Cracks expansion law of water-saturated coal-rock for fracture grouting: influence of confining pressure and superfine cement-based grout

This study conducted a splitting test on raw coal grouting under various confining pressures, focusing on the changes in viscosity over time of a cement-based grout. The results indicate that the chosen grouting direction for this experiment was perpendicular to the coal seam joints. Fracturing of post-grouted coal rock primarily occurred along transverse fissures, although some samples exhibited oblique fracturing due to internal fissures. This suggests that the initial crack arrangement in coal influences the final split shape and crack propagation pattern. As the confining pressure increases, so does the pressure at which splitting begins, with the latter being 1.2 to 2.5 times higher than the former. The highest grouting pressures at different confining pressures were as follows: 0.730 MPa, 1.268 MPa, 1.535 MPa, 2.106 MPa, and 2.430 MPa, representing increases of 42.4%, 17.4%, 27.1%, and 13.4%, respectively. Grouting pressure monitoring can be divided into three phases: the pressure rise phase, the pressure stabilization phase, and the pressure decrease phase. During the depressurization stage, rapid pressure drops occur due to complete penetration of the coal body, and pressure is promptly alleviated following grouting cessation. Injecting grouting material into fractures can widen their apertures, thereby enhancing grouting efficiency. However, fluid pressure may also cause irregular fracture extension and coal damage, reducing grouting efficiency. Scanning electron microscopy (SEM) results indicate that water saturation alters the pore structure of coal rock, transforming its robustly cemented original skeleton structure into a more pliant and replete one, making the sample more susceptible to fracturing under minimal grouting pressure to generate flow channels. Therefore, continuous monitoring of coal body fracturing degree and real-time adjustment of grouting pressure are crucial for achieving optimal grouting reinforcement efficacy.