Fault-slip rockburst in deep mining and hard rock tunneling is primarily induced by fault activation. In this study, the evolutionary process of the unloading-induced fault activation was analyzed by true triaxial unloading tests on pre-fractured specimens, and the influence of the unloading rate of σ3 (minimum principal stress) and initial σ3 value on fault activation was investigated. Intact specimens were also tested for comparison. The activation process was studied with the aid of acoustic emission and deformation monitoring. Our results indicate that asperities on original macro fracture have sufficient time to degrade gradually at a low unloading rate, and a higher unloading rate usually leads to a smaller unloading extent of σ3 at the moment of fault activation, resulting in less deformation ( \(\Delta \varepsilon_{3}\) ) in the direction of σ3. Moreover, the evolution of acoustic emission (AE) hit rate was divided into inactive phase and rapid increase phase during the unloading process, and both the duration of the rapid increase phase and the proportion of rapid increase duration to whole unloading duration decrease with increasing unloading rate. The initial σ3 value is inversely related to the unloading extent of σ3 and \(\Delta \varepsilon_{3}\) , while it is proportional to the post-peak stress drop and the amount of gouge within the post-failure specimens, which indicates that a larger initial σ3 intensifies the damage of the fault. Our results also show that the σ3 value at which fault activation occurs can be predicted by the single weak plane theory during the unloading of σ3. The time-delayed failure observed in three intact and one pre-fractured specimen shows that time-delayed rockburst in deep tunnels can be induced by the unloading of σ3. The strength for pre-fractured specimens in both loading and unloading tests is well fit by the Mogi-Coulomb strength criterion, and the cohesion is smaller while the friction is greater in the unloading tests than in loading-induced fault activation. The findings contribute to a better understanding on the unloading- induced fault-slip process and the mechanism of slip rockburst.
