Types, occurrence mechanisms and prevention techniques of roof disasters in fully-mechanized top coal caving face

The excavation thickness of the coal seam and the corresponding motion space of the overlying strata are so large in the fully-mechanized top coal caving face, that roof accidents are occur easily. In order to understand the mechanism of roof disasters in fully-mechanized caving working face, some cases of roof disasters in fully-mechanized caving working face are statistically analyzed. According to the roof conditions and disaster characteristics, the roof disasters in fully-mechanized caving working face are divided into two types: large-area roof cutting and support crushing for soft roof, large-area roof collapse with roof cutting and support crushing for hard roof. Taking Cuimu Mine and Caojiatan Mine as examples, the characteristics of two types of roof disasters are analyzed, and the corresponding roof disaster mechanism is presented: The roof in soft roof working face is of weak cementation. If the actual support stiffness is insufficient, the roof will break at the coal wall, and is easy to be unstable after breaking. As the same time, the insufficient support strength eventually leads to the leads to cutting and support crushing disaster of roof. The roof in hard roof working face is of good integrity and is not easy to break. When the suspended roof area is enough, the sudden breaking of roof will cause an impact on the support. If the breaking line is at the coal wall, it will easily leads to cutting and support crushing disaster of roof. Combined with engineering practice, prevention and control measures for two types of roof disaster are presented. For the soft roof working face, the measures such as reasonably improving the support stiffness, the supporting efficiency and the advancing speed of the working face, and the early warning and so on are used to prevent the occurrence of large-scale roof cutting and support crushing. For the hard roof working face, in addition to support management and rational mining design, it is necessary to improving roof conditions. For examples, the regional hydraulic fracturing technology is used to weaken the roof and prevent large-area roof collapse and support crushing disasters.