To characterize the hypermobility mechanism of rock avalanches, a series of rotary shear tests at different shearing velocities (Veq) 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 Veq ≥ 0.61 m/s, (2) high‐temperature rises and layers with high porosity were observed in the samples sheared at Veq ≥ 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.

To explore the mechanism behind the volume effect of rock avalanches observed in field data, a series of rotary shear tests were conducted at a shearing velocity of 0.87 m/s and varying normal stress levels (from 0.29 to 1.85 MPa) on soil collected from the basal facies of the Yigong rock avalanche in Tibet, China. This experimental study reveals that (1) as normal stress increases, the steady state apparent friction coefficient (μss) of the soil decreases, indicating a normal stress‐dependent feature of μss; (2) the higher the normal stress, the lower the decay rate of μss; (3) water vaporization induced by frictional heating is commonly observed in the tests accompanied by large decreases in μss, and excess pore pressure is generated in the shearing zones of the samples due to vapor accumulation; and (4) with increases in normal stress and decreases in soil permeability, an increasing feature of excess pore pressure is reached. Based on the experimental results, we propose that the coupled effects of water vaporization induced by frictional heating in avalanche basal facies and depth‐dependent permeability of avalanche basal facies should be contributed to the volume effect of rock avalanches.