Study On The Influence Of Coal Seam Spacing On The Mining Pressure Behavior In Middle Abandoned Coal Seam With "Upper Longwall-lower Pillar" Residual Areas

With the rapid development of social economy,the demand for coal is also increasing.As a non-renewable resource,the coal reserves are decreasing.In order to ensure the sustainable development of national economy,it is urgent to mine the residual coal resources under complicated conditions.Among them,the middle abandoned coal in compound residual mining areas is widely existed in many mining areas in China,and the rock layer movement of the middle abandoned coal stope is affected by many kinds of factors.The interval between coal seam is the main factor affecting the failure of middle abandoned coal roof and floor.Due to the difference of coal seam spacing,it will lead to the difference of strata behaviors of middle abandoned coal surrounding rock,and then affect the safe mining of middle abandoned coal.Only by grasping the law of mining pressure appearance of middle abandoned coal under different interlayer rock thickness conditions,can it play a broad guiding role in mining under similar conditions,and provide reference for the prevention and control of mine pressure disasters.On the basis of previous studies,this paper chooses the abandoned coal in the middle of the" upper longwall-lower pillar" composite residual mining area of Jinhuagong Coal Mine of Datong Coal Mine Group as the research object.By means of field investigation,theoretical analysis and numerical simulation,it studies the law of stress distribution,moving deformation and overburden structure about middle abandoned coal surrounding rock under different rock strata thickness conditions,and the relationship between the layer spacing and the failure of middle coal roof and floor is analyzed.From the perspective of the layer spacing,the evaluation method of middle coal mining feasibility is further discussed,and the theoretical basis of safe mining of abandoned coal in the middle of compound residual mining area is perfected.The main research contents and results are as follows:(1)The layer spacing effect of the surrounding rock stress distribution in the middle coal seam stope is mainly manifested as follows:as the spacing of the lower layer increases,the range of the pressure relief zone decreases,and the range of the stress concentration zone increases.When the spacing of the lower layer is 30 meter,the stress concentration factor is the smallest and the pressure relief effect is good.The upper layer spacing mainly affects the stress distribution at the position above the coal pillar.When the upper layer spacing is 12 meter,the stress concentration factor is the smallest.(2)The layer spacing effect of the surrounding rock movement deformation of the middle coal seam stope is mainly reflected in the position above the column mining area,but has little effect on the rock layer above the remaining coal pillar:the change rate of maximum subsidence of middle coal seam increases with the increase of interval between lower coal seams,the influence degree of lower coal mining on middle coal seams decreases gradually.The maximum displacement of middle abandoned coal decreases with the increase of upper layer spacing,but the change rate of displacement is different.When the distance between upper layers is greater than 9 meter,the rate of change is significant increase.(3)The layer spacing effect of the surrounding rock structure of the middle coal seam stope is as follows:the spacing of lower layer mainly affects the structure form of the the middle coal seam floor.When the lower layer spacing is greater than 30 meter,the lower control strata are affected by multiple mining of upper-lower and middle abandoned coal seams,and can still control the movement of overlying strata with good stability.Under the condition of the stability of the lower interlayer rock layer and the remaining pillars,with the mining of the middle coal,the different upper layer spacing has certain influence on the roof caving characteristics.When the upper layer spacing is greater than 12 meter,the upper interlayer rock layer do not break,forming the"rock-gangue" type arch structure;when the upper layer spacing is less than 12 meter,the upper interlayer rock layer cave and break.By simplifying the model,the relationship between the controlled roof distance and the direct roof tensile strength is analyzed.The minimum thickness of the upper interlayer rock layer is hx?(?)(4)According to the relationship between the lower layer spacing and the breaking distance of the hard rock layer,the critical lower layer spacing can be obtained.When the structure of lower control strata in the middle coal seam floor is exist,the critical lower layer spacing is h0=h·(lmax/Lmax)2.According to the relationship between the upper layer spacing and the depth of failure,the critical spacing of the upper layer in the presence of the structure of the roof bearing layer of the middle coal seam is obtained,that is[h0]=X·cos?/zcos(?/4+?/2)e(?/4+?/2)tan?.(5)From the perspective of layer spacing,the structural forms of the roof and floor of the middle abandoned coal can be divided into three types:the lower control layer is unstable,the lower control layer is stable but the upper bearing layer is unstable,and the upper bearing layer and lower control layer are stable.Further,a method for evaluating the mining feasibility of the middle abandoned coal in the "upper longwall-lower pillar" composite residual mining areas is formed.