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

Frictional instabilities in fluid-saturated granular materials underlie natural hazards, including submarine landslides and earthquake initiation. Experimental evidence shows distinct failure behaviors under subaerial and subaqueous conditions due to the coupled influences of mechanical deformation, interparticle friction, and particle–fluid interactions. We use three-dimensional coupled computational fluid dynamics–discrete element method (CFD–DEM) simulations to investigate the collapse and runout of dense and loose granular assemblies in both environments. Parametric analyses demonstrate that pore-pressure evolution controls the failure mode in saturated settings (fast vs slow sliding), consistent with prior laboratory experiments and lattice-Boltzmann–discrete element method simulations: dense assemblies stabilize via dilation, whereas loose assemblies compact rapidly and transiently fluidize. At the 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 the failure shear zone is rate-strengthening with respect to the inertial number (In, for dry medium) and viscous number (Iv, for fluid-saturated medium). We further utilize the mesoscale stress framework to compare the evolution of pore pressure in the CFD–DEM simulations of subaqueous slope collapse with an analytical solution for the development of the failure front, using inputs derived from numerical triaxial DEM tests on the same assemblies. The analytical model reproduces steady-state excess pore pressures and captures fluid–particle coupling; however, a mismatch near failure onset suggests a role for transient frictional behavior in grain–fluid interactions. These results support the development of physics-based models of natural hazards and advance our mechanistic understanding of saturated granular failure.