Effect of stress condition on the creep behaviour of Beishan granite

Abstract In China, a deep granite formation in Beishan area is currently considered as a potential host rock for high-level radioactive waste repositories. Creep tests were conducted by two modes, namely single step loading mode on several samples and multi-step loading mode on single sample. According to the experiment results, the granite specimens failed almost at the same axial strain and volumetric strain at a given confining pressure, despite of the differences in deviatoric stress, stress ratio or strain ratio. The increase in stress condition (deviatoric stress, stress ratio, or strain ratio) could accelerate the cracking process and creep strain rate, and consequently resulted in a shorter time to failure. The stress effect on the cracking process was also confirmed by the acoustic emission (AE) energy rate. Significant increases in AE energy rate and the percentage of AE events with high energy could be observed with the increase of deviatoric stress. It was indicated from the results that, the stress ratio and strain ratio were more reasonable when discussing the influence of stress condition, perhaps because they were defined by considering both the applied stress level and the mechanical properties of rock material.