Calculation method and evolution rule of the strain energy density of sandstone under true triaxial compression.

Deep rock masses are typically in complex stress states, and research on the evolution of their strain energy density is of highly important for understanding their failure characteristics. In this work, a true triaxial stress‒balanced unloading test is designed to analyze the u and u evolution of sandstone under true triaxial compression conditions. The study results indicate that as σ increases, the elastic strain decreases in the σ and σ directions, whereas the residual strain progressively increases, and the magnitude of decrease in elastic strain exceeds the magnitude of increase in residual strain. Throughout the loading process of σ, u progressively decreases in the σ and σ directions, whereas u gradually increases, and the magnitude of decrease in u surpasses the magnitude of increase in u. The u and u of sandstone under different stress levels were calculated via true triaxial stress‒balanced unloading tests, and the evolution of u and u in the three principal stress directions and the overall strain energy density of sandstone followed a linear energy storage law. On the basis of this law and the true triaxial stress‒balanced unloading test, a new method for calculating the true triaxial u and u was proposed. A study on the σ unloading stress path revealed that the σ unloading stress path significantly affects the storage and dissipation of the strain energy density in the three principal stress directions of sandstone. On the basis of the research results, the criteria for determining rockbursts were discussed.