Investigation On Protection Scope Demarcation Of Upper Protective Layer Using Pitching Oblique Mining Method

With the increase of mining depth, coal and gas outburst has become the natural disaster which seriously threats the safety of coal mine production in extracting low permeability and high gas coal seams in southwest China, and mining protective layer is the most valid and economical regional prevention measures. The key problem encountered in extracting protective layer is to determine the protected scope, especially in mining steeply inclined upper protective layer using pitching oblique methods. If the traditional methods mentioned in"Coal and Gas Outburst Prevention Regulation"were adopted to delimitate the protection scope, the phenomenal that the actual protected scope would differ from the method used above would occur. Based on the research background of steeply inclined upper protective layer using pitching oblique methods in Dongling coal mine, this paper researched into the delimitation problem of steeply inclined upper protective layer using pitching oblique methods, its main contents include as follows:Firstly, through conclusions and analysis of floor stress, floor deformation, failure law of floor and gas migration rules after mining, it set up the theoretical foundation for field testing and numerical simulation. The analysis indicated that, with the advance of working face the floor stress had gone through three phases: stress concentration before mining, rapid stress-relax after mining and stress recovery, corresponding to compressed deformation before mining, expanded deformation after mining and deformation recovery; shear failure tended to occur in the junction between compressed and expanded region, while bed-separated fissures and vertical fissures tend to occur in the expanded region. The dip of coal seam was the important factor which influenced the failure form and the distribution of floor stress on the dip direction. As regards with gently inclined and inclined coal seam, the stress of the downhill on the coal floor was larger than on the uphill, and fractured zones were larger in the near downhill; while as regards to steeply inclined coal seam, due to the filling of the collapsed shale, it confined the upside swell of floor, and it resulted that upward fractured zones were lager than downward region.Secondly, under the guidance of theoretical analysis, the change of gas pressure and gas flow were investigated in Donglin coal mine, and the change law of gas pressure, gas flow and permeability coefficient were all analyzed with the advance of working face, and the distribution law of initial gas pressure of protected seam and four gassy belt scope were all determined. The results showed that the protection angles on strike direction were variant ranging from 43.5 degree to 53.6 degree; while the bottom dip direction was 78.7 degree; and the advance distance of protective coal seam should not be less than 3.4 times the layer distance.Thirdly, based on the method of three dimensional fast lagrange method, the dynamic development process of stress distribution and deformation of protected coal seam with exploitation of steeply inclined protective seam of pitching oblique mining were simulated using FLAC3D. Besides, the law of the stress and deformation normal to the protected coal samdwich were analyzed with the advance of upper protective layer coalface; based on the protection rule of pressure-relieving and normal deformation of protected seam, the pressure-relief angle on the dip and strike were ascertained. The result showed that the protection angles on the upper and bottom dip direction were 81.5 degree and 74 degree respectively; while the protection angle on strike direction were variant ranging from 30 degree to 52 degree; the most largest pressure-relieving angle was in the middle pitching oblique coalface. Through comparison, the result of the numerical simulation was close to the field investigation, and it showed that it was feasible using numerical simulation.Fourthly, by synthesizing the above results, the final results were obtained as follows: the bottom dip direction was 74 degree, while the protection angles on strike direction were variant ranging from 30 degree to 53.6 degree; and the advance distance of protective coal seam working face should not be less than 3.4 times the layer distance.