RAPID ROCK MASS FLOW WITH DYNAMIC FRAGMENTATION: INFERENCES FROM THE MORPHOLOGY AND INTERNAL STRUCTURE OF ROCKSLIDES AND ROCK AVALANCHES

摘要

Dynamic fragmentation is a new hypothesis for the mechanism of rock-avalanche long runout. Low-strain-rate fragmentation is dominated by growth of a few Haws. It is the regime leading to initial failure of many landslides. Static rock strength is largely independent of loading rate. The dynamic regime is entered when growth of a few Haws does not relieve elastic strain fast enough, and stresses rise adjacent to the flaws, forcing many new ones to nucleate and grow. Strengths of dynamically fragmenting materials increase at about the 4th root of strain rate. Elastic strain energy, W, per unit volume, released at failure is given by W=Q2/(2E), where Q is strength and E is elastic modulus. Its explosive release as kinetic energy provides a large, isotropic, clast-dispersing stress, every time any clast is stressed to failure. Fragmentation-induced dilation is a positive granular "pressure", but also causes low pore-fluid pressure, and is incompatible with saturation of voids by liquids and therefore is incompatible with high pore pressure and undrained loading. Driven entirely by internal deformation within the avalanching mass, dynamic fragmentation propels the distal margins of large avalanches of brittle rock further than they could travel had they just collapsed to joint-bounded clasts.