Attenuation law of concentrated stress under coal pillar of close coal seams and its application.

When mining the multiple coal seams in close proximity, the coal pillar left in the goaf causes stress concentration in the floor. The layout of mining roadway in lower short distance coal seam is affected by the propagation of concentrated stress caused by the upper coal pillar. To determine the reasonable distance of the roadway in the lower coal seam outside the coal pillar, the attenuation law of concentrated stress outside the coal pillar boundary has been studied through simulation model, theoretical analysis, and example analysis. The results show that the concentrated stress of coal pillar decreases with the distance from the coal pillar. At the coal pillar boundary position, the stress change rate reaches the maximum in the floor with different depths, and the stress decreases rapidly in the floor strata outside the coal pillar. Under the same stress condition, the roadway layout in strata at different depths is different. The joint formula of stress and stress change rate was deduced to determine the reasonable horizontal distance of roadway outside coal pillar. The results obtained by the numerical simulation fitting formula and the theoretical calculation formula are close to each other when calculating an engineering example. The roadway pressure appearance is not obvious in the experiment and physical simulation, which indicates that the theoretical formula can satisfy the requirement of engineering calculation. The method provides a reference for roadway location selection under similar conditions.

Study of water-conducting fractured zone development law and assessment method in longwall mining of shallow coal seam.

Starting from the source of mining, scientific understanding of surface damage law and assessment method in longwall mining of shallow coal seam is conducive to solving the problems of geological hazards and deterioration of the ecological environment, and promoting the coordinated development of efficient coal mining and environmental protection. Based on numerical simulation and theoretical analysis, the surface damage process and spatiotemporal evolution of fracture field are discussed. The influencing factors and assessment method of surface damage are clarified. The results show that surface damage undergone the immediate roof caving stage, the fracture and instability stage of main roof, the spatial amplification stage of separation layer, the instability stage of surface damage control layer and the mining damage stability stage. Under the critical extraction condition, the cracks above the goaf are divided into the crack area outside the cut, the crack area inside the cut, the re-compaction area in the middle goaf, the crack area behind the longwall face, and the crack area in front of the longwall face. The overburden reaches critical failure ahead of surface critical mining. The sensitivity of loose layer thickness to surface subsidence coefficient is greater than that of mining thickness to surface subsidence coefficient. Surface damage control should be adjusted to local conditions, and finally realize zoning treatment and zoning repair. Through the three-step method of "longwall face rapid advancing method, local grouting reinforcement overburden method and zoning treatment ground fissures method", the surface damage control of 12,401 longwall face is realized. This research provides theoretical guidance and application value for surface ecological restoration in similar mining area.