Grain-based modelling of dynamic shear rupture of heterogeneous rock using a coupled continuum-discrete model

Abstract

Dynamic shear rupture of rocks plays a significant role in the formation of earthquake faults and the stability of underground engineering structures at depth. This paper aims to explore shear strength, progressive fracturing and seismicity, and underlying shear failure mechanisms of heterogeneous rock under dynamic loads. A lab-scale direct shear test is conducted on a cubic granite sample by using a Triaxial Hopkinson bar (Tri-HB) system, and a coupled continuum-discrete element method is applied to simulate the functionality of the full-scale Tri-HB system for dynamic direct shear tests. The evolution of shear stress and strain distribution shows obvious concentration around the shear rupture band, which verifies the feasibility of the designed testing and modelling configuration for characterising dynamic shear stress and deformation. A grain-based discrete element method (GB-DEM) is adopted to represent the mineralogical heterogeneity of granite and to reveal multi-scale fracturing and seismic activities under dynamic shear loads. The microcracking process shows a transition of the dominant failure mode from intergranular crack to transgranular crack during the shear process. The shear rupture zone is typically initiated from the discrete fractures inclined at certain angles, which are gradually coalesced to form a main shear fracture that breaks the rock into two main blocks. The seismicity of the shear rupture process shows a drop in the b-value before the peak shear stress, followed by a certain degree of recovery at the post-failure stage. We found that shear mechanical response and rupture behaviour show a strong dependency on both strain rate and initial normal stress. The number of transgranular cracks is increased with increasing strain rate, which consequently results in intensively crushed mineral grains. Inclined fractures become steeper under higher initial normal stresses which further widen the shear rupture band. The normal-shear stress curves show a common trend of two linear increase stages followed by nonlinear damage and failure processes. The shear strength can be approximated by the Mohr-Coulomb strength envelope under various initial normal stresses and strain rates.