Background Burnt rocks represent special geobodies formed by the spontaneous combustion of coal seams. Their pore and fracture systems provide preferential spaces for groundwater occurrence and migration while also posing severe water hazard threats to the safe mining of adjacent coal seams.

Methods To address this engineering challenge, this study systematically analyzed the void structures within burnt rocks using three-dimensional reconstruction technology. Through laboratory seepage and grouting experiments, this study revealed the seepage patterns and grouting-induced permeability mechanisms of typical burnt rock specimens.

Results and Conclusions The burnt rock specimens contained highly developed pores and fractures, with the void structures showing complex and various spatial distributions. The interconnected large pores and fractures accounted for 56.72% of the total volume of voids, suggesting moderate connectivity in primary pore and fracture networks. Seepage experiments indicate that the burnt rocks exhibited a significant nonlinear relationship between the hydraulic gradient and flow velocity of seepage, with their seepage dynamics behavior consistent with the patterns characterized by the Forchheimer equation. The permeability varied significantly along different seepage directions, with differences in permeability coefficients of about 50%. Clay-cement grout exhibited high injectability within pore-fracture systems in the burnt rocks. The effective diffusion distance of the grout was predominately governed by the geometrical confinement effect of the pore-fracture throats. The proportion of pores and fractures filled by grout gradually decreased with an increase in the distance from the grouting port. The burnt rocks exhibited significant permeability reduction after grouting. The hydraulic gradient and flow velocity maintained a nonlinear relationship after grouting. Within 14 days after grouting, the permeability reduction rate increased with time, eventually stabilizing. These findings provide a theoretical basis and technical reference for water hazard prevention and control during coal seam mining in burnt rock areas.