The study of failure of a rock under constant loading is essential to the design and construction of excavations in the rock for mines, tunnels, and nuclear waste repositories. The constant loading test on granite with acoustic emission monitoring clearly shows the progressive damage process. The inelastic volumetric strain is proportional to cumulative acoustic emission counts, which indicate that the damage within granite is attributed to micro-cracking or micro-fracturing within granite. Three creep phases can be determined more easily from the cumulative acoustic emission count. Source location analysis also shows the progressive damage process under constant loading. During primary creep, cracks distribute randomly and then cluster into several nucleation Zones. During secondary creep, some clusters become more active while others decay off. During tertiary creep, the activities of the active cluster become stronger until the sample fails.
Granite has been considered as an ideal site to construct nuclear waste repository to prevent the mitigation of radioactive species due to its high structural capacity and low permeability. However, preferred hydraulic pathways can be created if the rock around the excavation is damaged under sustained loading. Therefore, fundamental studies to evaluate the failure and damage mechanism of granite at a certain stress level are needed.
The development of cracks of Barre granite under constant loading had been studied by scanning electron microscope (Kranz, 1979). However, the production of sections often introduces new cracks, making the Identification of natural and experimentally stress- induced cracks difficult. Furthermore, cracks viewed in the un-stressed state may give false impression about the dimensions of the cracks when stressed. Laboratory long-term loading tests conducted on granite in Canada had shown that damage could be quantified through the measurement of irreversible volumetric strain and the degradation of static and dynamic elastic properties (Chandler & Lau, 2002). However, the dynamic elastic properties were derived from the zero-pressure ultrasonic velocities measured after the completion of each unloading sequence, and the static elastic properties were determined during reloading. The procedures were tedious and did not truly represent in-situ conditions. To overcome this limitation, Lin et al (2004) had measured the ultrasonic velocity of granite at its true stress state and found the degradation of dynamic elastic properties under constant loading. However, the transducers can be damaged easily by sudden shocks, it is not possible to carry out measurement to the verge of rupture. The application of the method is, therefore, also restricted.
Acoustic emission (AE) is defined as the transient elastic wave generated by the rapid release of energy from a source within a material. Since acoustic emission monitoring is essentially passive, it provides an ideal non-destructive method for studying failure mechanism of rock at its true stress state. One important aspect is that counting the number of AE events prior to sample failure. Lockner & Byerlee (1977) showed a correlation between AE rate and inelastic strain rate.