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Mechanical behaviour of fracture layer failure in secondary mining composite roof strata

Zhao Guozhen, Chang Fengyi, Wang Shuai

Archives of Mining Sciences

2024

Vol. 69, no. 4

655--674

In the extremely close distance of lower coal seam mining, affected by secondary mining, the fractures expand through the layer in the composite roof, and this process is complex and diverse. In this paper, a three-point bending load test of different strength rocks and their combinations is conducted by combining acoustic emission. The results indicate that the overall deflection angle in the lower roof is approximately 8° smaller than that in the upper roof. When the fracture extends from the brittle roof to the plastic roof, the deflection angle in both types of roofs increases. When the plastic roof extends to the brittle roof, the deflection angle of the brittle roof increases while that of the plastic roof decreases. The damage degree of the composite rock mass during the failure process lies between the two single rock masses that make up the composite rock mass. The damage is dense at the initial expansion moment of the fracture in the rock mass, but weak when the interface expands through the layer. The energy evolution of the composite roof mainly depends on the high-strength roof, and the horizontal migration process of the fracture at the interface does not release energy.