Influence of time-lagged unloading paths on fracture behaviors of marble using energy analysis and post-test CT visualization

Underground excavation usually results in the decrease of horizontal stress in the front of the working face, which may trigger the occurrence of rock bust hazards. Examining rock failure under confining pressure unloading conditions is necessary to reveal the fracture mechanism of rock mass subjected to stress disturbance. Although many investigations have studied the rock mechanical behaviors during unloading, the time-lagged triaxial unloading paths on the rock fracturing mechanisms are still poorly understood. Triaxial unloading experiments and the post-test CT scanning technique were conducted on fine-grained marble samples under different unloading conditions. The stress–strain behaviors, energy evolution characteristics, and the fracture pattern of samples were analyzed. The results indicate that obvious features at various stages can be identified from the deformation history curve of the elastic strain energy (U_e), total strain energy (U), and dissipated energy (U_d). U, U_e, and U_d almost all increase with sample deformation at the time duration stage, and the elastic energy and dissipation energy curves slightly decrease and increase again. After the unloading point, the dissipation energy sharply increases and the incremental rate of elastic energy becomes lower. The elastic strain energy increases with the increasing unloading rate for samples at the confining pressure unloading point and failure point, and it is influenced by the time-lag effect of confining pressure. The crack pattern in the CT images depends on the triaxial unloading rate and time effect, which is in good agreement with the energy dissipation and release analysis. The unloading strategy has an effect on the energy conversion during triaxial unloading tests; in other words, energy dissipation and release are dependent on the unloading rate and time.