Study on the mechanism of a new fully mechanical mining method for extremely thick coal seam

In order to achieve safe and efficient mining for extremely thick coal seam more than 20 m, a new fully mechanized top-coal caving mining method with first extracting the middle layer was proposed in this study. In this study, physical model with a similarity ratio of 60:1 and the corresponding numerical simulation were performed to explore the laws of fracture propagation, displacement and stress fields of top-coal. The results show that the top-coal fragmentation effect of this new mining method is better than that of slice caving mining, especially on the period of the secondary top-coal caving stage. During the stage of pressure-relief mining (stage Ⅰ), the top-coal collapse shape can be observed as approximately trapezoidal. Meanwhile, there is periodic behaviours for the broken and instability of the top-coal cantilever beam structure on the top of support along with the decrease of top-coal displacement. During the secondary stage of fully mechanized top-coal caving mining (stage Ⅱ), the origin cantilever beam structure of top-coal can be fully broken with loose structure. And the total displacement of the medium layer of top-coal is greater than that of the upper layer, while the vertical crack can be obviously observed in the lower layer of top-coal. In addition, the higher fractal dimension of top-coal fracture contributes on the broken of top-coal. The evolution characteristics of displacement and stress fields of top-coal and overlying strata in physical model are verified through numerical simulation.