Abstract
Extremely close coal seam groups are widely distributed in China, and the main mining method is downward mining. In the downward mining process of extremely close coal seam groups, the violent movement of overlying strata will cause the redistribution of surrounding rock stress. It not only produces stress concentration on the pillar but also causes the roof of the lower coal seam to be broken and difficulty in maintaining the mining roadway. In this study, the physical similitude modeling method and field observations were used to study the breakage and migration law of overlying strata in the downward mining of extremely close coal seams. Results show that in the process of mining upper coal seam, the first weighting step of the main roof is 37.5 m and the periodic weighting step is 12.5 m. The occurrence of strata separation is beneficial to the prediction of roof weighting. When the working face advances to 25 m, the rock stratum approximating a parallelogram of height 5 m does not collapse, and the working face is relatively dangerous. When mining the lower coal seam, the overall pressure of the working face is large, but the periodic weighting of the working face is not obvious. The first collapse step of the immediate roof is 15 m. When mining the upper and lower coal seams, the subsidence of the monitoring point increases significantly at 17.5 and 15 m, respectively. The roof collapse of the lower coal seam occurs 2.5 m ahead of that of the upper coal seam. The hydraulic value of the support, roof fall height, and sloughing depth in the entire working face reach the maximum at the coal pillar, and the extreme points at the coal pillar are relatively concentrated. This research provides some guidance for the safe and efficient mining of extremely close coal seams in the future.
Highlights
China produces and consumes the most amount of coal in the world [1]. e proportion of coal in primary energy production and consumption is approximately 70%
When the distance of coal seams is extremely close, the roof integrity of the lower coal seam will be damaged by the mining of the upper coal seam. e area above the roof is the caving zone formed by the collapsed immediate roof [7]
Comprehensive analysis shows that the average first collapse step of the immediate roof in 1001 working face is 15 m and no obvious periodic weighting is shown, which is consistent with the physical similarity simulation results. e hydraulic value of the support, roof fall height, and sloughing depth in the entire working face reached the maximum at the coal pillar, and the extreme points at the coal pillar were relatively concentrated
Summary
China produces and consumes the most amount of coal in the world [1]. e proportion of coal in primary energy production and consumption is approximately 70%. Advances in Civil Engineering and water and gas accidents [11, 12] Factors such as mining thickness, burial depth, and dip angle of coal seam are closely related to the law of overlying strata movement [13]. Based on physical similarity simulations, Huang et al [15] studied the characteristics of overlying strata movement and strata behavior law in fully mechanized coal mining and backfilling longwall faces. Based on the engineering background of the Wuhushan coal mine, the law of overlying strata breakage and migration in the downward mining of extremely close coal seams was studied using the physical similitude modeling method. Based on the floor failure mechanics model, Zhang et al [28] proposed a new method to monitor floor failure depth and successfully applied it to the Caocun coal mine in China. Studies regarding the breakage and migration law of overlying strata by physical similarity simulations are rare. erefore, the law of strata breakage and migration must be studied to realize the safe and efficient mining of extremely close coal seams
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