The concept that the scattering of the strength of rocks should be large has been widely accepted in engineering rock mechanics. However, carefully arranged experiments have shown that the coefficient of variation of uniaxial compressive strength for thirty-one specimens of Oshima granite is 1.9%. The effect of environment on the fracture process of granite was then investigated. The compressive strength of the air-dried specimens was higher, by about 20%, than that of the wet specimens. The dilatant strain rate under the wet condition was higher, by a factor of three, than that under the dry condition, at the same stress levels. These facts indicate the important effect of water. A model, based on the assumption that the slow crack growth due to stress corrosion plays an important role in the fracture process of rock under compression, explains quantitatively these effects. Since the different environment may bring about different water content of the specimens, the strength of rocks may scatter within 20%, in a usual room condition.
