Study On Three Dimensional Analysis Of Overburden Cracks In Fully Mechanized Mining Face Of Xinjing Coal Mine And The Optimum Location Of Ground L-Type Extraction Borehole

Ground L-type extraction borehole replace high-level drainage roadways to extract goaf gas,which has fast construction speed and low cost.However,because L-type boreholes are far smaller than high-level drainage roadways,it is necessary to accurately grasp the law of gas migration in goaf and overburden fissure zone after coal seam mining,and reasonably determine the spatial location of boreholes.Based on the actual geological conditions of 3213 panel in Xinjing Mine,the process of overburden stress field change,the process of fracture development and the evolution of fracture field distribution after mining are studied by using similarity simulation and CDEM three-dimensional discrete element simulation,and an accurate three-dimensional fracture field model is obtained.Combining the model with COMSOL,a multi-physical field coupling analysis software,the distribution of gas in the goaf of L-shaped borehole extraction with different vertical and horizontal distances from the return air roadway of the panel is simulated,and the following research results are obtained.(1)Similar simulation is used to analyze the development process of cracks and the distribution law of crack field in the mining process of panel.The height of fracture development above overburden increases with the occurrence of periodic weighting.Fracture field is trapezoidal distribution in the early stage of mining.With the advance of panel,the middle fracture is gradually compacted,and the fracture field becomes the middle compaction,and the two ends of the fracture develop trapezoidal.(2)CDEM numerical simulation software is used to analyze the changes of stress and displacement fields of overburden strata in the mining process.The development and distribution of the fracture field above the panel are analyzed by the pressure relief coefficient and the separation rate.It is determined that the fissures are circularly distributed over the working face.The width and height of the fissure circle are between 21-28 m and 38-45 m,and the width and height of the fissure at the end of the working face are slightly larger than those at the opening end.(3)According to the results of numerical calculation and physical similarity simulation,a three-dimensional model of rectangular terrace of mining fracture is constructed.A series of parameters,such as the distribution range of fissure zone,the height of fissure terrace and the width of fissure zone,are determined for Xinjing mine 3213 panel.(4)COMSOL numerical simulation results show that under U-type ventilation mode,the gas concentration in the upper corner of the working face is 9.114%,which exceeds the safety production limit stipulated by the state,and measures need to be taken.Under the condition of U-type ventilation and L-type surface boreholes,the guiding effect of boreholes on air flow makes the high concentration gas in the deep goaf and fresh air flow in the working face converge to the high extraction roadway,which changes the distribution of gas concentration and reduces the gas concentration in the upper corner to less than 1%.By comparing the extraction effect of L-type extraction boreholes in different spatial locations,the reasonable borehole layout range is determined as follows: vertical distance from the coal seam roof is 14m~29m,inclined direction is 5m~15m from the horizontal distance of the upper wall of the return air roadway.