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.

为实现20m以上特厚煤层的安全高效开采，提出了一种首先开采中间层的新型综放开采新方法。在这项研究中，采用相似比为60：1的物理模型并进行了相应的数值模拟，以探究其规律。顶煤的裂缝扩展，位移和应力场。结果表明，这种新的开采方法的顶煤破碎效果要好于片放开采，尤其是在二次顶煤开采阶段。在泄压开采阶段（Ⅰ阶段），顶煤塌陷形状可观察为近似梯形。同时，随着顶煤位移的减小，支护顶部顶煤悬臂梁结构存在破坏和不稳定性的周期性行为。在综放开采的第二阶段（Ⅱ阶段），顶煤的原始悬臂梁结构可以被完全破坏而具有松散的结构。并且顶煤中层的总位移大于上层的总位移，顶煤下层明显可见竖向裂缝。另外，顶煤断裂的较高的分形维数有助于顶煤的破碎。通过数值模拟，验证了物理模型顶煤及上覆地层位移和应力场的演化特征。