| The Datong mining area has Jurassic and Carboniferous double-series coal seams,and the upper and lower layers overlap widely in the area.The distance between the double-series coal seams is about 110~200m.The upper Jurassic coal seams have been mined,and the lower Carboniferous 3-5# coal seam.The 3-5# coal seam is an extra-thick coal seam.After mining,its overlying rock is damaged greatly,and the mining fissures are easy to communicate with the old gobs of the upper Jurassic coal seam,causing water and gas leakage disasters in the old gobs,which seriously affects the safety of underground production.In this paper,taking the 8106 fully mechanized mining face of Yongdingzhuang Mine in Datong mining area as the research object,aiming at the engineering problem of a large amount of air leakage in its goaf,the movement of the overlapping mining strata in double-series coal seams is studied by means of theoretical analysis,numerical simulation and field measurement.The fracture forms and dynamic evolution characteristics of fractures are analyzed,the controlling effect of key layers of overlying strata on water and gas leakage disasters is analyzed,and the connection mechanism of multi-layer goaf in double-series coal seams is mastered.The research on the distribution of spontaneous combustion "three zones" reveals the gas migration state in the goaf under the condition of air leakage and the influence of the air leakage volume on the spontaneous combustion "three zones" in the goaf.the result shows:(1)The internal fissures in the mined-out area of the Jurassic coal seam group are chaotic,and air leakage channels are most likely to be formed near the reserved coal pillars.There are two key layers between the two layers.,Periodic breaking and caving of the lower key layer and the upper key layer.After the upper key layer breaks and sinks,the double-series goaf is connected,which leads to serious external air leakage in the Carboniferous gob.(2)After the 3-5# coal seam is mined,on the cross section of the fissure area formed between the upper and lower key layers,the main air leakage channels are divided into dynamic fissures and edge fissures.The dynamic fractures are the vertical fracture fractures above and behind the working face.The dynamic fractures show the law of “appearance-closure” with the periodic fracture of the lower key layer,and their gas conduction capacity also changes dynamically.(3)Under the actual air leakage condition,the internal pressure of the goaf is higher than the external pressure,the upper pressure is higher than the lower pressure,and the air flow enters the goaf through the gas-conducting cracks,and the seepage direction of the gas in the goaf is from top to bottom,from inside.Towards the outside,the range and shape of the heat dissipation zone in the goaf have little effect with the change of the external air leakage.The distribution range of the oxidation zone is proportional to the air leakage.When there is no external air leakage,the source of the air leakage in the goaf is the working face,and the leakage of the working face is mainly concentrated.On the air inlet side,external air leakage will increase the seepage velocity and oxygen concentration on both sides of the inlet and return airways in the goaf.Therefore,both sides of the air inlet and return airways in the goaf are areas that need to be monitored and prevented from fire. |
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