Advanced Test Methods for Investigating the Time-Dependent Behaviors of Rocks

To assess the long-term stability of underground structures such as tunnels, mines, and high-level radioactive waste repositories, it is essential to understand the time-dependent behaviors of rocks. Loading-rate dependence of strength is one of the time-dependent behaviors and has close relation to the other time-dependent behaviors such as creep and relaxation. However, only the loading-rate dependence of peak strength has been investigated in the previous studies. In this paper, the theoretical background and the previous experimental studies of the loading-rate dependence of rock strength were described first. Then, the new test method in which alternating loading rates and cyclic loading are combined was proposed to investigate the loading-rate dependence of a complete stress-strain curve from a single rock specimen. The test results of Tage tuff showed that the stress-strain curve is time dependent in both the pre- and post-failure regions and that the degree of the loading-rate dependence is almost constant. These results will be helpful for clarifying the mechanism of the loading-rate dependence of both intact and fractured rocks and for formulating a new constitutive equation.