Self-healing law of mining fractured rock mass in Shendong coalfield

The objective phenomenon of seepage reduction and self-healing is easy to occur in the mining fractured rock mass after long-term evolution process of coal seam after mining. In survey of this phenomenon, taking the No. 12401 work face of Bulianta Coal Mine and the No. 52306 work face of Daliuta Coal Mine in Shendong as typical test areas, the engineering exploration and theoretical research on the self-healing law in different areas of overburden were carried out. The evolution law of long-term self-healing process of overburden after mining was revealed. The results show that the fractured overburden of the two coal mines had achieved good self-healing effects 15 years and 7.3 years after mining, respectively. The areas with water conduction within the original water-conducting fractured zone have been significantly reduced. Although the caved zone still conducts water, the height has also been reduced. By taking the ratio of the vertical length of the self-healing section in overburden from the original water diversion to the water isolation to the height of the water-conducting fractured zone as the self-healing recovery rate for statistics, it is obtained that the self-healing recovery rate of No. 12401 work face in the overburden near the mining boundary and in the middle of the dip has reached 59.5% and 53.5% respectively, and the self-healing recovery rate of 52306 work face in the corresponding area is 25.8% and 47.5% respectively. Due to that No. 12401 work face has mined for a longer time and has a larger coal seam burial depth, its self-healing effect of fractured overburden is obviously better. Affected by the occurrence difference of different strata in the vertical direction of the overburden and the initial damasge degree in different areas in the horizontal direction, the distribution of the self-healing area is often discrete and discontinuous, in which the interval of the unrepaired area generally corresponds to the position of the key stratum or thick and hard rock layer, and the shorter the post mining life and the closer to the mining boundary, the more significant the dispersion. The self-healing effect of mining fractured rock mass is actually a process of permeability reduction and evolution under the comprehensive influence of multiple factors. For the mining conditions in Shendong coalfield, the residual settlement caused by stress compaction plays a leading role in the initial process within 1.5 to 2 years after coal mining, which is relatively significant in the overburden in the middle of the mining area. Then, the self-healing process has been dominated by the water-rock or water-gas-rock interactions in goaf. That is, the filling and sealing effect of the derivatives produced in the relevant process on the fractures and the roughness reduction of the fracture surface induced by hydraulic erosion directly affect the self-healing results. Therefore, whether there is long-term water seepage in the overburden after mining determines the difficulty of self-healing of fractured rock mass. Water conducting fractures in the overburden of most mines in Shendong are generally directly connected to the Quaternary loose layer or surface, and in recent years, there is abundant rainfall, which provides superior conditions for the generation of water-rock or water-gas-rock interactions and the promotion of self-healing effect. © 2023 China Coal Society. All Rights Reserved.