Grain-scale numerical simulation of crystalline rock fracturing using Soundless Cracking Demolition Agents for in-situ preconditioning

Abstract

Soundless Cracking Demolition Agents (SCDAs) are becoming increasingly popular for near-surface underground rock fragmentation applications. Several studies have been carried out to accurately simulate the fracturing processes of SCDA under its volumetric expansion inside a borehole. These numerical simulations and experiments have been limited mainly to homogeneous and intact rock masses in most cases. In this paper, we present a numerical approach that assesses the influence of mineral heterogeneity and rock mass defects (pore structures) on the fracturing performance of SCDA at the grain size level. For the simulation, a numerical crystalline rock grain assembly was generated using NEPER (a polycrystal generation tool) to introduce spatial variability of grain size that closely mimicked the grain arrangement of granitic rock. The assembly was then imported to 3-Dimensional Distinct Element Code (3DEC) as a block-based model. Heterogeneity was introduced to the model in terms of both mineralogical spatial distribution and strength variation. Pore spaces were introduced to the model using random grain deletion in the rock model. Intergranular and transgranular fracturing of the assembly was also simulated by utilizing a dual-layer discretization technique in 3DEC. SCDA charged fracture simulation was carried out in the model using a single central injection well. The results suggest intergranular fracturing to be the dominant mode of fracturing with additional grain crushing (transgranular fracturing) in the vicinity of the injection well. Grain size and in-situ stress anisotropy largely affect the direction and geometry of radial fractures initiated around an injection well during SCDA charging. The stress concentrations introduced by the pore structures of the matrix were found to have a strong influence on crack deflection, additional micro-cracking in the matrix and final tortuosity of the fractures produced. The results presented in this paper suggest that the micro-mechanical heterogeneity of the rock mass significantly influences the final fracture pattern produced by SCDA charging and therefore should be given more attention during crystalline rock pre-conditioning applications.