Seepage evolution patterns and self-sealing characterization of mining-induced overburden fractures in ecologically vulnerable mining areas

ObjectiveThe ecologically vulnerable areas (EVAs) in West China have been significantly affected by coal mining. In these areas, aquifers overlying goaves have been damaged under mining disturbance, being prone to cause water resource loss and desertification. Hence, it is crucial to guide the self-sealing of fractures artificially to preserve and further restore the groundwater level.MethodsThis study investigated mining face 22108 in the Buertai coal mine in the Inner Mongolia Autonomous Region. Based on the physical simulation experiments using similar materials, numerical calculations, and theoretical analysis, this study delved into the seepage evolution patterns of fractures in the overburden of coal seam 2-2 in the ecologically vulnerable mining area under the coupling effect of mining disturbance and fracture water seepage. Moreover, this study characterized and analyzed the self-sealing effect of overburden fractures. Results and ConclusionsThe results indicate that the overall fracture development, derived using displacement difference coefficients, exhibited a bimodal distribution characterized by highly-developed fractures on both sides of the goaf and compaction in its central part. As the mining face advanced for 78 m, hydraulically conductive fractures propagated to the No.Ⅱ aquifer, corresponding to a displacement difference coefficient of 0.25 and an abrupt change in moisture content. In this case, the evolution and distribution of seepage began to correlate passively with the development of overburden fractures. As the displacement difference coefficient increased from 0.25 to 0.40, the average moisture content of aquicludes in the overburden increased by 2.0%. Concurrently, fractures near the mining boundary exhibited higher conductivity than overburden fractures in the central part of the goaf. After coal mining, the groundwater level in the No.Ⅱ aquifer declined by 13 m on average, and new aquifers were formed by the compaction of the goaf and the closure of mining-induced overburden fractures, suggesting the succession of the groundwater flow field. Under the long-term influence of multiple factors like residual subsidence and stress-induced compaction, the fractured rock masses in the overburden showed a self-sealing phenomenon as evidenced by seepage reduction in aquicludes. According to the simulation results of self-sealing 8.3 years after coal mining, the fracture self-sealing rates, calculated using the seepage velocity of hydraulically conductive fractures, ranged from 45.5% to 55.6% in the central part of the goaf and from 6.3% to 25.0% in the mining boundary areas. In contrast, the self-sealing rates calculated based on the moisture content of the overburden ranged from 33.3% to 35.7% in the central part of the goaf and from 10.0% to 18.2% in the mining boundary area. The results of this study provide a theoretical basis for revealing the evolution patterns of seepage in overburden fractures in ecologically vulnerable mining areas in West China and quantitatively characterizing the self-sealing capacity of fractures.