How Porosity Controls Macroscopic Failure via Propagating Fractures and Percolating Force Chains in Porous Granular Rocks

Abstract

This contribution aims to illuminate the micromechanisms that control the macroscopic failure of porous, granular, and cohesive rocks. With discrete element method simulations, we triaxially compress cohesive granular models composed of interlocking breakable grains, similar to sandstone and oolitic limestone. We track the morphology of the force network and the resulting spatiotemporal fracture distribution. To shed light on the varying applicability of the pore-emanated and Hertzian fracture models, we focus on differences in the micromechanics that develop between and within grains. For 5–20% porosity rocks, the cement between grains develops 10–50% higher mean forces than material within grains. Force chains that support the highest system-spanning forces are more localized prior to failure in 20% porosity rocks and are more diffusely spread in 5% porosity rocks. Confining stress reduces this localization, similar to the impact of confining stress on the macroscopic expression of brittle failure. The magnitude and rate of fracture development relative to the axial strain or axial stress increase toward failure, consistent with experimental observations. In contrast, the rate of fracture development relative to the amount of external work done on the system, Wext, is approximately constant. Higher porosity rocks require lower inputs of Wext to produce the same degree of fracturing. Decreasing the cement strength promotes fracture development within the cement, revealing the dominance of intergranular, pore-emanated fractures. These simulations provide predictions about how porosity controls the persistence of the stress field, with implications on the sequential development of deformation bands, and other deformational processes that modify porosity.