To clarify the effect of the intermediate principal stress on the strength and deformational behavior of rocks, three sandstones, marble, and shale were deformed under true triaxial stress state in which the intermediate principal stress is not equal to the minimum nor to the maximum principal stress. Failure strength increased with the relative increase of the intermediate principal stress except for the stress state where the intermediate principal stress approaches to the maximum stress. The relative increase of intermediate principal stress induced also changes the deformation of rock from the ductile mode to the more brittle mode. The difference between the intermediate principal strain and minimum principal strain increased markedly with increment of the intermediate principal stress. The fracture stability of rock specimen was suggested by the concept of the effective shear strain energy stored around the closed microcracks. The deformational anisotropy was indicated by the preferred orientation of stress-induced open microcracks.
The Physical behavior of rock under triaxial stress state is essentially important for the underground mining and for the utilization of underground spaces for example, tunnels, electric powerhouse, storages of gas and oil, open pit mine, and high level waste repository and so on), because the act of excavations changes the stress fields and other physical and mechanical environments of rocks around the underground open space. The underground rock body is subjected to the initial stress prior to excavation. In many cases, it can be adequately assumed that the vertical normal stress is equal to the weight of the overlying rock mass, usually, 25–27MPa/Km on the average. The magnitude of the horizontal stresses lie widely between 0.5 and 3.5 times as large as the vertical normal stress at the depth shallower than 1000m(Brown and Hoek,1978). These observations suggest that the three principal stresses are different each others for many cases of initial. In order to investigate the mechanical characteristics of rocks around the underground openings, we have to adopt the testing equipment with which three different principal stresses can be reproduced precisely. The so-called triaxial compression test has been proved to be the most adequate and convenient testing method for the study of the mechanical characteristics of rocks for wide variety of confining pressures. In the conventional triaxial compression test, the longitudinal axial load is applied parallel to the axis of a cylindrical rock specimen through steel end pieces and the other stresses(confining pressure) are the cylinder by a fluid medium. The relatively homogeneous stress distribution can be produced in the specimen with the conventional triaxial test(confining pressure test). However, the intermediate principal stress is fixed equal to the minimum principal stress(compression test under confining pressure), in fewer cases, to the maximum principal stress(extension test under confining pressure), because the stress state in the confining pressure test is essentially axially symmetrical. According to the confining pressure experiments, failure strength increased remarkably with increase of the confining pressure, and relative transition from brittle to ductile behaviors is observed by the increase of the confining pressure.
