Study on the mining stress field distribution law beneath isolated coal pillar in close coal seam and reasonable location of the roadway

In the process of mining deep close coal seams, it is influenced by isolated coal pillars and goafs in upper coal seam, which causes the selection of roadway position in lower coal seam to be a technical problem restricting safe mining of coal seam. This study is based on the gateroad to the No. 3 coal seam working face in a mine in Guizhou Province, China. The stress evolution characteristics in the floor following the mining of the upper coal seam is investigated using a comprehensive methodology that included theoretical analysis, numerical simulation, and field observations. The rational roadway position in the underlying coal seam was identified by analyzing the deviatoric stress tensor and the evolution laws of plastic zones at various roadway positions. The results show that: (1) by establishing an analytical mechanical model of the floor after stabilization of the goafs on both sides, an analytical expression for floor stress as a function of depth is derived, revealing the stress distribution law within the floor. Numerical simulations using FLAC3D are conducted to examine the evolution of principal stress beneath isolated coal pillars in the upper coal seam. The stress propagates downward in an inclined manner and gradually attenuates, with the distribution transitioning from a bimodal to a unimodal pattern. The principal stress difference is relatively low right below the isolated coal pillar, that is, at the roof position of No. 3 coal seam. The stress distribution law obtained from theoretical calculation is generally consistent with that obtained from numerical simulation. (3) The coordinated deflection mechanism between the deviatoric stress tensor and the morphology of the plastic zone is revealed, transitioning from a stable elliptical pattern to a highly unstable butterfly-shape as the roadway location shifts from the centerline of the isolated coal pillar to either side.(4) Based on butterfly-shaped failure theory, a roadway layout zoning method is proposed that incorporates the two main influencing factors of the principal stress difference and the principal stress ratio. The roadway layout area is divided into three zones, with zone R-III identified as the optimal range. After comprehensively considering the economic benefit and safety factors of coal mine, it is recommended that the gateroad of the No. 3 coal seam be arranged directly beneath the isolated coal pillar. Field practice demonstrates that when the mining roadway in the lower coal seam is arranged beneath the isolated coal pillar, the surrounding rock deformation remains within acceptable limits, ensuring normal roadway operation.