First, the large area of ​​the roof pressure phenomenon and characteristics The large area of ​​the top plate is mainly due to the fact that the area of ​​the hard top plate is over a certain limit value, causing severe dynamic pressure caused by large area falling. When the top plate is pressed in a large area, the area of ​​one fall is as small as several thousand square meters, and the other can reach tens of thousands or even hundreds of thousands of square meters. Second, the cause and mechanism of large-area pressure on the roof The large area of ​​the roof is formed by the large area of ​​hard rock. Such as sandstone and conglomerate layers, the unidirectional compressive strength can reach 8.0~16.0kN/cm 2 , even up to 20.0kN/cm 2 . These rock formations are generally thick monolithic structures, and the bedding, joints and fissures in the rock mass are not developed. Some of these hard rock directly overlying the upper coal layer, a thin layer of some smaller formation between seam strength. Since the top layer directly overlying the slate hard coal seam, in the early taken back face, roof breaking down pitches up to 50 ~ 70m, even up to 100m or more. In order to avoid the hazard of initial pressure formation, it is often used to carry out mining by means of a knife, that is, when the working face is advanced by a certain distance, a coal pillar is supported in the goaf to support the roof. Thus, it seems that it is safe for a stope, but as the area of ​​the goaf increases, the pressure on the overburden will be mainly borne by the remaining coal gangue. As a result, the mechanical properties of the coal pillar itself are often changed (creep), and the supporting ability to the top plate is lost. It is known from practice that when the ratio of the coal support area to the goaf area is less than 30%, this phenomenon is particularly serious, and it is easy to form a large area of ​​pressure. From the condition of energy accumulation, the hard roof is bent and sunk before breaking, so that the coal rock in front gathers a large amount of bending elastic energy. In the process of breaking, if the conditions are sufficient, it will be suddenly released, and the phenomenon of impact ore pressure such as the ejection or protrusion of the coal and the destruction of the roadway will occur. Figure 10-4 Front and rear activity diagram of the hard roof
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