| Quantitative description for the evolution of crack in rock is one of the fundamental problems of rock mechanics. Based on the field observation of the abutment pressure, the cracking process of top coal is dynamically simulated, and the coal sample ,whose size is 300mm X 300mm X 300mm, is press cracking by the simulated abutment pressure. The cracks are measured continuously. Through the fractal geometry, the evolution of crack is described quantitatively. The variety of crack fractal dimension reveals the crack propagation. The complete fracture of hard coal comes through two stages of crack density increment and two stages of crack expansion. The complete fracture of medium-hard coal undergoes two stages of crack density increment and one stage of crack expansion, and the denseness of crack is the key for medium-hard coal to fracture. To the soft coal, there are once crack density increment and once crack expansion, and the expansion is import to fracture.It is found that the relation between the fractal dimension of crack and the abutment pressure is polynomial expression. According to the damage mechanics, the constitutive equation of top-coal fracturing is obtained. The propagation law of crack is proved to be correct by the changing of damage variable. Based on the spot measure and coal sample fracture experiment, the distribution of the top-coal block is found. Coal block distribution is the foundation of simulation test for coal block drawing.Through similar simulation test of block top-coal drawing, the influence of coal block size and block position in top coal on the recovery ratio and drawing manner is studied. It is found that the drawing manner has the different adaptation to the top coal which has different block distribution. The caving capacity is mainly decided by the block size. The top-coal which has layer difference of block with the same size has different recovery ratio. It is shown, that, to the given top-coal blocks, the recovery ratio of uniformly mixed blocks is maximal in the manner of drawing at interval of one support. When top coal has increasing block size from the underside to the upside, the recovery ratio is medium, andthe best drawing manner is in order. When the big blocks are in the middle part of top coal, the recovery ratio will be minimum, in this case, drawing at interval is needed for medium-hard coal and soft coal, and drawing in order suits with hard top coal. The top-coal block distribution needs corresponding drawing manner. In order to increase the recovery ratio, the pre-treated measures are needed to soften the hard top coal.The arching mechanism is studied in this paper. The relation of arching parameters to top-coal block, shield girder angle, friction coefficient, and angle of waste rock after support is found, which shows that the limit angle of shield girder is 40, and the limit angle of waster rock in gob is 45 ~50. If the angle is less than the limit angle, the arch, formed by top coal blocks will be in high position or a half arch comes into being. It is difficulty to destroy the balance of high arch or half arch, as a result, the top coal will be lost. The ratio of arch height to arch span increases with the coal bock size increment and the waste rock pile angle decrease. Reduce the arch height is helpful to improve the top coal recovery.The longwll top coal caving with hard roof and hard coal faces two troubles: One is the impact of roof, the other is that the hard top-coal is difficulty to be drawn. On basis of the law of coal fracture, the control of top coal fracture is studied. It is important to reduce the span of the hard roof to avoid its impact load on supports. However, at longwall top coal caving face, the periodical weight of roof is a main factor that crushes the top coal, which is beneficial to the top coal recovery. How to use the roof pressure safely is of crucial importance in top coal caving. Based on field monitoring, theoretical analysis of strata movement, the numerical model of compound structure of hard roof stepped cantileve...
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