On the interaction of dilatancy and friction in the behavior of fluid-saturated sheared granular materials: a coupled Computational Fluid Dynamics--Discrete Element Method study

Abstract: Frictional instabilities in fluid-saturated granular materials underlie critical natural hazards such as submarine landslides and earthquake initiation. Distinct failure behaviors emerge under subaerial and subaqueous conditions due to the coupled effects of mechanical deformation, interparticle friction, and fluid interactions. This study employs three-dimensional coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations to investigate the collapse and runout dynamics of dense and loose granular assemblies across these settings. Parametric analyses reveal that pore pressure evolution plays a central role in governing failure mechanisms: dense assemblies stabilize through dilation, while loose assemblies undergo rapid compaction and fluidization, particularly under subaqueous conditions. Spatiotemporal analyses of coarse-grained fields further highlight strain-rate-dependent behavior driven by evolving porosity and effective stress. Both environments exhibit rate-strengthening behavior that scales with the inertial number (In) and viscous number (Iv), though driven by distinct mechanisms: subaerial systems are dominated by interparticle contact networks, whereas subaqueous systems are influenced by fluid drag, pore-pressure buildup, and lubrication. An analytical solution for excess pore pressure is compared with breaching-induced pressure distributions from CFD-DEM simulations, using input parameters derived from numerical triaxial DEM tests. The model captures fluid-particle coupling effectively, reproducing comparable excess pore pressures at steady state, while early-time discrepancies underscore the complexities of transient interactions. These findings advance the understanding of failure mechanics in saturated granular media and support the development of physics-based models for mitigating hazards associated with subaqueous granular flows.