Control mechanisms of pore-pressure dissipation in debris flows

Abstract

Debris flows are typically saturated mixtures of debris grains and interstitial slurry consisting of water and clay. Pore pressure in slurry plays a crucial role in the characteristic behavior and runout of debris flows. However, the mechanisms that cause pore pressures to diffuse are still uncertain. Here we report consolidation tests to investigate the effects of debris composition (uniformly graded and widely graded) and slurry density on the dissipation of pore pressure. Pore pressures of debris flows were found to dissipate at contrasting rates ranging from a few seconds to dozens of hours and dissipation time strongly depended on slurry density. We observed an abrupt change in pore-pressure dissipation time at a critical slurry density. We propose that a critical pore throat controls the permeability of a debris material and regulates the pore-pressure dissipation based on percolation theory. This hypothesis is verified by pore-network models from a micro-computed tomography analysis with high resolution. The critical pore throat of the uniformly graded debris material is larger than that of widely graded debris material due to the difference in the porosity. The permeabilities and hydraulic diffusivities of debris flows significantly decrease once critical pore throats are blocked, resulting in a change of pore-pressure dissipation. Critical slurry density is approximately linearly correlated with the porosity of debris flows. Our results highlight opportunities to use micro-structural properties for interpreting debris-flow behavior.