Study on the true triaxial compression mechanical behavior of fissured granite and its micro-fracture mechanism based on the grain-based model

Abstract

Understanding the mechanical properties and rupture mechanism of fissured rocks under true triaxial stress conditions is of great significance in guiding engineering practice and preventing geo-engineering disasters. In this study, the mechanical properties, deformation characteristics, and damage mechanisms of prefabricated fissured granite were investigated by laboratory true triaxial experiments and grain-based model (GBM) simulations. Granite specimens with seven different fissure geometries (rock bridge inclination and length and fissure inclination) were tested in true triaxial compression tests. As a complementary study, the mineral composition and microstructure of granite were modeled using GBM numerical simulations for purpose of analyze the evolution of microfractures. The obtained results indicate that, larger rock bridge inclination, smaller rock bridge length, and larger crack inclination decreased the peak strength (up to 76.31 %), damage threshold, and modulus of elasticity of the specimens. Based on the specific configuration of the fissures, the pattern of damage underwent a transformation. It shifted from tensile shear cracks to a type of mixed tensile-shear cracks where the tensile cracks were predominant, and these cracks fully traversed the rock bridge region. Acoustic emission (AE) analysis shows that the fissured samples exhibit earlier and more intense energy release due to accelerated microcrack formation and propagation, and this effect is more pronounced at larger rock bridge inclinations, smaller rock bridge lengths, and larger rift inclinations. Numerical simulations validate the ability of GBM to capture macroscopic deformation and failure modes, showing strong agreement with the experimental stress-strain response (the peak stress deviation < 4.04 %). The microcrack evolution analysis emphasizes that cracks mainly start at the fissure tip and that the rock bridge geometry has a key influence on the propagation trajectory. In addition, the intermediate principal stress (${\sigma }_2$) enhances the peak strength but weakens as the effect of cracking increases, especially at larger rock bridge inclinations or smaller fissure angles. This exploration offers new insights of view on the micromechanical fracture mechanism of cleft granite under complex stress states, and provides theoretical and practical guidance for the stability assessment of deep underground engineering.