Abstract: The surrounding rock of diversion tunnel in Danba Hydropower Station is mainly composed of quartz mica schist at the high-stress zone, which is typical transversely isotropic rock mass and has distinct layered characteristics with apparent dilatancy and high energy. Based on the MTS815 system, triaxial unloading tests were conducted to investigate the characteristics of dilatancy and energy change, and the loading direction was parallel and perpendicular to the schistosity of rock samples, respectively. The results demonstrate that the characteristic stress of both groups increased with confining pressure, which exhibited a good linear relationship with confining pressure. The crack closing stress, damage and expansion stress and peak stress of the parallel group were superior to those of the vertical group, whereas the crack stress was inferior to that of the vertical group. The dilatancy parameters of the parallel group were higher than vertical group. However, expansion characteristics of these two groups were weakened with the increase of confining pressure. Moreover, the index of expansion showed great agreement with confining pressure by an exponential distribution, and dilatancy angles had a linear relationship with the partial stress ratio. At the peak point and the residual point, the energy values of these two groups enhanced with confining pressure, and the energy values revealed good exponential distribution with confining pressure. Furthermore, under the same confining pressure, the energy characteristic value at the peak point and the residual released energy of the parallel group were greater than those of the vertical group. However, the residual dissipated energy of the parallel group was lower than that of the vertical group. For both groups, the relationship between the dissipated energy value and the dilatancy angle, together with the energy characteristic value and the dilatancy index, demonstrated good exponential distribution, which fit at the peak point as well as the residual point.

Key words: mica-quartz schist, characteristics of transversely isotropic, triaxial unloading, dilatancy, energy property, correlation analysis