LABORATORY STUDY OF FABRIC DEVELOPMENT IN SHEARING TILL: EXPLORING THE IMPORTANCE OF EFFECTIVE NORMAL STRESS AND SHEARING RATE

The alignment of particles in subglacial till has commonly been used to invoke the bed deformation model for glacier flow. To clarify the relationship between particle alignment and shear strain, laboratory experiments using a rotary-shear device have revealed a strong relationship where strong fabrics develop in the direction of shearing at relatively small strains (~10) and remain strong to high strains (>50). Although these experiments were performed under reasonable glacial conditions, it is unknown how the fabric would vary as function of effective normal stress (ice load minus the pore water pressure) and shearing rate. To isolate the importance of these two variables in the development of fabric, two sets of experiments were performed where we independently varied the effective normal stress from 30-150 kPa and the shearing rate from 100-860 m/yr. Both set of experiments were run to high strains (~90) and anisotropy of magnetic susceptibility (AMS) was used to characterize fabric strength. The results from these experiments indicate that regardless of the effective normal stress and shearing rate, strong fabrics still develop in the direction of glacier flow as represented by S₁ eigenvalues of the maximum principle susceptibility (0.92 to 0.98). Thus it appears that fabric magnitude is independent of the effective normal stress and shearing rate indicating that shear strain remains the most important variable in fabric development.