Abstract:

In view of the low top coal recovery rate in fully mechanized top coal caving mining of extra thick coal seams with hard roof, taking the fully mechanized top coal caving mining of “two hard” coal seams as the actual field conditions, theoretical analysis, numerical simulation, field tests and other methods are used to analyze the corresponding relationship between roof structure and top coal stress state, to reveal the top coal space damage characteristics, and evaluate the application effect of surface fracturing double effect weakening technology. The results show that the roof structure can be divided into “combined short cantilever beam” + “articulated rock beam” hybrid bearing structure, “combined short cantilever beam” independent bearing structure, and “articulated rock beam” independent bearing structure according to the stress state of the top coal. After implementing fracturing measures, the stability of the overburden structural stability worsens, the failure height of the overburden increases, the thick sandstone overhang disappears, the peak value of the advance bearing pressure decreases significantly, and the integrity of the top coal is damaged, weakening measures can effectively reduce the suspended length, stress concentration, and integrity of the top coal caused by the thick and hard roof during the mining process of the working face. Within the range of fracturing damage area, the end of cycle resistance decreases by 25. 4% during non pressure periods, and the cycle resistance decreases by 15. 3% during pressure periods. After surface fracturing, the degree of damage to the overlying rock layers is relatively high, and the hard and thick rock layers become thin layers, resulting in an improvement in the recovery rate of the top coal, the recovery rate of the top coal increased by 6. 1% compared to the top coal weakened without surface fracturing.