Experimental research on the seepage characteristics of water-sand flow in crushed sandstone of the coal mine collapse-type water inrush and sand gushing

In the high-intensity mining process of shallow buried coal seams in the Shendong and North Shaanxi mining areas, the collapse-type water inrush and sand gushing has become a typical geological hazard that threatens the prevention and control of roof water damage in mining engineering. Thoroughly studying the water and sand seepage characteristics of fractured rock masses is of great significance for revealing their disaster mechanisms and corresponding disaster prevention and control. In view of this, using the self-developed water-sand seepage test system of broken rock mass, based on the fractal dimension calculation method of cumulative mass of rock fragments proposed by Xie Heping, the broken sandstone samples with different particle size ratios were prepared according to the Talbot continuous gradation formula. The water-sand seepage test with variable porosity and particle size ratio was designed. The variation of permeability of water and water-sand mixture in broken sandstone with porosity and particle size ratio was analyzed. The permeability expression considering porosity ratio and fractal dimension was established, and the influence of the viscosity of water-sand mixture on permeability calculation results was quantified. The variation law of sand inrush mass of broken sandstone with porosity and particle size ratio is revealed, and the critical sand inrush criterion driven by permeability and fractal dimension is constructed. Taking the 22402 working face of Halagou Coal Mine as an engineering example, the accuracy of the criterion is verified. The results indicate that: ① The water-sand seepage process in crushed sandstone can be divided into three stages: initial fluctuation, relative stability, and decay. Permeability increases with porosity and Talbot’s power index, while water seepage exhibits higher permeability than water-sand seepage. ② The outflow mass of sand increases with porosity and Talbot’s power index, showing a near-linear relationship with permeability. ③ In existing research, many scholars have used simplified methods that use water viscosity as a substitute for water-sand mixture viscosity may introduce a maximum error of 18.3% in the calculations of this study, which could amplify to over 800% when upscaled to field conditions of water inrush and sand gushing disasters. ④ The introduction of fractal dimension can reduce the fitting error of fractured sandstone permeability by about 56.43%. The critical sand collapse curve proposed based on the variation law of permeability and fractal dimension on sand collapse quality has been applied well in the 22402 working face of Halagou Coal Mine.