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

Abstract The concealability of grout reinforcement engineering significantly complicates the challenges associated with grouting operations in underground engineering. Achieving grouting precision is particularly difficult due to the uneven distribution of ground stress fields, varying degrees of pore and fracture development across different rock masses, and the complex and dynamic environments through which grouting slurry propagates. This study conducted a splitting test on raw coal samples subjected to grouting under various confining pressures, with a focus on the time-dependent changes in the viscosity of cement-based grout. The results indicate that the selected grouting direction in this experiment was perpendicular to the coal seam joints. Post-grouting fracturing primarily occurred along transverse fissures, although some samples exhibited oblique fracturing due to pre-existing internal fissures. This observation suggests that the initial crack configuration in coal influences the final fracture morphology and crack propagation path. As confining pressure increases, so does the splitting initiation pressure, which was found to be 1.2 to 2.5 times higher than the confining pressure. The peak grouting pressures under different confining pressures were recorded as 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 categorized into three distinct phases: pressure rise, pressure stabilization, and pressure decline. During the pressure decline phase, rapid pressure drops occur once the coal body is fully penetrated, and pressure is promptly relieved after grouting cessation. Injecting grouting material into fractures can widen their apertures, thereby improving grouting efficiency. However, fluid pressure may also lead to irregular fracture extension and coal matrix damage, ultimately reducing grouting effectiveness. Scanning electron microscopy (SEM) results demonstrate that water saturation alters the pore structure of coal rock, transforming its originally well-cemented skeleton structure into a more flexible and porous configuration, making the sample more prone to fracturing under minimal grouting pressure and facilitating the formation of flow channels. Therefore, continuous monitoring of the fracturing degree of the coal body and real-time adjustment of grouting pressure are essential for achieving optimal grouting reinforcement outcomes.