It is important to evaluate the long-term stability of rock materials for the purpose of ensuring the long-term integrity of structures in a rock mass. In this study, we showed an evaluation method of the long-term strength of rock considering the change of the surrounding environmental conditions. It was shown that the long-term strength of rock decreased remarkably when the environmental condition changed from air to water condition and from low temperature to high temperature condition. This is due to the acceleration of subcritical crack growth in rock, because the relationship between the crack velocity and the stress intensity factor for subcritical crack growth is dependent on the temperature and existence of water. It is also recognized that the values of long-term strength of rock are affected significantly by the highest temperature and water if the rock can be surrounded by water. It is concluded that the long-term strength of rock under water environment with high temperature is important for ensuring the long-term integrity of structures in a rock mass.
