Exploring the role of sample size on fracture growth mechanisms in intact rock: insights from 3D DEM-DFN analysis

The effect of sample size on the deformation characteristic and fracture growth mechanism of intact rock subjected to unconfined compressive stress state is examined in this study via discrete element method (DEM) coupled with discrete fracture network (DFN). Three-dimensional (3D) numerical models are first developed based on reported laboratory experiments and verified to realistically replicate the effect of sample size on the macro-mechanical response of the intact rock. This includes strength behavior, fracturing activities, and energy budgets. Micromechanical analyses are then performed to understand the role of sample size on the deformational behavior and damage progression in intact rock. Emphasis is placed on the distributions of coordination number, evolutions of crack density and the degree of crack anisotropy with regard to invariants of crack tensors. In addition, the 3D distribution of microstructure and contact networks are also presented. Results reveal that deformability and strength of intact rock are vastly reliant on the sample size. Increasing sample size facilitate the accumulation of induced microcracks due to the increased probability of interparticle bond failure. It is found that the increase in sample size can result in more tensile openings with strong dilatancy, as evidenced by local-scale porosity based on the implementation of Voronoi cells. Finally, the monotonical increase in seismic b-values with the increase in sample size suggests an acceleration in the number of small-magnitude acoustic emission (AE) events.