Investigating frictional instability due to pressurization in granular media: insights from coupled computational fluid dynamics discrete element method

Abstract

Fluid pressurization can reactivate subcritically stressed granular layers in faults, slopes, and injection-perturbed reservoirs, but grain-scale feedbacks among pressure diffusion, drainage, and contact-network degradation remain unresolved. Here, 3D coupled CFD-DEM simulations investigate pore-pressure-induced reactivation of confined, fluid-saturated granular shear layers under imposed shear stress. Strain-controlled tests define the Mohr-Coulomb strength envelope; stress-controlled simulations then impose subcritical shear stresses while basal pore pressure increases under drained and undrained conditions. Instability is governed not by pore pressure alone, but by its coupled evolution with effective stress, drainage, dilation or compaction, hydraulic connectivity, and granular fabric. Undrained boundaries retain excess pore pressure, whereas drained boundaries maintain vertical gradients and suppress excess pressure. Internal fields reveal alternating dilation and compaction bands and reorganization of a porosity-derived permeability proxy, showing that hydraulic pathways evolve during deformation. Micromechanical diagnostics identify localized particle rotation, force-chain reorganization, porosity redistribution, and coordination-number variations controlled mainly by imposed shear-stress level rather than drainage. Second-order fabric metrics show that post-failure weakening coincides with loss of directional force-chain organization, especially at lower shear. Friction-velocity and friction-porosity trajectories indicate a transition from dilatancy-dominated strengthening to pore-pressure-driven weakening. Viscous-number scaling partially organizes the low-Iv creeping response, 10^-8 <= Iv <= 10^-5, but not onto a unique local rheology. These results clarify how drainage-controlled hydromechanical feedbacks and fabric degradation convert pore-pressure forcing into instability.