Laboratory study of the frictional rheology of sheared till

Abstract

Deformation of till produces power law creep for low strain at stresses high enough to cause permanent deformation but below the shear strength. Experiments were conducted on till (a mixed size granular material) from Matanuska Glacier, Alaska, and the Scioto (Ohio) Lobe of the Laurentide Ice Sheet (Caesar till). We deformed till in double direct shear under fixed shear velocity or shear stress (creep). Normal stress ranged from 50 kPa to 5 MPa at shearing rates ranging from 1 to 300 μm/s for 1 cm thick samples. Creep was induced via small step perturbations in stress. Fabric development within till layers was investigated by varying shear strain prior to creep tests. In velocity‐controlled experiments, till deforms as a nearly Coulomb plastic material with slight velocity strengthening, corresponding to a stress exponent, n > 60. Creep experiments conducted well below the shear strength indicate lower n values, increasing as shear stress increases. With increasing initial strain and inferred fabric development, the creep strain rate decreases while n increases. Experiments at a normal stress of 1 MPa and no initial strain show n = 6.8, increasing to n = 17.5 at higher shear strains; however, strain rate was still decreasing and thus these values represent maximum estimates. Our data show that in the absence of dilatant hardening till exhibits rate sensitivity at strain of order 1 or less. At low strains, n likely depends on consolidation state, permeability, and dilation. Deformation is nearly rate insensitive (Coulomb plastic) at shear stress near the shear strength or at high strain.