Addressing high-pressure fault rupture limitations in PFC3D: a dynamic weakening approach

Abstract

Discrete element method (DEM) codes were developed in the field of rock mechanics. Compared to continuum codes, it has many advantages, such as allowing larger grain displacements, detachment of grains, and simulation of discrete fractures. DEM has long been used to model dynamic instability on faults. However, the disadvantage of DEM codes in the simulation of higher confining pressure triaxial tests has not been discussed previously. This work aims to investigate these shortcomings and provide workable solutions for an existing numerical framework to reproduce realistic fault rupture behaviors. Our study primarily comprises of two parts. In part one, we explored how the non-Dirac delta distribution of contact forces controls the fault rupture initiation and its impact on fault rupture propagation under high confining pressure. To resolve the discrepancies between the experiments and the simulations in standard PFC3D code, a novel local dynamic weakening model was proposed, motivated by the cohesion zone model from fracture mechanics and the cohesion loss model from rock mechanics in part two. The dynamic weakening model is incorporated into the smooth-joint (SJ) contact model and is tested with simulations of experiments conducted under high confining pressures. It successfully reproduces realistic fault rupture behaviors, and the synthetic acoustic emission (AE) characteristics including magnitude–frequency relationships and fractal dimensions match those in the experiment. This study illustrates that cohesion loss within granular materials and the softening around rupture tips are quintessential mechanisms that promote fault rupture.