Study On The Law Of Mining-induced Fracture Evolution Of Overlying Strata And Relieved Methane Delivery And Its Engineering Application In Fully-mechanized Top Cola Caving

The pre-degasification effect is less in China, because most of coal-bed methane existing "three high-low" features. It is an important tackling methane direction that using moving influence and mining-induced fracture evolution features of mining overburden rock to degas. The paper systematically summarized and reviewed plenty of achievements of rock strata movement and structure evolution, the migration laws of coal-bed methane and degasification methods, it can get that the mining overburden rock fissure patterns and distribution of methane migration law are the theoretical foundation of safety and simultaneous extracting of coal and gas, which are one of the study key points for the future.The micro-structural characteristics and influencing factors of coal sample is studied with scanning electron microscope, the whole process of stress and strain of the permeability of coal samples is analyzed through the electro-hydraulic servo seepage test. It is got that ground pressure is the main controlling factor. The first weighting step and period weighting step were derived by elastic thin plate theory when overburden rock was broken, the distinguishing method of key strata based on thin plate theory was obtained, and the corresponding distinguishing procedure was compiled.Based on simulation experiment and FLAC3D numerical simulation, the space-time law and distribution pattern of mining overburden rock and scopes and features of relieved pressure are analyzed. On the basis of the predecessors'research achievement, combined the key strata theory, it is proposed a simple engineering model which a dynamic mining-induced fracture round-rectangle trapezoidal zone would be formed if the bed-separated fissure and broken fissure were connected. It is analyzed that the dynamic evolution characteristics, mechanical mechanism and influencing factors of mining-induced fracture zone. It is obtained that the zone width of starting cut is about the first weighting step, the zone width of face is changing between 2~3 times of period weighting step, the zone width near intake entry and return entry is about 0.7~0.8 times of the first weighting step, the height of trapezoidal surface is basically subject to the location of the key strata and the deformation, breaking and instability form of the voussoir beams structure.On the basis of analyzing the relieved methane source and flow pattern in mining-induced fracture zone, the relieved methane delivery mathematical model is established by the theory of porous media fluid dynamics and mechanics of porous flow, and the relationship between relieved methane migration and dynamic evolution of mining-induced fracture is obtained. FLUENT software was applied to stimulate the methane delivery laws in the mining-induced fracture zone, which were in the ventilate condition of U, U+L, U and strike high roadway, and U+L and high roadway pattern. It is obtained that the discharge methane effect of tail roadway is best when the connection roadway interval is 1.35 times zone width of mining-induced fissure zone near face, and the degasification effect of strike high roadway is best where locate near the return entry, which the parallel distance from dip breaking line is 0.46 times zone width of mining-induced fracture zone near return entry, the vertical distance from caving zone boundary is 2.8 times mining height.Based on field observation, the influence factors of methane outflow were analyzed such as daily mining distance, daily coal output, air distribution, mining production processes, degasification quantity, ground pressure and overlying strata moving. It was obtained that the absolute and relative methane outflow of the first weighting are 1.25 times, the absolute methane outflow of the period weighting is 1.22 times, and relative methane outflow of the period weighting is 1.39 times than before weighting. The methane outflow is periodic changing which the step basically equity to period weighting. The seven stage laws of methane outflow with the key strata moving are analyzed. Through the field test in the 103 working face of Shanxi Heshun Tianchi Energy Co.,Ltd., it is showed that the degasification parameter of relieved methane is reasonable. The paper can provide a certain theoretical and technological foundation for optimizing achieving degasification system of relieved methane and safety and simultaneous extracting of coal and methane.