In most of the underground facilities, the existence of water and anisotropic stress are very serious problem in excavating and maintaining the underground open space. Previous studies are not sufficient for estimating the real rock mass behavior with considering the mechanical and hydro-geological aspects. The fractures in situ underground environment are not distributed randomly. Most of fractures in situ occur in a systematic manner with obeying the geologic and tectonic rules. Previous studies are also insufficient for evaluating the history of fracture occurrence and its development. In this paper, we describe about the importance of laboratory experiment, especially useful advantage for adopting True Triaxial Test to investigate the effect of the intermediate principal stress. We focus on the importance of the intermediate principal stress and introduce the observations related to the mechanical characteristics on the wide range of stress state from the confined triaxial compression to confined triaxial extension.

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