Solid-liquid interaction caused by minor wetting in gravel-ice mixtures: A key factor for the mobility of rock-ice avalanches

Rock-ice avalanches pose serious risks to humans and infrastructures due to their extremely large volumes and high mobility in glacierized mountain regions. The mobility of rock-ice avalanches is supposed to be influenced by water, while the solid-liquid interactions induced by minor wetting remain an open issue. A series of flume tests was performed to investigate the influence of a small amount of water (no more than 4% of the gravel weight) on the behavior and mobility of gravel-ice mixtures. The test results show that intergranular refreezing bonds were created with a preliminary addition of water. The breakup of some weak refreezing bonds during particle rearrangement likely dissipated some kinetic energy, and thus, the mixture mobility was reduced. As the water content increased, the thermal conductivity could likely not remain efficient enough to refreeze all the trapped water and thus allowed unfrozen water to lubricate particle corners, which enhanced mixture mobility. With a further increase in the water content, suction-induced cohesion cemented particles as clusters and reduced mixture mobility. However, a high ice content resulted in mobility enhancement with increasing water content because more gravel was refrozen into the ice matrix, and thus, the basal friction was lowered by the occupation of ice cubes at the bottom. The redistribution pattern of trapped water during mass propagation changed at a characteristic water content. When the water content was lower than the characteristic content, superficially cohered water detached from the surface asperity of one particle and transferred to another during particle rearrangement, i.e., local redistribution of trapped water. When the water content was higher than the characteristic content, the downward transfer of a considerable amount of trapped water was triggered by particle rearrangement under the influence of gravity, i.e., bulk redistribution of trapped water. The water experiencing bulk redistribution influenced the mixture mobility by exerting intergranular drag force via the build-up and break-up of liquid bridges, while locally redistributed water influenced mixture mobility by altering the surface properties of particles. This study is expected to be helpful in better understanding the propagation mechanisms of water-poor rock-ice avalanches and in improving relevant hazard assessments.