Three-dimensional spatial distribution of porosity and permeability, and key controlling factors for grout diffusion in deep coal mine goafs

To address water accumulation in old goafs of deep coal mines, which readily forms water inrush hazards and makes grout diffusion difficult to predict, we established a three-dimensional continuous spatial distribution model of porosity and permeability in the goaf based on O-ring and key stratum theories. A multiphysics-coupled numerical model combined with response surface methodology (RSM) was used to quantitatively analyze the effects of influencing factors and their interactions on grout diffusion behavior. The results show that grout diffusion within the goaf is strongly asymmetric: the horizontal diffusion distance exceeds 25 m, the upward vertical diffusion distance exceeds 5 m, and the downward vertical diffusion distance exceeds 2 m. The permeability of the porous medium and grouting pressure promote diffusion, whereas grout viscosity inhibits it. The RSM-based grout diffusion volume prediction model was applied to the design of a fly-ash grout backfilling project in a goaf of a coal mine in Shaanxi Province. Considering pressure attenuation along the flow path and the heterogeneous seepage field, a borehole spacing of 40 m was determined, providing a theoretical basis and technical support for water-hazard control and optimization of grouting parameters in similar old goafs of deep coal mines.