| There exist a series of uncertain factors in primary face of thick and high-gas seam. Besides, The lack of examples for reference increased the difficulty of gas prevention. The paper chose the primary working face of N2105in Yuwu mine as engineering background. The rule of overlying strata movement was analyzed from three aspects of the lithology of roof and floor in overburden, the mechanism of pressure relief and overburden movement, and the distribution of two zones. And the analysis on the law of gas migration in stope was from gas flowing in coal and gas two aspects. Based on the distribution of the face strata, the strike and dip model were established by UDEC2D(universal distinct element code). The law of fissures evolution and mining-induced mechanical characters were studied by the strike model. The distribution of highly developed fissure zones was determined by dip model. They laid foundation for the determination of layout parameters of boreholes for coal seam and the drilling field for fissure zone. Through the analysis on the principle of gas drainage and the gas accumulation, the associated and comprehensive pattern for gas extraction were obtained. The key parameters of boreholes in coal seam and drilling field for fissure zone were determined. Then it applied to engineering test and some conclusions were obtained as follows:1) The development of fractures in overburden tended towards high horizons with the mining-induced area enlarging and roof weighting. It reaches the highest horizons while the range of50-70m distance from cut-hole in gob was compacted.2) With the process of the stress redistribution caused by mining, the bearing stress of the coal body in front of the working face increased sharply. When it reached the strength of the coal, it would decrease to the residual stress. The phenomenon of relief appeared and the maximum stress move towards the deep gradually.3) The well-developed fracture area in overburden:The intersection of the range spacing upper and under end of gob0-50m and the range spacing seam roof30～50m can be the gassy area.4) The associated and comprehensive pattern of gas extraction was established through the formation of theories’ and techniques’system of gas extraction.5) The gas extraction rate of coal seam can be improved by increasing the recovery rate of boreholes and utilizing the effect of pressure relief. The borehole bottom position of drilling field was the range of35-40m above the coal seam. The distance of drilling field in air return way was40m and the one in belt roadway was80m, which intercepted the gas migrating in an fissure circle collaboratively.6) The gas drainage quantity of coal seam is influenced by the advancing speed of working face. To increase the advancing speed of working face appropriately can improve the gas extraction of coal seam, which is good for safety production.7) The gas drainage quantity of drilling field for fracture zones was at low level during initial mining. Mining-induced fractures at the initial mining of N2105face develop gradually and the quantity of gas extracting in NO.50drilling yard increases to highest level of10m3/min when the cumulative footage ranges of100-120m, which means fissures reach the maximum of horizons.8) The most prominent problem of primary face is to get through the initial mining period time. It is obtained that the formation of the compaction area marks an end of initial mining, and some relevant proposals are putted forward. |
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