A new research system for caving mechanism analysis and its application to sublevel top-coal caving mining

• BBR system was established following a long-term detailed study of the top-coal drawing mechanism.
• In SLTCC, the drawing extent index decreases linearly with increasing sublevel height (Hs).
• With the increasing of Hs, the top-coal recovery ratio decreases after an initial increasing phase.
• Most of the residual coal in the current sublevel will not be recovered in the next sublevel.
• Drawing from the floor to the roof decreases the coal loss near the floor greatly in SLTCC.

The study of top-coal drawing characteristics is key to improving the top-coal recovery ratio and to reducing the waste of coal resources in longwall top-coal caving (LTCC) mining. This paper introduces a Boundary-Body-Ratio (BBR) based research system established by the authors following a long-term detailed study of the top-coal drawing mechanism. For BBR system, a simplified parabolic model is proposed to describe the development process of the boundary of top-coal in normal top-coal drawing cycles, where the drawing body is a cut variant ellipsoid (CVE) that can be described by the modified Bergmark-Ross model. A method for improving the recovery ratio and reducing the rock mixed ratio of top-coal by selecting reasonable drawing technique parameters and controlling the shape of the boundary of the top-coal is given for the BBR system. Based on the BBR research system, the drawing mechanism for loose top-coal in sublevel top-coal caving (SLTCC) in steeply inclined coal seams is studied for different sublevel heights and drawing direction conditions using distinct element numerical calculations and loose top-coal drawing experiments. The results show that the boundary curve of top-coal can be fitted by a parabola and that the drawing body remains as a cut variation ellipsoid in SLTCC, as is the case for a flat coal seam. The convex point of the boundary of the top-coal moves toward the goaf with increasing sublevel height, which would result in the incomplete development of the top-coal drawing body. The drawing extent index decreases linearly with increasing sublevel height, and the top-coal recovery ratio decreases after an initial increasing phase. Approximately 90% of the residual coal in the current sublevel will not be extracted during the drawing process of the next sublevel. Drawing from the floor to the roof expands the overlap area between the drawing body and the boundary of the top-coal, improving the recovery of the top part of the top-coal. Such a drawing sequence decreases the coal loss near the floor by 38.2% compared to drawing from the roof to the floor and greatly increases the resource recovery ratio.