Self-healing mechanisms of water-conducting fractures：The influence of chemical precipitation induced by the mixing of different groundwater

In the long-term evolution process after the development of the water-conducting fractures in overburden, the self-healing phenomenon will occur, in which the water-conducting seepage capacity will gradually decrease. It is of great significance to study the self-healing mechanisms of water-conducting fractures for scientifically guiding the ecological function restoration of groundwater in mining areas. Based on the engineering detection results of self-healing of water-conducting fractures in No. 12401 working face of Bulianta coal mine in 15 years after mining, the mechanisms of repairing and reducing permeability of water-conducting fractures caused by chemical precipitation generated by the mixing of different groundwater from different layers after mining are studied. The results show that the water-conducting fractures in the overburden had directly developed into the Quaternary loose layer, which had connected multiple aquifers in the strata. Due to that the shallow groundwater contains more Ca²⁺, while the bedrock groundwater contains more CO₃ ²⁻ and HCO₃ ⁻, the CaCO₃ precipitation will be produced when the two kinds of groundwater meet and mix in the mining overburden. The sediment migrates with water and is continuously adsorbed on the fracture surface, the scaling process of inclusion co sedimentation consolidation occurs. After a long time of accumulation, the scale or inclusion with certain corrosion resistance is finally formed to plug and repair the fractures. The indoor test shows that the self-healing and permeability reduction effect of the water-conducting fractures caused by this process is stable and rapid. The absolute permeability of the fractured rock specimen is reduced from 0.09×10⁻¹⁵ m² to 0.0025×10⁻¹⁵ m² after nearly two months of mixed flow of simulated groundwater with two different water quality, and there is no obvious permeability fluctuation under the condition of 1.5 MPa water pressure. Because the confluence and mixing of different groundwater mainly occur in the fractured rock mass near the mining boundary, the self-healing process of water-conducting fractures and its effect in different areas of overburden will be significantly different. The self-healing of overburden water-conducting fractures in the middle area of No.12401 working face is mainly caused by the water-rock or water-gas-rock interactions caused by rainfall infiltration, while the water-conducting fractures near the mining boundary are dominated by the confluence mixed flow reaction of different groundwater and the water-rock or water-gas-rock interactions, so the self-healing effect of the latter should be obviously better. The field detection results show that after 15 years of self-healing evolution, the water-conducting fractures in the overburden near the mining boundary have been self-repaired in the depth range of 86.7 m to 179.1 m, while the overburden in the middle of the mining area still has local unrepaired phenomena in the corresponding self-healing area, with the depth range of 110.0 m to 118.6 m, which confirms the good repair effect of the chemical precipitation generated by the intersection and mixing of different groundwater.