Effects of initial unloading level on the mechanical, micro failure and energy evolution characteristics of stratified rock mass under triaxial unloading confining pressure

In order to study the strength, micro crack distribution and energy evolution characteristics of stratified rock mass under unloading confining pressure conditions, triaxial compression tests and triaxial unloading confining pressure tests at different initial unloading levels were carried out. The experimental results indicated that the triaxial unloading confining pressure plus axial stress is a stress path between the stress state of uniaxial compression and triaxial compression. With the increasing initial unloading level, the specimen is closer to the stress state of triaxial compression, the peak stress, peak axial strain, peak circumferential strain and peak volume strain increase linearly, Poisson’s ratio decreases linearly, failure type of specimens shows 4 typical characteristics. When the bedding plane inclination is 45°, 3D reconstruction and quantitative characterization of micro cracks of stratified rock mass specimens are conducted based on CT scanning results. Variations of area fraction along the specimen height show 3 typical characteristics. The stratified rock mass mainly stores and dissipates energy before the peak stress, and mainly releases and dissipates energy after the peak stress under different conditions. With the increasing initial unloading level, the input energy, elastic energy, dissipative energy and energy dissipation rate at the peak stress increase approximately linearly.