Research on the law of multiple mining in close coal seams and the roadway support of lower coal seam.

After multiple mining of the upper and lower coal seams in the near vicinity, the formation of multi-layer coal pillars and multi-layer goafs will affect the roadway layout of the coal seam under pre-mining. Taking the 11,103 working face of Fenxi Mine as the engineering background, the evolution law of multiple mining deviators in four stages, including the goaf on one side of the coal pillar in the upper coal seam, the double goaf on the side of the coal pillar in the upper and lower coal seams, the double goaf on the side of the coal pillar in the upper coal seam, the goaf on the side of the lower coal seam, and the double goaf on both sides of the coal pillar in the upper and lower coal seams, and the deformation and failure mechanism of the surrounding rock in the lower coal roadway were analyzed by numerical simulation, and the reasonable layout position of the lower coal roadway was obtained.The results show that: (1) The main reasons for the failure of the surrounding rock of the lower coal roadway are the changes in the concentration, propagation range and disturbance characteristics of the deviatorial stress of the surrounding rock of the multi-layer coal pillar and the goaf floor during multiple mining. (2) The deviator stress characterization of pure shear failure is analyzed theoretically. In the two stages of goaf on one side of the upper coal seam pillar and double goaf on the side of the upper and lower coal seam pillars, the peak deviator stress zone propagates to the deep part of the floor in a similar "arc slip line" and "half-crescent arc slip line". In the two stages of double goaf on one side of the coal pillar in the upper coal seam, double goaf on the side of the lower coal seam, and double goaf on both sides of the coal pillar in the upper and lower coal seams, the peak deviator stress zone is concentrated in floor of the coal pillar, showing a failure form similar to "flat ellipse and circular ellipse". (3) In order to obtain the relative stability area of the roadway, among the influencing factors, with the increase of horizontal dislocation, the peak value of the deviator stress of the surrounding rock of the roadway decreases, and the reduction range of the deviator stress of the surrounding rock of the roadway is ranked as follows: the coal pillar > the floor > the solid coal > the roof, and with the increase of the vertical dislocation, the reduction range of the deviator stress of the surrounding rock of the roadway is ranked as: the solid coal > the roof > the coal pillar > the floor, and the reasonable suggestions for the optimal layout of the roadway are provided in combination with the engineering practice, which provides reference and reference for similar engineering conditions.