Influence of soil density on the solid-to-fluid phase transition in flowslide flume experiments

Flowslides are extremely hazardous due to their sudden failure and rapid flow-like motion. Focusing on the transient solid-to-fluid phase transition, we carried out a series of flume studies following various experimental scenarios, simultaneously recording granular velocity distributions, pore-water pressures, microseismicity, and acoustic emissions with precise time resolution (±0.01 s). The solid-to-fluid phase transition was recognized from the transition from solid-like shear deformation to flow-like velocity distribution. The variation in excess pore pressure in slopes during failure differed depending on initial soil state parameters, resulting in different failure patterns and distinct solid-to-fluid phase transition dynamics: (i) diffuse failure in loose slopes, the soil instability caused by increased local pore pressure (far from liquefaction) can trigger the whole failure of the slope; (ii) Mohr-coulomb failures in denser slopes, the soil mass slid along a localized shear zone, and progressively evolved to a diffuse shear throughout the slide mass. and (iii) retrogressive failures in very dense slopes. During the rapid motion process, the soil mass behaved as a fluid even after the excess pore pressure had dissipated, demonstrating that mechanisms other than soil liquefaction governed the fluid-like behavior. We inferred that the fluidization originated from the inherent shear-rate-dependent behavior of granular materials. Additional evidence for this hypothesis was provided by ultra-high frequency acoustic emissions (approximately 100 kHz) suggesting grain-scale collisions accompanying the fluidized runout.