| Protective layer mining is currently recognized as the most economical and effective measure to prevent coal and gas outburst.Although there have been many research results on this technology,there are still many problems to be explored.Based on the mining of the protective layer 864 working face of Tongting Coal Mine,this thesis studies the deformation law of overlying coal and rock mass and the delineation of pressure relief protection range by means of theoretical analysis,laboratory experiment and numerical simulation,and verifies it through field investigation.The main research results are as follows :(1)The physical parameters and coal body characteristics of 7 coals were measured by experiments,and the results showed that the coal samples belonged to medium volatile coal,with maximum solidity coefficient of 0.42,maximum initial velocity of dispersion of 13.6 mm Hg and maximum gas content of 7.38 m~3/t,indicating that the coal body has the characteristics of high crushing strength,strong adsorption capacity and fast gas dispersion.From the scanning electron microscope and liquid nitrogen adsorption experiments,it is found that the surface pores and fissures of the coal body are not sufficiently developed and mainly microporous,indicating that the coal seam has poor permeability and there is a risk of coal and gas protrusion.Combining with the coal seam assignment,the 8 coal seam has no risk of protrusion,therefore,the protection seam mining unloading pressure and increasing penetration method of mining the 8 coal seam and protecting the 7 coal seam is proposed.(2)Using FLAC3 D and UDEC to establish numerical models,based on MooreCullen and Cullen slip joint models,we simulated the deformation and fracture distribution law of overlying coal rock after mining of the protected seam,and the results show that the settlement range of overlying coal rock is larger than the range of the mining area,and the expansion and deformation rate of coal seam reaches 20.5‰.Between the traditional pressure relief boundary line and the coal tunnel plumb line at the working face,the protected coal body experiences the stress evolution process of "vertical stress increases and horizontal stress decreases".Under the influence of horizontal stress relief,the overburden fissures are further expanded and a large number of vertical fissures are generated at the above location,which provide good channels for gas transportation.Under the reasonable form of extraction,it can effectively reduce the coal seam gas content and realize the simultaneous elimination of protrusion.(3)By analyzing the two idealized coal-rock fracture geometric models in the elastic stage and the inelastic stage,the permeability evolution model related to the stress in all directions in Tongting Coal Mine is derived,and the relationship between permeability and three-dimensional stress is analyzed.The results show that between the traditional pressure relief boundary line and the vertical line of the working face coal roadway,the coal permeability can be increased by up to 900 times.Combined with the numerical simulation results,from the middle to the outside,the permeability distribution range of the protected layer is divided into : permeability greatly increased area,permeability increased area,permeability decreased area and original permeability area.(4)The effect of residual gas content and pressure,expansion and deformation rate and unloading gas extraction were investigated by arranging boreholes for field tests,and the results showed that the maximum residual gas content was 3.26m~3/t and the projected maximum gas pressure was 0.27 MPa,and the gas content and pressure were effectively reduced.The expansion and deformation rate of coal seam reached 21.6‰,which is consistent with the numerical simulation results.the total extraction volume of 7 coal gas was 1,733,400 m~3,of which 68.4% was extracted by surface drilling,and the gas extraction rate of 7 coal seam reached 71.9%,the extraction volume was larger than the original storage volume of 7 coal gas within the protection area,indicating that the protected coal seam has been fully depressurized.(5)Through the analysis of field measurement and gas extraction statistics,it is found that under the enhanced extraction by surface drilling,the residual gas content and pressure of the protected seam are less than the critical value between the traditional pressure release boundary line and the coal tunnel plumb line at the working face,and the difference is smaller than that within the traditional pressure release boundary line.It shows that the permeability of the coal seam in this area has increased,the degree of pressure release is high,and the pressure release range of the protected layer is larger than the theoretical pressure release range.Combining with the numerical simulation results and considering certain safety factor,the final unloading angle is 90° in the direction of strike,the upper part of the tendency and the lower part of the tendency.The thesis has 82 figures,27 tables,and 79 references. |
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