Uniaxial and triaxial compression tests were conducted for five rocks with the strain rate alternately switching between C1 and C2 at every predetermined strain interval. It was found that the strain interval had to be appropriately selected according to the confining pressure and the ratio, C2 / C1, in order to obtain the loading rate dependency precisely. Two stress-strain curves corresponding to the strain rates C1 and C2 were obtained applying the spline interpolation to the experimental data for just one specimen.
Concerning Tage tuff, Sanjome andesite and Akiyoshi marble, the loading rate dependency of peak strength estimated by the proposed method was reasonably well coincided with the published data. So it can be said that the proposed method is useful to evaluate the loading rate dependency of peak strength.
Loading rate dependency of residual strength was discussed for Tage tuff, Kimachi sandstone and Horonobe mudstone. Under the confining pressure, the ratio, (stress at the strain rate C2) / (stress at C1), was almost constant in the post-failure region. It is the most important finding of this study.
Time-dependent properties of the rock are indispensable to estimate the long-term stability of underground structures. In this study, the accurate and easy method was proposed to obtain the loading rate dependency. The advantage of the method is that a few rock specimens are enough for testing.