Grouting and re-bearing behaviors of sandy mudstone in the residual stage

Grouting is an essential technique for reinforcing tunnel rock masses following deformation and failure. However, the mechanisms and effectiveness evaluation of grouting in fractured rock masses that have experienced substantial deformation and transition into the residual stage remain insufficiently understood. To elucidate the relationship between grouting effectiveness and pre-cracking strain, grouting and subsequent re-fracturing tests were conducted on sandy mudstone specimens with varying levels of pre-cracking strain. Additionally, a model was developed to determine the optimal grouting timing during the residual stage. The results indicate that the failure mode of specimens in the residual stage exhibits banded and localized distribution patterns. As pre-cracking strain increases, both the maximum fracture aperture and the relative grout injection ratio increase, with the increases becoming more pronounced at higher strain levels. After grouting, the consolidation coefficient and strength enhancement coefficient exhibit a positive correlation with pre-cracking strain, although the rate of increase gradually decreases. Grouting does not alter the initial failure mode of residual-stage fractured specimens but effectively suppresses secondary crack propagation in regions distant from primary fractures. At the microscale, grout bonds within the rock matrix form cavity structures that delay tensile failure and generate an interconnected network, thereby enhancing crack resistance. Based on the evolution of rock damage and the efficiency of grouting materials utilization, a method is proposed to determine the optimal grouting timing for fractured specimens in the residual stage. At the optimal timing, specimens exhibit moderate damage while maintaining high reinforcement efficiency per unit mass of grout.