Preferential Flow and Mechanical Stability of Model Sandy Soils

Preferential flow occurs as rain permeates a soil, where unstable pathways of enhanced fluid flow coexist with drier regions. The instability is triggered by a competition between gravitational, capillary, and viscous forces that governs the spatial extent and the kinetics of the preferential flow pathways. Preferential flow has been linked to increased occurrence of landslides following heavy rainfall events and earthquakes, yet the mechanism by which preferential pathways trigger failure remains poorly understood. Using a custom-built laboratory-scale experimental setup, we probe the infiltration of water through a porous medium of tunable grain size and grain surface properties. We induce mechanical perturbations to the material to study the failure modes of the wet granular channels. Complementing these experiments with a rheological characterization of a preferentially wetted granular medium, we elucidate the impact of preferential paths on the stability of the granular medium.