Characteristic stress and strain precursor information for fine-grained granite during failure process under triaxial loading and unloading conditions

The unloading effect by excavation may cause irreversible and severe damage to the surrounding rock masses in underground engineering. In this paper, both conventional triaxial compression (CTC) tests and triaxial unloading confining pressure (TUCP) tests were conducted on fine-grained granite to study its triaxial compression failure processes due to unloading. Based on the crack volumetric strain (CVS) method, the crack axial strain (CAS) method and crack radial area strain (CRAS) method were proposed to identify the failure precursor information (including stress thresholds and axial strain at the initiation point of crack connectivity stage) during the rock failure processes. The results of the CTC tests show that the stable crack development stress ${\sigma }_{sd}$, unstable crack development stress ${\sigma }_{usd}$, and crack connectivity stress ${\sigma }_{ct}$ identified by the CAS method are 6%, 74%–84%, and 86%–97% of the peak stress, respectively. For the TUCP cases, as the confining pressure increases, the stress thresholds, axial pressure at failure and axial strain at the start of the crack connectivity stage increase, while the time ratio of the crack connectivity stage to the entire unloading stage decreases. This indicates that fine-grained granite is prone to generate more cracks and leads to fail suddenly under high confining pressure. Furthermore, this new method demonstrates that the point at which the derivative of the radial crack area strain transitions from stable to a sudden increase or decrease is defined as the precursor point of rock failure. The results of axial strain at the starting point of the crack connectivity stage are very close to those predicted by the AE method, with β₁ no more than 11%.