Time-dependent cracking of Beishan granite under the effect of temperature and stress condition

In China, a deep granite formation in Beishan area located in Gansu Province of northwestern China is currently considered as a potential host rock for high-level radioactive waste repositories. With the purpose to investigate the time-dependent behavior of Beishan granite under the effect of temperature and stress condition, a series of creep tests were conducted by two modes, namely mono-step loading mode on several samples and multi-step loading mode on a single sample. The microcracking process in both time and space were obtained with the help of a 3D acoustic emission monitoring system. From the experimental results, a similar volumetric strain can be found when the rock samples failed at the same confining pressure, although the stress condition and temperature loaded are different. The increase in stress condition or temperature may accelerate the time-dependent cracking process and increase the volumetric strain rate, and consequently reduce the time to failure. The stress ratio apparently to be more reasonable when it comes to discuss the influence of stress condition. When the same stress ratio is conducted on different samples, the steady state volumetric strain rate is observed much lower in the stress-stepping creep test than that in mono-step creep test. The effects of temperature and stress condition on the time-dependent cracking are also confirmed by the recorded Acoustic Emission (AE) events. A good accordance is noticed between the recorded AE count and volumetric creep strain. However, before the appearance of the large increase in volumetric strain, the rapidly increase of acoustic emission activity has occurred. According to the spatial distribution of AE events, the most high amplitude cracks are found to occur on the circumferential failure surface of the specimen.