Study on pressure relief range of overlying strata and accurate gas extraction of adjacent strata in superimposed mining of coal seam

Aiming at the mining problem of No.9 coal seam with strong outburst danger in Xinyuan Coal Mine, we proposed that to mine No.3 coal seam as the upper protective layer first, by analyzing the failure mechanism of roof and floor after coal seam mining, the maximum depth of No.3 coal seam floor failure and the height range of No.9 coal seam fracture zone are predicted respectively. In order to restore the real mining situation of the working face, the numerical simulation scheme of goaf filling is studied, and the stress and displacement evolution law of overlying strata of No.9 coal seam during No.3 coal seam mining is analyzed. The migration of overlying strata in single coal seam and double coal seam mining is compared, and the proportion of gas emission in mining layer and adjacent layer is calculated. Therefore, the gas extraction scheme using medium gas extraction roadway to extract gas from adjacent layers is proposed, and its extraction effect is verified. At the same time, the pressure relief effect of No.9 coal seam is investigated. The results show that the maximum failure depth of No.3 coal seam floor is 59.52 m, which is greater than the spacing between No.3 and No.9 coal seams. It is preliminarily verified that the mining of No.3 coal seam has a pressure relief effect on No.9 coal seam, and the height range of No.9 coal seam fracture zone is 11.92-46.88 m. After the mining of No.3 coal seam, No.9 coal seam is distributed in four areas: original rock stress area, stress concentration area, pressure relief area and re-compaction area. The pressure relief range increases first and then decreases due to the roof pressure of the working face, and finally tends to be stable. The vertical stress of No.9 coal seam decreases by more than 40%, the maximum pressure relief rate is 0.54, and the maximum expansion deformation rate is 4.78‰. The residual gas pressure of No.9 coal seam is reduced to less than 0.23 MPa, and the permeability coefficient of coal seam is increased by 62 times, which eliminates the outburst risk of protected layer. During the stable extraction period of the middle lane, the gas concentration in the return air lane is in the range of 0.18%-0.38%, which ensures the safe production of the working face.