Analysis of Pressure Relief Effect on the Protective Layer of Hard Roof and Extra-Thickness Coal Seam Mining

The roof of Tashan coal mine was hard and difficult to fall. In the process of fully mechanized coal caving in the Carboniferous extra-thick coal seam, the strong pressure and rock burst appeared in the working face. To obtain the pressure relief effect of the exploitation of Shan 4# protective seam in Tashan coal mine, The stress distribution and displacement variation of the roof and floor of Shan 4# protective layer were obtained by numerical simulation. And by mine pressure observation in the 8107 working face of the un-mining protective seam and in the 8108 working face of mining protective layer, the pressure relief effect of mining Shan 4# protective layer was analyzed. It was shown that the lateral of the open-off cut and the rock mass in front of the working face produced stress concentration when 3–5# coal was mined separately. The stress concentration coefficient of k was 3. The liberated layer was in the floor relief area from the open-off cut after the upper protective layer being mined owing to internal misaligned arrangement of the liberated layer when the protective layer and the liberated layer were mined. The stress concentration of the lateral of open-off cut and working face of the liberated layer was greatly reduced, and the stress concentration coefficient was about 1.5. The middle of the goaf would be re-compacted with the advance of the working face. The maximum principal stress of the liberated layer was W type distribution. The expansion deformation of 3–5# coal was M type distribution. The working resistance of the 8107 working face was mainly distributed in 8000–12,000 kN. The working resistance of the 8108 working face was mainly distributed in 4000–10,000 kN. The vertical stress of the roof of the 3–5# coal was decreased by about 30% after mining the protective layer. The mining of the protective seam effectively reduced the pressure strength of the roof of the fully mechanized coal seam. The reference was provided for the rational layout of coal seam safe mining with impact tendencies.