Study On Reasonable Size Of Small Coal Pillar For Hard Roof Driving In Extra-Thick Coal Seam

Based on the engineering background of 3-5#coal seam mining in Tongxin Coal Mine,this paper studies the reasonable size of small coal pillar in gob-side entry driving under the condition of multi-layer hard roof and thick coal seam.Due to the large mining height of the extra-thick coal seam,the large gas emission from the working face,and the large amount of residual coal in the adjacent goaf,the paper studies the reasonable size of the small coal pillar from two aspects:the stability of the roadway and whether it has the characteristics of isolating the harmful gas in the adjacent goaf of the same layer.In this paper,field investigation,theoretical analysis,numerical simulation,experimental research and engineering application are combined to determine the overburden fracture characteristics and mining stress field distribution characteristics of 8210 working face in Tongxin Mine.The overburden structure model and the superposition mechanical model of internal and external stress fields of the double key strata in the hard roof and extra-thick coal seam of the 8210 working face were established.The reasonable size range of small coal pillars in the 8210 working face to ensure the stability of the roadway under the condition of small coal pillar protection is 5.5m～6.2m;using FLAC^3Dnumerical simulation software,it is determined that the reasonable size of small coal pillar which can ensure the stability of roadway under the condition of small coal pillar protection in hard roof and extra-thick coal seam is 6m.Through the analysis of permeability,dimensionless permeability,stress sensitivity coefficient and strain of small coal pillars,it can be seen that small coal pillars that have undergone multiple mining effects still have the ability to maintain the stability of roadways.In the first and second stages,the ability to isolate harmful gases in the same layer near the goaf is slightly reduced.In the third stage,it does not have the ability to isolate harmful gases in the same layer near the goaf.The engineering application shows that the theoretical analysis of overburden fracture characteristics is consistent with the measured results.The deformation monitoring of surrounding rock in 5210roadway shows that 6m small coal pillar combined with corresponding support means can ensure the stability of roadway during mining.The ANSYS Fluent software was used to compare and analyze the air leakage gas exchange between the small coal pillars with and without grouting and the goaf.It can be concluded that the grouting plugging of the coal wall cracks on the side of the 5210 roadway near the goaf can effectively prevent the gas in the 5210 roadway from flowing with the gas in the 8305 goaf,so that the CH₄gas concentration in the upper corner is less than 1%.The field measured gas concentration data are consistent with the numerical simulation results.On the whole,the reasonable size of the small coal pillar that can ensure the stability of the roadway under the condition of small coal pillar roadway protection in the extra-thick coal seam with hard roof is 6m,and the 6m small coal pillar with grouting modification and permeability reduction has the ability to isolate the harmful gas in the same layer adjacent to the goaf,and realizes the safe mining of the working face.The research results can provide reference for the retention of small coal pillar size in gob-side entry driving mining technology under similar conditions.It is of great significance to improve the recovery rate of coal resources and the economic benefits of enterprises,and to realize safe and efficient mining under the condition of small coal pillar protection.The paper has 61 figures,9 tables and 60 references.