It is important to understand time-dependent deformation and fracturing in rock to evaluate its long-term strength (LTS); subcritical crack growth (SCG) provides insight into the weathering of a rock mass over the long term. The LTS of rock is commonly evaluated under the same environmental conditions. However, in practice, the environment is constantly changing and must be accounted for in evaluating the LTS of rock. In this study, we developed a method to evaluate LTS under changing environmental conditions, with a focus on the influence of water on the LTS of rock. LTS decreased rapidly when the environmental conditions changed from air to water. In a case where the environmental conditions changed repeatedly from air to water at various duration intervals, the value of the LTS was similar to that in a continuous water environment. Because a dramatic decrease in the LTS occurred when the environmental conditions changed from air to water, we conclude that the effect of water on the acceleration of SCG in rock should be considered in the long-term use of rock structures. SummaryIt is important to understand time-dependent deformation and fracturing in rock to evaluate its long-term strength (LTS); subcritical crack growth (SCG) provides insight into the weathering of a rock mass over the long term. The LTS of rock is commonly evaluated under the same environmental conditions. However, in practice, the environment is constantly changing and must be accounted for in evaluating the LTS of rock. In this study, we developed a method to evaluate LTS under changing environmental conditions, with a focus on the influence of water on the LTS of rock. LTS decreased rapidly when the environmental conditions changed from air to water. In a case where the environmental conditions changed repeatedly from air to water at various duration intervals, the value of the LTS was similar to that in a continuous water environment. Because a dramatic decrease in the LTS occurred when the environmental conditions changed from air to water, we conclude that the effect of water on the acceleration of SCG in rock should be considered in the long-term use of rock structures.

Granite rock masses are used for various geomechanical and engineering purposes such as the construction of underground repositories for radioactive waste. Investigating the fracturing in granite, especially time-dependent fracturing, is therefore crucial to consider the long-term integrity of granite rock mass surrounding structures. Subcritical crack growth is one of the main causes of the time-dependent fracturing in rock materials. For the long-term integrity of the structures in a rock mass, it is essential to investigate subcritical crack growth in rock, which is influenced by the surrounding environmental conditions. Numerous cementitious materials are used to construct various structures such as underground repositories for radioactive waste, underground power plants, etc. In these cases, the calcium ion concentration in the ground water in a rock mass surrounding these structures is high. Therefore, investigations on the subcritical crack growth of rock in water with a high calcium ion concentration are necessary. In this study, subcritical crack growth in granite is measured in distilled water and a calcium hydroxide solution to investigate the influence of calcium ions on the time-dependent fracturing in rock. Oshima granite was used as a rock sample. The load-relaxation method of the double-torsion test was used to measure the crack velocity and the stress intensity factor. All measurements have been carried out under controlled temperature and relative humidity. It was shown that the crack velocity in a calcium hydroxide solution is lower than that in distilled water even though the pH was high. It was also found that precipitation of calcium compounds closes the crack, suggesting that precipitation decreases the crack velocity. Subcritical crack growth index of granite obtained in a calcium hydroxide solution was higher than that in distilled water, which brought about higher long-term strength and longer time-to-failure. It is concluded that a water environment with a high calcium ion concentration is suitable for the long-term integrity of granite rock mass.