A theoretical model for drawing body in sublevel top coal caving mining and its validations

The sublevel top coal caving (SLTCC) mining is widely employed in steeply inclined and thick coal seams. In SLTCC, the large coal drawing height and the influence of roof and floor boundaries introduce significant complexity into the coal extraction process. The study demonstrates that, in an SLTCC working face, the granular nature of top coal gives rise to a 'locking-embedding' effect and a 'loosening-compaction' cycle during the drawing process. These phenomena invalidate several core assumptions of the conventional Bergmark-Roos (B-R) model, which is no longer fully established, and the maximum moving angle changes dynamically. Therefore, an improved B-R model was proposed to account for the dynamic variations in particle shape and maximum transport angle, and the corresponding boundary equations of the drawing body were derived. Additionally, a novel numerical simulation method based on the rolling resistance linear model (RRLM), which takes into account particle shape, has been introduced. Validation was conducted through numerical simulations and physical experiments. Both numerical and physical tests confirmed that the improved B-R model more accurately captures the drawing body shape in SLTCC. Field measurement data in Wudong coal mine in Xinjiang, China, further substantiated the model's validity. In addition, as sublevel height increases, the drawing body exhibits markedly different evolution trends on the roof- and floor-side boundaries. Formulas for the improved B-R model at various sublevel heights were established. The research may provide new approaches for SLTCC simulations and theoretical calculations of drawing body shape at different sublevel heights.