soft bed that have reached quite different conclusions. Brown a nd oth ers (1987) used the sliding theory ofLliboutry (1979) a nd a Coulomb-pl as ti c ideali zation for the rheo logy of till to stud y the basal moti on of th e Pugct Lobe of the Cordilleran ice sheet. They co ncluded th at onl y ploughing, the dragging of c1as ts through till, could have reduced the drag on large pa rticles at the ice/bed imerface sufficiently to a ccount for the sm all basal shear stress inferred from a reconstructed pro file of the lobe. Alley calculated that ploughing might occ ur over a na rrow ra nge of basal shear stress, he a rg ued th at till is best idealized as a viscous fluid and that its effecti ve viscosity is too large to a llow a la rge pl oughing vcloeity without bed d eform ati on
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