Fracture initiation mechanism of true triaxial fracture grouting under stress and crack influence

The deep formation are characterized by complex stress environments and intricate fracture distributions, resulting in unclear fracture propagation behavior during split grouting under triaxial stress and an empirical approach to parameter design. To rationally design split grouting parameters, elucidate fracture initiation and propagation mechanisms, and enhance grouting reinforcement effectiveness, split grouting tests were conducted on rock-like specimens under varying stresses and distances from pre-existing fractures. By analyzing acoustic emission (AE) parameters, we obtained the variations in grouting pressure and fracture propagation characteristics under different principal stress ratios (σ_y/σ_z) and distances from pre-existing fractures. The discrepancies between the experimental fracture initiation pressures and those calculated by different splitting theories were discussed. Fracture toughness corresponding to various test conditions was derived by inverse analysis based on fracture mechanics, and the inversion results were incorporated into a numerical model to validate the experimental outcomes. Finally, the practical implications of the beneficial findings from the tests were summarized. The results indicate that as the principal stress ratio σ_y/σ_z increases, the fracture initiation pressure during split grouting exhibits a “rise-fall” trend, and the fracture propagation mode shifts from unidirectional to multidirectional expansion. The fracture initiation pressure is proportional to the distance L between the pre-existing fracture and the grouting hole. As L increases, the influence of the pre-existing fracture on the propagation path of the split fracture diminishes. At a distance of 70 mm, only deflection of the propagation path occurs, without interconnection between the split fracture and the pre-existing fracture. The fracture propagation process during split grouting is predominantly characterized by tensile failure. When fractures exist within the rock mass, the proportion of tensile cracks during propagation increases slightly. At a fracture distance of 10 mm from the grouting hole, the proportion of tensile cracks rises significantly. The fracture toughness K_\mathrmIc^* under triaxial stress is proportional to the principal stress ratio σ_y/σ_z. The closer the ratio is to 1, the higher the fracture initiation pressure. Pre-existing fractures alter the stress distribution in the stratum, weakening the overall stress level of the rock mass and reducing the fracture initiation pressure to some extent. However, at certain distances, they can increase the minimum principal stress around the grouting hole, thereby raising the fracture initiation pressure.