Analysis on the top-coal caving structure of extra-thick hard coalseam with shallow depth in fully mechanized sublevel caving mining

In order to further investigate the top-coal caving structural characteristics of extra-thick hard coal seam with shallow bury depth in fully mechanized sublevel caving mining,authors studied the No. 3 coal seam mined in Qian- shuta Coal Mine at Yuyang District,China. The caving structure of hard top coal was studied by using numerical simu- lation,theoretical analysis,in-site measurement,etc. Cantilever structure of top coal exits at the face end position. The internal stress of cantilever top coal was analyzed,and the cantilever length was also simulated. The results indicate that different broken lengths have ansignificant impact on the caving and releasing of top coal. When broken length is less than the horizontal projective width of coal caving port,top-coal caving are relatively easy. On the contrary,it is difficult for top-coal caving,and the total time for handling big lump coalincreases. The results explains the macro- structural factors of difficulty on hard top coal caving.