In order to examine the influence of the water as stress corrosion agent on the strength of rocks, a series of uniaxial compression tests under the water vapor pressure ranging from 10-3 to 103 Pa is conducted on Kitagishima granite and Kumamoto andesite. The values of the stress corrosion index of both rocks are determined.
At the beginning of the twenty one century, the large rock cavern for radioactive waste disposal will be constructed within a rock mass at a depth of more than 1000m, in Japan. The stability of such large rock cavern must be guarantee over very long timescale. The prediction of long-term strength of rocks is indispensable for insuring long-term stability of the rock cavern. The strength of rocks under compression is affected by the density, the shape and the direction of cracks, the strain and stress rate of loading, the stress state, the temperature and the existence of water in vapor phase or in liquid phase, and so on. Some researchers have indicated that the strength of rocks is dependent on the subcritical crack growth due to stress corrosion and is proportional to the water vapor pressure in atmosphere. However, few experiments to verify this fact have been performed. In this paper, to examine the influence of the water as the stress corrosion agent on the strength of rocks, a series of uniaxial compression tests under the water vapor pressure ranging from 10-³ to 10³ Pa is conducted on Kitagishima granite and Kumamoto andesite. Furthermore, the stress corrosion index is determined, then the estimation of long-term strength of rock is discussed.
In general, it is considered that the stress corrosion plays a major role in the subcritical crack growth at the fracture of rocks, ceramics. The water, namely aqueous environments, is one of important agents for the stress corrosion of rocks. The expressions of crack velocity including the effect of the stress corrosion have been proposed to explain the subcritical crack growth, where a is the crack length, p is the water vapor pressure, E* is the activation energy, k is the Boltzman's constant, T is absolute temperature, nw and n are experimentally determined constants and E* is the tensile stress near the crack tip for a very narrow elliptical crack. The value of nw is to be almost unity (Wiederhorn 1969). Sano performed a series of the experiments in which the strain rate varied under atmosphere, then the stress corrosion index of Oshima granite was determined to be 32. The values of stress corrosion index of other rocks is determined by John (1972), Sangha (1972), and Peng (1973) based on uniaxial compressive strength obtained from the same testing method. These values were 32 to 62. The values of the stress corrosion index determined by them were 13 to 70. However, there is a few experiments to examine the influence of the water vapor pressure on the uniaxial compressive strength.