The working face will continue to advance. When the working face knocks off the old tops that have been displaced, the working face will be under the cantilever protection of the old top, and then return to the normal state. Since this pressure is periodic, it is often referred to as periodic pressure. Sometimes even if the rock formation can form a masonry beam balance, the rock mass A is suspended to break, and the amount of deformation also changes periodically. to sum up: (1) As the working face advances, the A rock will enter the fracture state from a steady state. (2) The rotation of the A rock block and the reverse rotation of the B rock block. (3) Due to the rotation of the rock block, the top plate of the working face will inevitably sink and the load on the support will increase. (4) The performance of the bracket must be adapted to this process, that is, there is a certain amount of hungry and shrinkage, and can prevent the top plate from cutting along the coal .
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When the working face advances, the rock block A is first in a suspended state, and the working face support will be protected by the rock mass A, and the force is very small. But when the ultimate span is reached, the rock formation breaks. At this time, if the rock mass A does not bite with the rock mass, the rock layer and the overlying rock layer will be formed by one end supported by the bracket and the other end supported by the fallen meteorite.