Abstract: By analyzing the broken, caving characteristics and motion criterion of top coal and immediate roof behind the shield, the discrete element numerical simulation model for top coal was established in this paper. Meanwhile, from the moving characteristics of coal-rock blocks and evolvement rules of contact force, the top-coal arching mechanism behind shield was discussed. The kinematic mechanism of top coal arching is that there exits a high-speed area of top coal above the coal draw-point, and the moving speeds of coal-rock blocks on the two sides of high-speed area are smaller. The top coal silts at both sides of the coal draw-point, and develops unstable contact force arching above the coal draw-point, which can result in moving speed of top coal near the coal draw-point decreasing. When large-scale top coal blocks in the higher level surge up to the coal draw-point, they can be easily squeezed and form an arch in mining process. The mechanics mechanism of top coal arching is that the drawing process of granular top coal can be seen as a formation and destruction process of the contact force arch. When the upper contact pressure is greater than the frictional resistance formed by the arching extrusion blocks, the contact force arch may damage, otherwise the stable contact force arch appears, and the coal draw-point may be jammed. Field measurements have verified the theoretical analysis.

Key words: top-coal caving, arching mechanism, numerical simulation, field measurement