Study on the influence of close range coal seam mining relationship on the deformation law of surrounding rock in lower roadway

When the mining relationship between the upper coal seam working face and the lower coal seam roadway in the close distance coal seam changes, the deformation and instability mechanism of the surrounding rock of the roadway will be more complicated. At present, there is little research on the dynamic evolution law and instability characteristics of the roadway when the upper coal seam working face and the lower coal seam roadway advance in different directions. Taking the close range coal seam of Nengdong Coal Mine in northern Shaanxi as the research object, a combination of theoretical analysis, numerical simulation, and on-site measurement is used to study the stability of the lower coal seam roadway after the upper coal seam working surfaceis mined. Theoretical analysis shows that the depth of the floor cracks generated after mining the upper coal seam working surfaceis 22.5 m, and they have not developed to the lower coal seam. According to the spatial relationship of mining, the mining face and the roadway are divided into three states: facing, intersecting, and advancing in the opposite direction. Numerical simulations show that when the spatial relationship between the roadway and the working face changes, the deformation of the surrounding rock of the lower coal seam roadway is affected. The results show the following points. ① When the mining relationship between the upper coal seam working surface and the lower coal seam roadway is intersecting and advancing in the opposite direction, the stress of the surrounding rock of the roadway shows a trend of first increasing, then decreasing, and then increasing again. When the length of intervals of travel is 90 m, the maximum stress value is 6.5 MPa, and the stress concentration factor is 1.49. When the length of intervals of travel is 100-110 m, the stress reduction of the surrounding rock of the roadway is the largest, decreasing by 53.2%. When the length of intervals of travel is 150 m, the minimum is 0.95 MPa, and then it continues to increase until it returns to the original rock stress. ② The displacement of the surrounding rock in the roadway increases significantly when the length of intervals of travel is between 100-150 m, and reaches its maximum displacement of 0.036 m at 150 m. As the roadway approaches the boundary coal pillar, the displacement of the roadway decreases. The on-site measurement results show that when the upper coal seam working surface passes through the lower coal seam roadway, the displacement of the roadway increases significantly, and the maximum displacement of the roof is 3.41 cm. It is consistent with the numerical simulation results. If the geological conditions are simple during the process of intersecting advancement, the advancement speed can be appropriately accelerated to reduce the impact of upper coal seam working surface mining on lower level roadways.