Damage Evaluation with P-Wave Velocity Measurements during Uniaxial Compression Tests on Argillaceous Rocks

In the study of time dependent behavior of rock, the main difficulty is to predict delayed failure, which is of the utmost importance in assessing the safety of underground structures, such as deep underground facilities designed for high-level radioactive waste disposal. In this context, the viscoplastic behavior associated with the rock damage must be taken into account. As the longitudinal and transversal wave velocities are related to the physical and mechanical characteristics of materials, ultrasonic measurements can give valuable information about the development of damage. In this study, P-wave velocity measurements were used to monitor damage evolution during uniaxial strain in controlled compression tests and long-term creep tests. These measurements were performed using sensors in a piezoelectric copolymer of polyvinyl-difluoride, which were placed on both ends of cylindrical rock specimens. Throughout the experiments, the dilating behavior of an argillite could be correlated with a decrease of the P-wave velocity. Our results show that during a creep test, P-wave velocity measurements allow the three different phases of creep to be distinguished. During primary creep the P wave increases because of pore closure. The secondary creep phase, characterized by a constant strain rate, is identified by a linear decrease of the wave velocity; this trend accelerates during tertiary creep.