Influence mechanism of thick hard layer on fracture and energy release characteristics of composite roof

Investigating the instability and energy release characteristics of layered roof structures is crucial for elucidating the mechanism of rockburst induced by strata fracture. In this study, three-point bending tests were designed and conducted on composite roof structures under different conditions. The instability and energy release characteristics of the composite roof were investigated using digital speckle and acoustic emission monitoring techniques, revealing the mechanism of rockburst induced by thick hard rock layers in the roof. The results show that the presence of thick hard rock layers alters the failure mode of the layered roof. The failure process of the composite roof with thick hard layers can be divided into four stages: overall bending deformation, low-strength rock beam cracking, stress adjustment, and unstable fracture. The presence of thick hard rock layers also significantly affects the strength and energy release of the composite roof during fracture. Specifically, the load-time curve of the composite roof transitions from a "single peak" to a "double peak," and the fracture mode shifts from gradual failure to instantaneous unstable fracture. Additionally, the presence of thick hard rock layers causes a sharp increase in both the stored strain energy before fracture and the energy released during fracture. These findings highlight the importance of considering thick hard rock layers in the prevention and control of roof rockburst.