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 natural hazards, including submarine landslides and earthquake initiation. Experiments show distinct failure behaviors under subaerial and subaqueous conditions due to coupled deformation, interparticle friction, and particle fluid interactions. We use three-dimensional coupled computational fluid dynamics, discrete element method (CFD - DEM) to investigate collapse and runout of dense and loose granular assemblies in both environments. Parametric analyses show that pore pressure evolution controls failure mode in saturated settings (fast vs slow sliding), consistent with prior laboratory experiments and lattice Boltzmann discrete element simulations: dense assemblies stabilize via dilation, whereas loose assemblies compact rapidly and transiently fluidize. At mesoscale, we coarse grain particle contact statistics and Eulerian fluid fields to define apparent friction and normalized pore pressure, and organize inertial and viscous responses using log10(In/Iv). Spatiotemporal analyses of these coarse grained fields reveal strain rate dependent behavior governed by evolving porosity and effective stress. In both environments, friction in failure shear zone is rate-strengthening with respect to inertial number (In, for dry) and viscous number (Iv, for fluid-saturated). We further utilize mesoscale stress framework to compare evolution of pore pressure in CFD - DEM of subaqueous slope collapse with an analytical solution for development of failure front, using inputs derived from numerical triaxial DEM tests on same assemblies. The analytical model reproduces steady-state excess pore pressures and captures fluid-particle coupling, but mismatch near failure onset suggests transient frictional effects. These results support physics-based hazard models and improve mechanistic understanding of saturated granular failure.