Small coal pillar technology in fully-mechanized top-coal caving face of multi layer hard roof and extra thick coal seam

In order to study the stability of small coal pillar roadway protection and its characteristics of blocking disasters in adjacent goaf areas at the same layer, the mining of extra-thick coal seams with hard roofs in the Carboniferous mining area in Datong Mining Area was used as the engineering background, and theoretical analysis, experimental research, numerical simulation, etc. were used. From the two perspectives of the stability of the small coal pillar roadway protection tunnel and its ability to block harmful gases in adjacent goaf areas at the same layer, a comprehensive analysis is made of the reasonable size of the small coal pillar and the evolution characteristics of its permeability in different mining stages. Aiming at the specific mining conditions of small coal pillar mining in gob-side entry driving in 8210 working face of Carboniferous coal seam in Tongxin Coal Mine, a mechanical model of superposition of internal and external stress fields in double key strata is established, and the calculation formula of reasonable size of small coal pillar in gob-side entry driving under the condition of double key strata is deduced. The reasonable size of small coal pillar in 8210 working face is theoretically determined to be 6.0 m. The permeability evolution characteristics of small coal pillars in different mining stages were studied by using DJG–II coal rock seepage test equipment. It was determined that the permeability of small coal pillars in the third mining stage was 23 times higher than that in the initial test, and the small coal pillars in this stage basically lost the ability to block the harmful gases in the adjacent goaf. According to the theoretical research results, 6 m small coal pillars were selected for industrial test. According to the experimental results, during the mining test, the small coal pillar was modified to reduce the permeability, and the thick layer concrete (100 mm) was sprayed on the surface and roof of the small coal pillar within 1 500 mm from the coal pillar. The practice shows that a certain amount of deformation has occurred in the mining roadway of the small coal pillar roadway protection during the mining process of the working face, but the deformation of the roadway is within the safe and controllable range, the roadway is stable, and safe mining can be realized; during the mining process, the CH₄ gas concentration in the upper corner of the 8210 working face is much lower than that in the goaf of the adjacent 8305 working face, indicating that the small coal pillar has the ability to block the harmful gas in the adjacent goaf of the same layer after the modification and permeability reduction. The mining practice also further verifies the rationality and scientificity of the theoretical and experimental research results. The research results can provide reference for the popularization and application of small coal pillar roadway protection technology under similar conditions.