Effects of Normal Stress, Shear Rate and Granular Material Types on Steady-State Shear Resistance and Viscosity in Rapid Dry Granular Flows

Understanding the rheological characteristics of rapid granular flows is essential for elucidating numerous geological phenomena, including abrupt fault sliding and high-velocity landslide movements. This study employed rotary shear tests on diverse granular specimens, covering a broad shear velocity spectrum from low to high magnitudes and under varied normal stress conditions, to explore the variation of mechanical properties across different flow regimes. Experimental outcomes indicated that under shear velocities below 1 m/s, the steady-state shear resistance exhibited dependencies on both normal stress and material constituents, accompanied by a consistent velocity-related pattern. Specifically, the steady-state shear resistance of the tested samples underwent a transformation: transitioning from velocity-strengthening behavior at low shear velocities (≤ 0.1 m/s) to velocity-weakening behavior when shear velocities exceeded 0.1 m/s. Notably, when shear velocities surpassed 1 m/s, the steady-state shear resistance became insensitive to normal stress and material composition, converging to a comparable steady-state value for both crushable and non-crushable granular materials. While normal stress and mineral composition exerted minimal impact on steady-state shear resistance under high shear rates, they significantly modulated the weakening rate—defined as the transition process from peak strength to steady-state shear resistance. This weakening rate was found to be closely associated with the material's crushability, quantified by the Weibull modulus. These findings offer valuable insights into the underlying mechanisms controlling the hypermobility of large-scale landslides and the rapid dynamic processes of geological granular flows.