Abstract
The puzzle of the unexpectedly high mobility of large geophysical flows has been reported as "solved" many times since Albert Heim drew attention to it after a disastrous landslide at Elm, Switzerland. However, the mechanism of high mobility remains mysterious.
A series of high-speed rotary shear experiments on particles of different minerals was conducted to explore the high mobility flow-like behavior of crushable dense granular flow. We found that the flow behavior was more explicable when the shear resistance was considered as a viscous resistance rather than as a frictional resistance.
We also found that for the crushable material, the viscosity dramatically decreased and reached a constant and relatively low value, which controlled the high fluidity and hypermobility of large geophysical flows such as rock avalanches. More precisely, there are two phases of the flow behavior, that are separated at a weakening point in the accumulating strain for crushable material.
The first phase was a simple Newtonian or non-Newtonian-like flow. The second phase was more complex, in which the flow viscosity decreased profoundly and reached a constant viscous resistance at a considerable strain.
This finding is important for explaining the hypermobility of many large geophysical processes, such as rock avalanche motion, natural faulting and crater collapse. In particular, we demonstrate that the behavior of rock avalanches is similar to that of complicated fluids with extensive weakening and that the viscosity of this special "liquid" is as low as 500 Pa.s.
This finding can also help improve the accuracy and reliability of the numerical simulation of rock avalanches by using the viscous model obtained from the experiments.