Abstract
AbstractA ring-shear device was used to study the factors that control the ultimate(steady) strength of till at high shear strains.Tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26.3° and 18.6° for tills containing 4% and 30% clay-sized particles, respectively Other tests at steady normal stresses and variable shear-strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate. This weakening may be due to small increases in till porosity.These results provide no evidence of viscous behavior and suggest that a Coulomb-plastic idealization is reasonable for till deformation. However, viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till. We hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure. During a deformation event, transient dilation of discrete shear zones should cause a reduction in internal pore-water pressure that should strengthen these zones relative to the surrounding till, a process called dilatant hardening. Consequent changes in shear-zone position, when integrated over time, may yield the observed distributed strain.
Highlights
Since th e provocati\'e geophysical work on Ice Stream B, Antarctica (Bl ankenship a nd others, 1986), which sugges ted that fast fl ow of the ice st ream might be facilitated by pervasive d eform ati on of th e underl ying till (Alley and others, 1986), th ere have been m any st udies a im ed at modelling the flow of modern and pas t glaciers o n deformable beds (e.g. Bo ulLon and Hindm a rsh, 1987; C larke, 1987; Alley, 1989; M acAyeal, 1989; K am b, 1991; J enson and others, 1996)
Some studies have foc used on sedim ent transport in deforming beds (A ll ey, 1991; Boulton, 1996; Hooke a nd Elverh0i, 1996), and some models have emphasized that suc h transport may be a n importa nt factor in co ntrolling the long-term stability of ice sheet (e.g. M acAyeal, 1992)
Th e T wo Ri \"Crs till b e hm·ed sim il a rl y in so me experiment s, but in other ex p er im e nts d isplayed a post-pea k redu cti o n in s tre n~ th th a t is ty pi cal of clay-ri c h sed iment, in which clay pa rticles a lig n duri ng shea ring, the reby reducing in te rn a l fri c ti on (Skem pto n, 1985)
Summary
Since th e provocati\'e geophysical work on Ice Stream B, Antarctica (Bl ankenship a nd others, 1986), which sugges ted that fast fl ow of the ice st ream might be facilitated by pervasive d eform ati on of th e underl ying till (Alley and others, 1986), th ere have been m any st udies a im ed at modelling the flow of modern and pas t glaciers o n deformable beds (e.g. Bo ulLon and Hindm a rsh, 1987; C larke, 1987; Alley, 1989; M acAyeal, 1989; K am b, 1991; J enson and others, 1996). Some studies have foc used on sedim ent transport in deforming beds (A ll ey, 1991; Boulton, 1996; Hooke a nd Elverh0i, 1996), and some models have emphasized that suc h transport may be a n importa nt factor in co ntrolling the long-term stability of ice sheet (e.g. M acAyeal, 1992). Differences of opinion regardi ng th e form of this relati on have led auth ors to advocate dispa ra te constitutive relati on for till deforma tion. These can b e grouped as either Coulomb-plastic or viscous relation s
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
