Ring-shear studies of till deformation: Coulomb-plastic behavior and distributed strain in glacier beds

A 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 degrees and 18.6 degrees 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 he 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 sheer strain observed in subglacial till. We hypothesize that deformation may become distributed in till that is deformed cyclically In response to fluctuations in basal wafer 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 aver time, may yield the observed distributed strain.