(1. College of Resources and Safety Engineering，China University of Mining and Technology(Beijing)，Beijing 100083，China；
2. Top-coal Caving Mining Research of Coal Mining Industry，China University of Mining and
Technology(Beijing)，Beijing 100083，China)
Abstract：In order to achieve safe and efficient underground mining of extremely thick coal seam above 20 m，a fully mechanized top-coal caving mining method after extracting the middle slice in extremely thick coal seam was creatively proposed. The similar simulation test with a large scale of 60∶1 was performed to study breaking and moving characteristics of top-coal，displacement and vector fields of top-coal，and support load effects of the fully mechanized top-coal caving mining after extracting the middle slice in extremely thick coal seam above 20 m. The results show that，during the stage of pressure relief mining，the top-coal collapse shape is approximately trapezoidal，the breaking and instability of the stepped top-coal cantilever beam structure above the support have a periodic effect，the displacements of the upper，the median and the lower of the top-coal decrease in turn，and cutting top press support accident occurs after the first weighting of the main roof. During the fully mechanized top-coal caving mining stage，the short cantilever beam structure is finally broken into a loose structure and the total displacement of the median top-coal is greater than that of the upper top-coal. Due to supporting effect of the support structure，vertical cracks occur obviously in the lower top-coal，and the thickness of the top-coal affected by the support is approximately 10 cm. The evolution characteristics of the displacement and vector fields of the top-coal are in consistent with the experimental results. The research results lay a theoretical foundation for the development of mining technology of extra-thick coal seams above 20 m in China.
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