Damage Evolution Characteristics of Fractured Sandstone under True Triaxial Cyclic Loading and Unloading

The hard roof exhibits a considerable caving step, large block size, and high mine pressure, making it essential to investigate its damage and fracture characteristics for effective strata control. In this study, sandstone samples were carefully selected from the working face of Tashan Mine in the Datong mining area of Shanxi Province. These samples were deliberately prefabricated with cracks at various angles. A true triaxial cyclic loading and unloading test was then developed to simulate the actual mining conditions at the face of the mine. Subsequently, a numerical calculation tool based on the material point method and a strain softening constitutive model were used to establish a plane loading model incorporating different angles of prefabricated cracks. Comparison between the model calculations and experimental results revealed the capability of the model to accurately replicate the formation of real macroscopic physical cracks in fractured sandstone during cyclic loading and unloading, capturing the complex physical processes involved. Furthermore, a simulation study was conducted to examine the cyclic loading and unloading damage evolution process of fractured sandstone under varying angles of prefabricated cracks and different confining pressures. The findings indicate that: (1) The deflection angle of macroscopic crack propagation increases with the angle of the prefabricated crack. (2) Stress concentration areas in the sandstone samples originate from both ends of the crack during cyclic loading and unloading, extending to the rock's edge and diagonal in the stress concentration region. (3) As cyclic loading and unloading progress, dissipation energy steadily rises, especially with the development and propagation of macroscopic main cracks. Peak stress and dissipation energy at complete failure increase with higher confining pressures and angles of prefabricated cracks. These research finding can provide theoretical support for the control technology of hard roof in coal seams.