Optimal sublevel height and drawing process for horizontal sectional combined mining in steeply inclined, and closely spaced coal seam groups

The extraction of steeply inclined and closely spaced coal seams induces intense strata pressure disasters owing to cross-layer fracturing and collapse, rendering strike longwall fully mechanised mining unsuitable. An innovative horizontal sectional combined mining method with simultaneous coal-rock drawing is therefore proposed. Using the combined mining of the middle coal group at Tianshun Coal Mine, Xinjiang, China, theoretical analysis and numerical simulation were employed to evaluate the effects of sublevel height on the working face safety-production performance. A "mining-induced pressure relief-plastic failure" model for the top coal-rock mass was established, together with the coupled "stress release rate (SRR)-plastic deformation zone index (PDZI)" evaluation index. A three-component decision-making framework, comprising preliminary selection, multi-dimensional evaluation, and economic decision-making was proposed for optimal sublevel height determination. The flow behavior of the coal-rock mass and drawing body morphology distortion under the "subsidence-interlayer leap" effect were investigated. A multi-cycle intermittent drawing process from the roof side towards the floor side was proposed and validated through industrial trials. Results indicate that at a sublevel height of 25 m achieves optimal balance between pressure relief, plastic failure, and production capacity. Compared with a single steeply inclined coal seam, the drawing body morphology exhibited distorted characteristics: expansion towards the interburden area on the roof side, constriction within the interburden zone, and an upward deflection on the floor side. Industrial trials achieved single-cycle production of approximately 900 t and 85% coal-rock recovery rate, supporting fully mechanised top-coal caving mining of steeply inclined, closely spaced coal seam groups.