Rheological behavior of Crushed Rock Flows

The enduring mystery surrounding the unexpectedly high mobility of expansive geophysical flows has persistently tantalized researchers since Albert Heim's investigation following the catastrophic landslide at Elm, Switzerland. Despite numerous claims of resolution, the mechanism underpinning this remarkable mobility has remained elusive. To delve into the flow dynamics of crushable dense granular material exhibiting high mobility, a series of high-speed rotary shear experiments was conducted using various mineral particles. Our findings revealed a more explicable flow behavior when interpreting shear resistance as viscous rather than purely frictional. Notably, we observed a dramatic decrease in viscosity for crushable materials, stabilizing at a consistently low level, crucial in dictating the remarkable fluidity observed in large-scale geophysical flows like rock avalanches. The flow exhibited two distinct phases, demarcated by a critical point of weakening within accumulated strain for crushable material. The initial phase reflected a simple Newtonian or non-Newtonian-like flow, while the subsequent phase was more intricate, displaying a profound viscosity drop stabilizing at a constant level under substantial strain. This discovery holds significant implications for understanding hypermobile geophysical phenomena, including rock avalanche dynamics, 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.