EVOLUTION OF BOUNDING FAULTS DURING PERSISTENT LANDSLIDE MOVEMENT

Movement of landslides is primarily controlled by characteristics of their shear boundaries. According to granular critical-state theory, bounding shear zones may undergo shear-induced volumetric change during the first several meters of displacement and consequent changes in pore-water pressures may affect shear strength. If initially loose, shear-induced contraction and consequent pore-water pressure elevation and shear strength reduction may result in rapid motion, whereas dilation of initially dense soils and consequent pore-pressure reduction and strength elevation may slow motion. However, such dilatant strengthening decays with displacement. Monitoring evidence suggests that dilatant strengthening restricts accelerating movement of the Slumgullion landslide in Colorado, although the landslide’s hundreds of meters of cumulative displacement exceed distances required to reach critical-state conditions (less than a few meters). We tested the dilatant strengthening hypothesis at Slumgullion along its lateral shear boundary using laboratory measurements and surveying over a six-year period during which the landslide moved 4 to 15 m at the study locations. We found that soil porosity increased and the ground surface moved upward near the boundary, suggesting shear-induced dilation. We observed episodic, localized relocation and changed character of some parts of the boundary indicating that shearing of consolidated (relatively dense) soil locally occurred, while shearing consistently occurred within the same soil in other locations. Hence, it appears that dilatant strengthening restrains motion of the landslide only locally, with locations and degrees of strengthening changing through time. These changes likely occur due to varying normal stresses along bounding shear zones from fluctuating landslide volume and pore-water pressures. We expect that the landslide’s basal boundary and boundaries of other landslides display similar characteristics.