To investigate deformation characteristics under different stress states and loading paths, we conducted a true triaxial compression test using a rectangular specimen of 35×35×70 mm, a confined triaxial extension test and a confined triaxial compression test using a cylindrical specimen of ϕ50×100 mm.We also investigated the differences in boundary conditions between fluid pressure and solid piston pressure, during σ1=σ2 loading. The true triaxial test is superior in obtaining continuous deformation behaviors from ductile under a compression regime to brittle under an extension regime. The maximum principal stress increased by a finite amount in changing from a compression state to an extension state. The brittleness is defined as the reduction in themaximumprincipal strain observed under increases in the intermediate principal stress.Transition deformation characteristics are also observed when changing from a compression state to an extension state.
It is well known that stress states in an underground environment are generally a 3 dimensional field and the three principal stresses are different from one another. In laboratory experimental techniques, there are two experimental approaches to realizing in situ stress states in an underground environment; the confined triaxial compression test and the confined triaxial extension test. The former has the limitation that the minimum principal stress is always equal to the intermediate principal stress, similarly the latter has the limitation that the intermediate principal stress is equal to the maximum principal stress. We have little data relating to confined triaxial extension tests, especially for deformation characteristics under different stress paths. The extension test is defined by a stress system in which the longitudinal stress is less than the two equal radial stresses, and greater than zero. Their aims were focused on understanding the natural deformation behaviors of a deeply buried rock body.