Pressure Relief Functional Principle Of Stope Underlying Coal-Rock Mass And Application In Gas Extraction Of Protected Coal Seam

When a coal seam has been mined, the stress equilibrium around the stope is redistributed, which makes an good effect for gas control in outburst coal among the underlying coal-rock mass. Based on theories of the elasticity, rock mechanics, damage mechanics, seepage mechanics and ect, using theory analysis, similar simulation test, numerical Simulation, field application, the paper studies on pressure relief characteristic, move deformation, fracture developmentand seep characteristic of underlying coal-rock mass, still introduces the gas flow law of lower protected layer and pressure relief gas extraction method. The research has an important significance for safety production and subtainable development in coal and gas outburst mines.The stress change mechanical model for underlying coal-rock mass of stope was founded in the paper, combined with the numerical simulation, we obtain the sress change and distribution law of underlying coal-rock mas. Along the strike, coal-rock mass come through original stress area, support stress area, stress decreasing area and stress recovery area. Both sides coal pillar of working face and coal-rock mass below coal pillar are located in support stress area, supporting stress concentration factor gradually reduces with the increasing of depth, but the affection areas gradually expand to both sides. Underlying coal-rock mass below the goaf keeps pressure relief state, the pressure relief grade and effective pressure relief area becomes smaller with increasing of layers space. Underlying coal-rock mass stress gradually is recovered with mining proceeding ahead and stability of roof and floor strata movement in the back of goaf.Simultaneously, we obtained the deformation law and fracture development characteristics of underlying coal-rock mass movement, and the deformation characteristics and permeability evolution of lower protected layer. The rock layer underneath coal pillar takes on entire subsidence because of the support stress, rock layers below goaf form an oblique movement. With increasing of layers space, coal-rock mass deformation gradually reduces. The underlying coal-rock mass under an effect of upper protective layer mining was divided into the floor heave fracture zone and the floor heave deformation zone in this paper.The lower limit of floor heave fracture zone is 15~25m, in which fractures develop completely. The vertical cross and parallel fracture form the network channel of gas migration. The lower limit of floor heave deformation zone is 50~60m, which is made of parallel fracture mostly. With mining the protective layer, the lower protected layer experiences compression deformation first, then expansion deformation. The permability change law of coal seam as follows: The original value→smally decreasing→increasing greatly→stability→reducing at last.Combined with the fracture zone characteristics of underlying coal-rock mass, this paper analyzed the gas flow and collection characteristics of the lower protected layer, and lower protected layer pressure-relief gas drainage methods and related technical parameter were obtained. The main way for lower protected layer pressure-relief gas drainage that using network form upward borehole drilling through layers in floor rock roadway. By upper protective layers mining test in Shenyang Hongling coal mine and Huainan Xieyi coal mine, gas content of protected layer decreased effectively and risk of outburst coal seam was eliminated completely, which assured the safely and high effectively mining of outburst coal seams. According to investigation result, the expansion deformation degree of protected layer was 7.2‰when No.12# coal seam deformed steadily in Shenyang Hongling coal mine, and permeability coefficent in 12# coal seam increased 1010 times. The lower protected layer gas drainage rate could reach 77.5% in protected area. And the gas pressure decreased from 7MPa to 0.35MPa. The gas content decreased from 22.5m3/t to 5.06m3/t. The relative expansion deformation degree of protected layer was 4.0‰when the C13 coal seam deformed steadily in Huainan Xieyi coal mine. The gas drainage rate could reach 68% in C13 coal seam. And the pressure in C13 coal seam decreased from 4.5MPa to 0.5MPa. The gas content decreased from 16.5m3/t to 4.3m3/t. Additional, the conclusion obtained by the method of theory analysis, similar material simulation and numerical simulation and others was considered reliable and reasonable according to investigation and verification in field test.