ABSTRACT
ABSTRACT: I became attracted to the issue of the effect of intermediate principal stress on the strength of rocks after reading Prof. William Brace’s paper on brittle fracture of rocks published in 1964. During my stay at Prof. Brace’s laboratory at the Massachusetts Institute of Technology in 1965-1966, I refined the design of rock samples used in confined compression and confined extension tests and carried out experiments whose results clearly contradicted the earlier view that the effect of σ2 on rock strength is negligible. These important findings and fruitful discussions with American geophysicists both on the east coast (Brace, Walsh) and on the west coast (Griggs, Heard) encouraged me to design a true triaxial rock testing machine capable of loading rectangular prismatic rock samples independently in three different directions. The aim of my design was to meet the following requirements: (i) high stresses could be generated in rock samples, (ii) stress distribution in a sample would be homogeneous and (iii) the center of a sample would be kept in the same position during the test independent of the deformation of the sample. The true triaxial apparatus was built in 1968 and installed at my laboratory at the Earthquake Research Institute of the University of Tokyo. In 1969 I reported the first experimental results at the Annual Meeting of the Japan Seismological Society in Tokyo. Detailed discussions of the fracture and flow of rocks under general triaxial stress state were then presented in my papers published in the Journal of Geophysical Research and in Tectonophysics in 1971-1972. A broad overview of all of my investigations on the behavior of rocks under general triaxial compression conditions was also given in my recent book on experimental rock mechanics (Mogi, 2007).
