An experimental study on the seepage characteristics of sandstones with different numbers of fractures before and after grouting

Background Grouting in surrounding rocks serves as a conventional approach to controlling disasters in coal mine roadways. The developmental degree of fractures significantly influences the reinforcement and sealing effects of grouting.

Methods To determine the changes in the impermeability of fractured surrounding rocks before and after grouting, this study investigated sandstones—the most common sedimentary rocks in coal mines. Using laboratory experiments and numerical simulations, this study explored the permeability variations of sandstone specimens with varying fracture numbers under different confining pressures and assessed the impact of grouting on their seepage performance. Through triaxial compression-seepage experiments using a Rock Top multi-field coupling experimental apparatus, this study investigated the stress-strain behavior and permeability variations of sandstone specimens with different numbers (1, 2, and 3) of fractures before and after grouting under confining pressures of 6 MPa, 8 MPa, and 10 MPa.

Results and Conclusions The ratio of the permeability of the fractured sandstones after grouting to that before grouting is defined as the grouting repair coefficient (Z_s). Experiments indicate that a lower grouting repair coefficient is associated with a higher repair degree of the permeability. Under the same confining pressure, the permeability of the sandstone specimens increased to 27.6 to 283.4 times and decreased by 64.32% to 98.47% compared to their original permeability before and after grouting, respectively as the fracture number increased, with the grouting repair coefficient exhibiting a power-law decreasing trend. Under the same fracture number, when the confining pressure increased from 6 MPa to 8 MPa and 10 MPa, the permeability of the sandstone specimens decreased by 48.42% to 85.30% before grounting and by 53.89% to 90.14% after grouting. Regarding the failure characteristics before and after grouting, fractures in the sandstone specimens propagated gradually from their ends to adjacent fractures and thus were interconnected with the latter as the fracture number increased, leading to the formation of more complex failure patterns and more secondary cracks. Based on engineering practice, random fractures were generated at a ratio of 1:100 relevant to the original fracture numbers (i.e., 1, 2, and 3) using software COMSOL and Matlab. The analysis of water flow velocity and grouting effects of the mining face roof and the bottom boundary of the sandstone aquifer verified the conclusion that more fractures within a certain range corresponded to a higher repair degree for sandstone permeability after grouting. This conclusion will provide strong technical support and scientific guidance for the safe mining of coal resources and disaster prevention and control.