Motion Mechanisms for Top Coal and Gangue Blocks in Longwall Top Coal Caving (LTCC) with an Extra-Thick Seam

Using longwall top coal caving (LTCC) in an extra-thick seam, many problems appear, such as large broken top coal blocks, frequent arch formation in the drawing process, poor balance of drawing top coal and low top coal recovery, which seriously affect safe, efficient and green mining. In this study, the size distribution of top coal blocks was measured in LTCC with an extra-thick seam firstly. Based on the measured result, discrete element calculations and physical experiments of the top coal drawing process were conducted to systematically investigate the mixed motion mechanisms of top coal and gangue blocks in an extra-thick seam. The results show that with increasing drawing height, the height-width ratio of the drawing body increases in a parabolic form, indicating that the drawing body shape gradually changes from a short fat sphere to a high thin ellipsoid. In the whole drawing process, the height of rotation points of the top coal boundary increases first and then decreases within a range of one times the support height; however, the rotation angle of the top coal boundary rise gradually. The displacement field and velocity field of top coal and gangue particles have significant asymmetry characteristics in an extra-thick seam. Under the excessive drawing condition, when the mixed gangue ratio is about 10%, the pure top coal recovery can be greatly improved. In addition, the corresponding measures to improve the top coal recovery are given in LTCC with an extra-thick seam; the fine caving technology is also put forward, that is, single-opening caving method at the end and double-opening caving method in the middle, which provides a new approach for high resource recovery mining in LTCC with an extra-thick seam.