The role of rock fragmentation in the motion of large landslides

Abstract By considering the implications of the comminution generally associated with very large landslides, we arrive at a simple explanation for the remarkably low frictional resistance to motion demonstrated by large intact blockslides (e.g. Waikaremoana, New Zealand), volcanic debris avalanches (e.g. Socompa, Chile) and large rock avalanches (e.g. Falling Mountain, N.Z.), which allows such mass movements to achieve unexpectedly high velocities and long runout distances. During rapid grain flow under high direct stress, the overall grain motion generates stresses in many force chains that strain individual grains to failure; most of the elastic strain energy accumulated in these force chains before failure is returned at failure to the resulting grain fragments, resulting in apparent instantaneous pressures of ~ 3Q on the surroundings, where Q is the ambient strength of the previously intact grains (~ GPa). These intense pressures support some of the direct force on the shear layer, so that the effective (intergranular) stress in the shear layer is reduced. Because frictional resistance is proportional to effective stress, this reduces the overall frictional resistance to shear. The steady-state effective stress is that which just allows fragmentation to continue; the resistance to motion estimated from this relationship explains to much better than order-of-magnitude accuracy the reported motions of the large, rapid mass movements. We also deduce that grain fragmentation can be sustained for sufficiently long to explain the phenomena without reducing the mean grain size by volume in the granular layer to unrealistically small values. The presence of pore fluid does not appear to influence the effect of fragmentation dynamics in a major way. The proposed mechanism requires further laboratory and simulation studies to reduce its current dependence on limited field data, but its success suggests that it is worthwhile investigating further as an explanation for large mass movements in the types of brittle rock in which fragmentation occurs.