1. School of Engineering and Mining,China University of Mining and Technology(Beijing),Beijing　 100083,China; 2. Top-coal Caving Mining Research of Coal Mining Industry,Beijing　 100083,China

A large number of extra-thick coal seams with a thickness of more than 20 m have been found in western China. Most of them cannot be mined by using open-pit mining. No mature theories and technologies can be referenced in the world. The fully mechanized caving mining with pressure relief for extra-thick coal seams was proposed for achieving a safe and efficient mining of extra-thick coal seams with a thickness of over 20 m. The large-scale of 60 ∶ 1 physical analogue test was used to study the distribution characteristics of the top coal caving and broken blocks,and the distribution characteristics of top coal displacement field. In addition,the test studied the support resistance and its effect on the top coal braking in the fully mechanized caving mining with pressure relief for extra-thick coal seams with a thickness of over 20 m. The results show that the number of top-coal blocks is denser within 500 cm3 ,the block ratio is about 94% and the number of blocks is about 860 when the pressure relief working surface is extracted to 75 cm. The cumulative proportion of top-coal blocks is about 96% within 500 cm3 and the number of blocks is about 1 700 when the pressure relief working surface is extracted to 135 cm. The proportion of the top-coal blocks increased by 2%and the number of blocks increased by 840 when compared with that of pressure relief working face at 75 cm. It con- tains almost all the top-coal blocks when the cumulative volume reaches 500 cm3 ,the block ratio is about 99. 9% and the number of blocks is about 4 810 when the top-coal working face is extracted to 17 cm. The top-coal blocks within 500 cm3 is the most densest,the proportion of the blocks increased by 3. 9% and the number of blocks increased by 3 110 when compared with that at 135 cm. The effect of secondary crushing is remarkable,which can ensure the smooth release of the top coal. In the stage of pressure relief mining,the displacement of upper top-coal is larger than that of the median top-coal and the displacement of the lower top-coal is the smallest. In the secondary fully mecha- nized top-coal caving mining stage,the lower top-coal displacement is the largest,and the effect of the top-coal collapse and fracture is the best. Many obvious vertical cracks were generated in the lower top-coal for the supporting effect of the support,and the thickness of the top-coal which affected by the support is approximately 10 cm. The repeated sup- porting effect of the support significantly promoted the broken effect of the top-coal.

WANG Jiachen,LÜ Huayong,WANG Zhaohui,et al. Technical principle and experimental study on fully mechanized top-coal caving mining after extracting the middle slice in extremely thick coal seam[J]. Journal of China Coal Socie- ty,2019,44(3):906-914.