Dilation on granular flows: Insight for friction weakening on debris avalanches

Debris avalanches are a major concern due to their high mobility. However, the mechanism of friction weakening in debris avalanches remains poorly understood. This study systematically investigates the friction weakening mechanism of granular flows using rotation drum experiments, large-scale chute experiments, and numerical simulations. Notably, dilation of granular flows is a characteristic feature associated with friction weakening. The results indicate that dilation occurs synchronously with friction weakening during the motion of granular flows, as evidenced by the motion patterns and force interactions of debris avalanches. Collision contacts were identified as the primary driver of particle dilation. An optimal collision strength can induce dilation of granular flows, reducing contact between the sliding body and substrate, thereby leading to friction weakening. The peak collision strength of granular flows during movement is determined by fragment size. The critical condition for triggering friction weakening in debris avalanches is identified as the peak Savage number ( N s,p ) greater than 1.06. A mathematical model based on the granular inertial collision-friction coupling equation was developed. This study provides compelling evidence that the fractal dimension of various types of high-speed debris avalanches tends to stabilize within a narrow range.