Study On Protection Scope And Effect Factors In Exploiting Steep Under-Protection Layer

With the increasing of exploitation depth for coal mine, coal and gas outburst became the major factor of restricting the safe and effective exploitation to a mine, while mining the protective layer as the most effective measure of preventing the coaland gas outburst in the regional in the world was widely used in a serious outburst coal mine. The determination of protecting scope was an important problem when exploiting the protecting layer, which closely associated with many factors such as the existent condition, the mechanical properties of surrounding rock, mining method and roof management method.In the Southwest, the majority coal seam was steep incline and serious coal and gas outburst, and the geological structure was complicated. Furthermore, informal mining methods were adopt. Consequently, the determination of protection scope was much more difficult. According to the traditional method, which referred to the regulation of preventing coal and gas outburst, the protection scope was different from the real protection scope. So it's essential to determinate the protection scope for mining the steep incline under-protection layer renewedly.Taking the exploitation of steep incline under-protection layer of Donglin coal mine in Nantong Mine company as researching project background, combining with the test in laboratory, theoretical arithmetic, numerical simulation and industrial tests, the motion and breaking regulation of cover mass and the protection scope of protected layer after exploitation the under-protection layer was studyed by this paper. The sesults of this paper could be concluded that:①The basic physical-mechanics parameters of coal and rock arround the protection layer at investigation area were confirmed by the mine-site sampling, processed into standard test pieces, the compression test, the tensile test and the shear test.②A three-dimensional finite element numerical simulation model to simulate the exploition of oblique under-protection layer was established. The distortion and pressure-relieving degree of overlying rock srtatum was anslyzed and researched by numerical simulation after the exploition of oblique under-protection layer, and the effective protection scope of protected layer was analyzed and calculated on the basic of the stress protection regulation and the distortion protection regulation for pressure-relieving of protected coal seam. The result indicated that the upside and downside pressure-relieving protection angles along the incline were 90°and 80°respectively when exploiting the under-protection layer. The pressure-relieving protection angle along the strike distributed unevently on the upside and middleside and downside along the incline, and valued from 44°to 68°. For the reasons of security, it was reasonable to determinate the pressure-relieving protection angle along the strike by 44°.③Through the field industrial examination for the exploition of steep oblique under-protection layer in Donglin coal mine, the field test result of protection scope of the exploition of under-protection layer in the investigation region was obtained. The result of examination showed that: the investigation drill could be sealed by use of the method of Malisan curing agent combining with the concrete and plaster slurry mechanism sealling, and the test results of kinetic parameter for gas were more exact. The pressure-relieving protection angle of downside on the incline after the exploition of the under-protection layer was 83°, and the pressure-relieving protection angle along strike was 45°. The minimal leading distance between the coal face of protection layer and advancement face was 53 m. The result of numerical simulation calculation was near the result by field investigation and leaned toward the security directon. Consequently, ascertaining the protection scope of the exploition of steep incline oblique under-protection layer by use of numerical simulation was feasible.④The caving recrement filled the gob unevently along the incline after the exploitation of steep incline coal seam. There was no filling in the up-incline, weak in the middle-incline, and dense in the under-incline. The support load from filling recrement was analyzed and studyed, and the function of support load to the roof was established.⑤Based on the elastic thin plate theory and the unevently filling of gob, the mechanics model of distortion and breaking for basic roof after the exploitation of under-protection layer. The deflection function of basic roof was deduced, and the terminal span for the first breaking of basic roof was analyzed and calculated on the basis of Marcus modify solution. Roof control method of the exploition of oblique under-protection layer when the first weighting of the basic was analyzed and calculated.⑥According to the Key Strata Theory, the key strata of roof stratum was determinated after the exploition of steep incline under-protection layer, and it's first breaking interval was analyzed. Combining with the field test, the impact of the key strata's breaking to the distortion of overlying rock srtatum and the distributing of absciss layer and cranny and the gas effusing from protected layer and it's protection scpoe was analyzed.