Velocity-dependent frictional weakening of large rock avalanche basal facies: Implications for rock avalanche hypermobility?

To characterize the hypermobility mechanism of rock avalanches, a series of rotary shear tests at different shearing velocities (V_eq) ranging from 0.07 m/s to 1.31 m/s and at a normal stress of 1.47 MPa were carried out on soil sampled from the basal facies of the Yigong rock avalanche that occurred in the Tibetan plateau in China. Through conducting these tests, the macroscale and microscale features of the deformed samples were analyzed in detail with the following valuable conclusions being reached: (1) soil subjected to rotary shear exhibits a clear velocity-dependent weakening characteristic with an apparent steady state friction of 0.13 being reached at V_eq ≥ 0.61 m/s, (2) high-temperature rises and layers with high porosity were observed in the samples sheared at V_eq ≥ 0.61 m/s, and (3) the cooperation of thermal pressurization and moisture fluidization induced by friction heating plays an important role in explaining the marked frictional weakening of the soil. In addition, the appearance of nanoparticles due to particle fragmentation should facilitate the weakening of the soil but is not the key reason for the marked frictional weakening.