The Basic Theoretical Research Of Rock Stratr's Control In Mining Middle Abandoned Coal Seam With Composite Pillar Residual Arers

Middle abandoned coal seam with composite pillar residual areas(CPRA)distributes in many mining areas across China, which has considerable reserves and high value of exploitation. It has increasingly become the focus of attention among scholars and engineers in recent years. Due to superposition pillar mining effect of multiple coal seams, strata behavior in recovering middle abandoned coal seam with CPRA is obviously different with the one in upward mining to recovery residual coal seam, pressure-relief mining of protective coal seam,distressing mining of high burst liability coal seam and mining of closed distance seam group. Meanwhile, there is no mature theoretical system and scientific technical program to guide the safety mining of middle abandoned coal seam. Therefore, it's urgent and necessary to carry out the basic theoretical research of rock strata's control on mining middle abandoned coal seam with CPRA.The middle abandoned coal seam No.7 with CPRA in Baijiazhuang coal mine, which belongs to Xishan Coal Electricity Group, is selected as the research object. The research methods include field investigation, theoretical analysis, similar simulation and numerical simulation. The influence of upper and bottom pillar mining on stress distribution, moving deformation and structure evolution are explored firstly. Then, temporal and spatial evolution of“stress field—displacement field—structure field” is studied when the middle abandoned coal seam with CPRA is extracted by caving mining method. Thirdly,the stability of coal pillars abandoned in composite residual areas was analyzed.Finally, Safety-mining of middle abandoned coal seam with CPRA was evaluated. The basic theory of rock strata's control will form based on the above researches, which will provide guidance for safety and high efficiency mining.The main research contents and results are as follows:(1)The stress distribution evolves from initial balance to wave balance after pillar mining of coal seam No.8, while it converts from initial wave balance into new wave balance after pillar mining of coal seam No.6. In the mining process of middle abandoned coal seam No.7, the abutment pressure gather and aggregate in left-right sides of working face, while the pressure in the middle of working face is released obviously.( 2) The moving deformation evolves from initial static state to‘VVVV'state after pillar mining of coal seam No.8, while it converts from initial‘VVVV'state to new ‘VVVV'state after pillar mining of coal seam No.6. In the mining process of middle abandoned coal seam No.7, the vertical displacement of roof strata above working face will obviously increase, while it will decrease weakly in floor strata because of drum effect.( 3) The structure is mainly consisted of bearing layer above bottom residual mining area after pillar mining of coal seam No.8,while it is consisted of control layer under upper residual mining area. The former one's mechanical model can be simplified to rectangular flat plate with uniform load and clamped edges on elastic compression bars, while the later one's mechanical model can be simplified to rectangular flat plate with concentrated load and clamped edges on elastic compression bars. The stability of bearing layer can be determined by the strength, while the stability of control layer can be analyzed by the deflection.In the mining process of middle abandoned coal seam No.7, the structure is consisted of bearing and control layer and the continuity and integrity of them can be kept.(4)When safety factor 1.5sf ?, the coal pillars abandoned in compositeresidual areas is stable based on ultimate strength theory. When width of elastic region is consistent with2 a ?0.65 a or120.65 a aa??, the coal pillars abandoned in composite residual areas can keep long-term stability based on progressive failure theory. When the ratio of the width of yield region and coal pillar is more than 88.08%, the coal pillars abandoned in composite residual areas will be catastrophe failure based on catastrophic theory.(5)The determining technical method about safety-mining evaluation of middle abandoned coal seam No.7 with CPRA is formed based on upper control layer and bottom bearing layer.