Abstract:High stress is the controlling factor that influencing the deformation mechanism and stability of surrounding rock mass of deep-seated underground caverns. Stress relief and deviatoric stress state are the main mechanical effects induced due to excavation in rock masses where in situ stress exists. Tensorial nature of in situ stress and the intersection angle between longitudinal direction and major stress determine the magnitude of stress relief and the anisotropic level of induced stress. A general formula was derived for the magnitude of stress relief due to excavation. Thus, the variation law of the magnitude of stress relief was studied as intersection angle between the cavern axis azimuth and maximum principal stress direction varies. An index called “contribution degree” was purposed to quantify the contributions of different principal stress components to the magnitude of stress relief by excavation, and then to ascertain the dominant one. In order to study the variation characteristic of deviatoric stress state, the variation of the characteristics of stress ellipses with the intersection angle between cavern axis and maximum principal stress direction were illustrated graphically in combination with quantitative data. The method has been applied to the underground powerhouse of Jinping fist stage hydropower station. The results showed that when the longitudinal axis of underground caverns parallel to the major stress in horizontal plane, the magnitude of stress relief reached minimum. However, minimizing stress relief and deviatoric level at the same time was difficult. The study showed that due to the complexity caused by tensorial nature, in general case, it was the composition characteristics of geostress tensor that determined how and to what extent the geostress would affect the deformation and stability of surrounding rock mass of underground openings. Therefore, attention should be paid to the comprehensive analysis of the spatial characteristics of the geostress tensor.