Fines-controlled drainage in just-saturated, inertial column collapses

The wide particle size distributions, over several orders of magnitude, observed in debris flows leads to a diverse range of rheological behaviours controlling flow outcomes. This study explores the influence of different scale grains by conducting subaerial, fully saturated granular column collapse experiments with extreme, bimodal particle size distributions. The primary particles were of a size where their behaviour was controlled by their inertia while a suspension of kaolin clay particles within the fluid phase acts at spatial scales smaller than the pore space between the primary particles. The use of a geotechnical centrifuge allowed for the systematic variation of gravitational acceleration, inertial particle size and the degree of kaolin fines. Characteristic velocity- and time-scales of the acceleration phase of the collapse were quantified using high-speed cameras. Comparing tests containing fines to equivalent collapses with a glycerol solution mimicking the enhanced viscosity but not the particle behaviour of the fines, it was found that all characteristic dynamic quantities were dependent on the degree of fines, the system size, the grain fluid-density ratio and the column– and grain-scale Bond and Capillary numbers. We introduce a fine-scale Capillary number showing that, although surface tension effects at the column scale are negligible, fines do control the movement of fluid through the pore spaces.