Study on the Instability Mechanism of Coal and Rock Mining under a Residual Coal Pillar in Gently Inclined Short-Distance Coal Seam with the Discrete Element

The collapse of overlying rocks caused by the instability of residual coal pillars during lower coal seam mining significantly impacts its safety. This paper focuses on the gentle dipping coal seam group and utilizes the discrete element method (DEM) as the basis to comprehensively consider multiple factors through orthogonal experiments. In so doing it reveals the influence mechanisms of various factors on mining at a close distance under a residual coal pillar. Firstly, the process of lower coal seam mining under residual coal pillars in gently-dipping coal seams was simulated and analyzed based on a case study at the Baoping coal mine. Comparing the evolution characteristics of coal–rock fractures, stress changes, and displacement changes during the mining process reveals the mechanism of the joint instability of the lower coal seam, interlayer rock, coal pillars, and overlying strata under the disturbed conditions of lower panel mining. Secondly, an orthogonal simulation experiment was established using the width of the coal pillar and the thickness of the lower coal seam as variables. By comparing the development process of cracks, stress distribution, and rock displacement under different conditions, the research results indicate that the width of coal pillars has an impact on the maximum amount of coal pillar subsidence, while the thickness of the underlying coal seam has an impact on the time of subsidence, when hd ≥ 4.2 m (hj/hd ≤ 4) and w ≤ 14 m (w/hm ≤ 2), a large-scale collapse of the overlying strata of the coal pillar occurs.