The role of fines in the dynamics of just-saturated, inertial column collapses

Debris flows are subaerial, gravity-driven mass movements of water, soil and rocks.  High fluid volume fractions and the presence of a wide particle-size distribution lead to highly heterogeneous flow states, and the mechanisms giving rise to this phenomenology open to debate. For tractable modelling, assumptions around the interaction between grains and fluid must be made, but it is not clear whether those assumptions are reasonable across the wide range of length-scales observed. For example, recent studies have shown that the inclusion of a significant proportion of fine granular material within the flow’s composition limits the dissipation of excess pore pressures. Here we explore the possibility that these crucial pore pressure processes are governed at length scales that might otherwise seem insignificant to the macroscopic flow behaviour. Hence, we aim to provide insight on the underlying mechanisms controlling pore pressure through a scaling analysis describing the idealised scenario of sub-aerial axisymmetric column collapses of just-saturated fluid-grain mixtures. Glass beads provide the prototype for inertial particles within the debris flow, and Newtonian fluids carrying varying mass concentrations of fine kaolin clay particles provide the microscopic processes that can control the pore spaces. A geotechnical centrifuge permits elevated gravitational acceleration that when varied alongside particle size, fluid viscosity and mass concentration of fines, allows a wide parameter space to be explored. Pore pressure measurements from these collapses indicate two competing mechanisms, stemming from drainage related pore pressure dissipation and inertial collision related pore pressure generation. An empirical description of these processes is proposed based on our experimental data. This expression is then implemented to describe the fluid-particle coupling within a multiphase Saint-Venant inspired central-upwind scheme in an attempt to simulate the experimental observations.