| When a coal seam has been mined, the stress equilibrium around the stope is redistributed, and the caving zone, the fractured zone and the bending zone are formed in overlying strata resulted by collapsing. In certain altitude range, pressure relief effect happens on the coal-rock masses in the"three zones", which makes a good and active effect to the gas control of outburst coal seams. Based on over 120m extremely thick igneous rock exists above the distant protected layers in the outburst coal mine, we used several methods to research fractures and separations dynamic evolution, the mechanical characteristic, pressure relief characteristic, move deformation, and seep characteristic of overlying coal-rock masses, such as experiment research, theories analysis, similar simulation test, numerical simulation and field application. We still proposed several methods for draining pressure relief gas, and the chosen methods were successfully applied to the practice. The research has an important significance for safety production, and provides theoretical basis and reference for distant protective layer mining technology and pressure relief gas drainage in coal mines with similar geological conditions. The paper's innovative points are as followed:1)Considering the characteristics on thickness and intensity of igneous rock, combined with the action by rock load and sel-gravity, the mathematical model of the extremely thick igneous rock by forces had been founded. At the same, we analysed the separation producing theories using thin plate theory. The mechanical experiment and numerical simulation tests showed that the extremely thick igneous rock was the main key stratum of coal mine, which had a big broken drawing pace and controlled the whole movement of overlying strata.2)The paper analysed the separations and fractures dynamic evolution and deformation laws of protected layers using similar simulation and numerical simulation tests. The results showed that the separations and fractures could keep open for a long time under the support of the extremely thick igneous rock. The fractures became the channels of gas migration and separations became the enrichment areas of pressure relief gas. The pressure relief effect was equal to the protected layers, the average value of relative expansion deformation was 28‰, which resulted in an increase of over 3000 times of coal permeability coefficient.3)In the paper, we obtained the relations between fractures evolution and gas migration & storage. We proposed the"cap distribution"of fractures and separations, and the concepts of two enrichment areas of pressure relief gas, namely the fractured zone and separation zone in bending zone. Systematically and organically combined with gas migration and storage, rock movement, fractures and separations evolution, we got the coupling law of mining fractures field and gas migration field. 4)In the paper, we got the gas migration law of distant protected layers under the extremely thick igneous rock. The pressure relief gas flowed directly or along vertical fractures to the separations, making the separations be enrichment areas of pressure relief gas and easy for gas drainage. The back calculation of drainage data in field test proved that the separations full of gas existing nearby the pressure relief coal seams.5)In the paper, we developed an special gas drainage method, namely the distant penetration boreholes, which successfully applied to the practice in the Haizi coal mine. The practice in the Haizi coal mine has shown that, the amount of gas drainage stayed at above 4m3/min in the regular period gas, drainage rate could reach 73% in the middle coal group by draining for a year, with a gas drainage radius over 100 m, and the outburst danger of protected layers were eliminated. The method was not used in other coal mines at home and abroad.
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