ABSTRACT
Large, slow-moving landslides in the Interior Plateau of British Columbia, hitherto referred to as earthflows, display distinctive features such as discrete lobes, lateral deposits, flow bifurcation, plug-flow, and irregular accumulation fans, which are commonly observed on much smaller-scale mudslides and debris flows. The possible origins of the features are examined by considering material properties and the mechanics of deformation. Surface displacements show systematic variation related to convergence and divergence of the moving mass. Extending zones, analogous to those on glaciers, are associated with acceleration of the moving mass and attainment of the active Rankine state. Compressing zones, in areas of reduced gradient, show morphological and subsurface evidence of slow thrusting along shear planes, which are at residual strength. Field moisture contents are close to the plastic limit, even to depths of 10 m. This produces a rigid debris mass, with most of the downslope displacement occurring at well-defined boundary shear zones. Vertical velocity profiles show only slight internal deformation down to depths of 14 m. These deformation properties suggest the term “earthslide” is more appropriate than earthflow. The latter term should be applied to undrained failures in sensitive soils that exhibit true flow.
