Research on impact failure characteristics of coal-rock combination bodies under true triaxial loading and unloading conditions

Loading and unloading experiments on coal-rock combination bodies were carried out by a true triaxial test system to explore the mechanical behavior response characteristics of the combination bodies with different coal thickness proportions and to analyze the influence of the coal thickness proportion on impact failure characteristics of combination bodies. The results show that, as the proportion of the coal thickness increases, many X-shaped conjugate shear cracks form in the bodies, the strength shows a significant decrease trend with a gradually decreasing gradient, and the impact failure characteristics gradually become obvious. The deformation mainly occurs in the initial stress loading stage and the vertical stress reloading stage, and the maximum principal strain plays a dominant role in deformation. The characteristics of the full stress-strain curves of combination bodies with different coal thickness proportions are similar. As the proportion of the coal thickness increases, however, the plastic deformation gradually increases. According to the experimental results, under similar mining and geological conditions, the burst trend of the floor increases with a gradually decreasing gradient as the bottom coal thickness increases. When the bottom coal thickness increases to a certain value, the increase of the bottom coal thickness has no obvious superimposed effect on floor burst. The research results can provide a reference for further study of occurrence mechanisms of floor burst, the construction of pilot test platform for prevention and control technology, and the development of engineering simulation. © 2020, Science Press. All right reserved.