Experimental Study of Mechanical Behaviors and Failure Characteristics of Coal Under True Triaxial Cyclic Loading and Unloading and Stress Rotation

The top coal of extra-thick coal seams is susceptible to irreversible failure under cyclic loading and stress rotation caused by longwall top coal mining. To ensure efficient mining and avoid wastage of coal resources, the failure evolution process of coal under cyclic loading and stress rotation must be investigated. Hence, true triaxial conventional loading, true triaxial graded cyclic loading and unloading, and true triaxial main stress rotation path tests were conducted to reveal the mechanical behaviors and failure characteristics. First, the evolution of deformation, elastic modulus, Poisson’s ratio, and peak strength were analyzed. Then, the fracture volumetric strain, dissipated energy, and failure modes were characterized. Finally, the relationship between the fracture evolution of top coal under cyclic loading and stress rotation was studied. The results showed that the mechanical characteristics of top coal are complex. The deformation and expansion characteristics under three paths were different. Additionally, the elastic modulus and Poisson’s ratio showed negative correlation. Moreover, the internal coal damage was more severe and the failure was more violent under the true triaxial graded cyclic loading and unloading path. The failure mode of coal was dominated by shear failure and supplemented by tensile failure. The failure mechanism of top coal under true triaxial cyclic loading and unloading and stress rotation was revealed. Compared with static loading failure under conventional conditions, the longitudinal fractures caused by cyclic loading and the transverse fractures caused by stress rotation penetrated each other, affording the crushing of the top coal.