Height-limited mining with intelligent coal drawing technology on aquifers protection in ultra-thick coal seams

To address the challenge of controlling the drawing height in top coal caving mining under aquifers within ultra-thick coal seams, this study employed an integrated approach combining theoretical analysis, numerical simulation, and field testing at I0216302 working face of Mindong No.1 Mine. The results demonstrate that coal drawing is influenced by several key parameters: coal drawing angle, axis deflection angle, eccentricity, and horizontal distance from the drawing outlet to the coal break line. Specifically, the axis deflection angle follows an exponential relationship with drawing time, and the drawing body height exhibits logistic growth relative to time. The coal drawing process also exhibits dynamic evolution of multi-layered force chain arches that continuously form and collapse. Force blocks evolve through strong-to-weak arch transitions, with distinct spatial distributions: strong arches predominantly develop within 5-8 m ahead of the face and above hydraulic supports in the goaf area, while weak arches are confined near the drawing outlet. These force chain structures significantly influence drawing velocity by creating periodic resistance variations during arch collapse, causing particle speed fluctuations between 0.10–0.21 m/s. Regarding geometric morphology, when the drawing height is less than 2 times coal cutting height, the drawing body assumes a prolate ellipsoid shape with vertical-dominated development, where particles directly above the outlet move fastest. At 2–3 times coal cutting height, it transitions to an oblate ellipsoid with lateral-dominated flow, accelerating particles toward the upper-right goaf and upper-left canopy regions. Through the intelligent coal drawing system, by setting the coal cutting height at 3 m and the coal drawing time at 110 s, the coal drawing height can be controlled within 6 m, and fractured zone height can be maintained within 85 m. This effectively prevents water and sand inrush accidents during extraction.