The intrinsic mobility of very dense grain flows

Abstract

High mobility of natural disasters with severe consequences is often reported in large landslides and in earthquake fault ruptures. The mechanism of high mobility keeps to be mysterious to date. In this study, we used high-speed rotary shear experiments under normal stress 1 MPa and with a rotary speed of 2 m/s on grains of four different minerals to show that the tested dry dense grain flows with crushable grains are highly mobile intrinsically. Granular flow quickly weakens to a very low viscosity (about 500 Pa.s), which is independent of grain composition. We find that when flow begins, grains are crushed to a special fractal structure in which most larger grain is surrounded by much smaller grains. This special structure provides a favourable condition for generating and propagating acoustic energy from the abrasion indicated by the scratches on the grains. The scratching generates very high frequency elastic energy (vibration) in the larger grains, which profoundly weakens all grain contacts so that the grain mass flows like a giant flood. Chatter-marked scratches made by smaller grains scouring across boulders in rock-avalanche deposits suggest that the same processes occur in nature. This finding is important for the explanation of the behaviour of dense grain flows at large strains and high strain rates.