| When the first mining layer of close coal seam group in high gas coal mine is exploited,there are some problems arising,such as large gas emission in the goaf of the working face and the likelihood to exceed the limit of gas in the upper corner,which poses a severe threat to the safety and efficiency of mining operations.As for the existing gas overrun control technology applied in goaf,there has been good application results achieved under certain geological conditions.However,due to the complex and constant-changing occurrence of coal seam groups,it remains common for gas overrun accidents to occur in the mining face under the context of the first mining layer of close coal seam group.At present,it is still necessary to further explore the temporal and spatial evolution law of cracks in the top and bottom slate of the first mining layer of the coal seam group,ensure the accurate selection of gas control methods,and identify the key parameters of gas drainage in the goaf with super large diameter boreholes.Therefore,this paper adopts various research methods in combination,such as theoretical calculation and analysis,numerical simulation,physical simulation test and engineering practice.With the Xiaohuigou Coal Mine in Shanxi Province as the research object,a study was conducted to explore the mechanism of gas drainage by hole instead of roadway in super large diameter borehole of the first mining layer of coal seam group.The main conclusions drawn from this study are as follows:(1)The physical simulation test and numerical analysis were carried out under the context of close coal seam group mining.Besides,a simulation was conducted to reveal the temporal and spatial evolution law of fracture development,as well as the characteristics of mining displacement and mining stress distribution for close coal seam group mining.It was concluded that in the mining process for coal seam NO.2 in Xiaohuigou Coal Mine,there were significant vertical and separation fractures occurring at both ends of the goaf connected with the overlying coal seams NO.03 and NO.02,with the goaf within 50 m behind the working face as the priority area for pressure relief gas control.(2)The damage evolution model of coal and rock mass based on element strain was established,while the tensile and shear failures were corrected by plasticity.In addition,the fracture gas seepage model was constructed to determine the complex coupling relationship between fracture opening and shear displacement,seepage parameters and stress field,which provides a theoretical reference for the fracture seepage process and numerical simulation under the context of compression and shear forces applied simultaneously.(3)Based on the analytic hierarchy process,a goaf gas drainage method model was constructed by considering the gas emission of this coal seam,the proportion of gas emission of upper and lower adjacent coal seams,recovery rate,ventilation mode and other influencing factors.On the basis of expert weight evaluation,the ranking of gas drainage methods by importance in goaf under 6 different working conditions of absolute gas emission was performed.To be specific,the absolute gas emission at the2201 working face in Xiaohuigou Coal Mine falls within the range of 20～30m3/min,with the combined extraction method of super large diameter drilling and high roof drilling as a preferred option.In the meantime,the mathematical model of multicomponent gas flow and concentration dispersion considering goaf boundary conditions was constructed to analyze the working principle of gas drainage in goaf through super large diameter drilling.(4)An analytical model was established for the mechanical failure occurring to the gas drainage boreholes in coal pillar,for deducing the distribution equation of failure depth of surrounding rock in drainage boreholes and the critical judgment condition of failure and penetration of surrounding rock between boreholes.Besides,it revealed the characteristics of impact caused by mining stress concentration coefficient,lateral stress coefficient,original rock stress,borehole diameter,cohesion and internal friction angle on the failure depth of hole wall surrounding rock.With the increase of mining stress concentration coefficient,original rock stress and borehole diameter,there was an incremental increase in the failure depth of hole wall surrounding rock,which shows a decreasing trend with the increase of lateral stress coefficient,cohesion and internal friction angle.According to the geological conditions of mining at Xiaohuigou Coal Mine,a theoretical analysis was carried out to show that the safety hole spacing is 7.8 m for the super large diameter extraction drilling with a diameter of Φ580 mm.(5)Based on the setting of porous media parameters,the fluent numerical simulation software reflecting gas flow was adopted to explore the effects of drainage hole flow,hole number,the horizontal distance between the first hole and the upper corner,hole spacing and other factors on the distribution of gas concentration in goaf.In this way,those key parameters were obtained given different volumes of single hole extraction,such as the reasonable number of holes,the horizontal distance between the first hole and the upper corner,and the hole spacing in 2201 working face of Xiaohuigou Coal Mine.(6)The research results of gas extraction from goaf by super large diameter borehole were applied to the 2201 working face of Xiaohuigou Coal Mine.It was found out that the net amount of gas drainage from large-diameter boreholes with diameterΦ580 mm reached 580 times that of the boreholes with a diameter of Φ113 mm.As the number of combined pumping boreholes increased,the daily output improved progressively,which is conducive to the effective gas control in the upper corner of the working face,with a satisfactory outcome of gas control achieved.Figure [90] Table [22] References [185]... |
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