Abstract: Time-dependent deformation of rock mass is obviously found under high stress condition during excavation unloading process. To investigate the unloading creep characteristics of fractured rock, specimens were prepared by marbles from Jinping I hydropower station. Then unloading creep experiments were performed in the laboratory under constant axial pressure and stepwise unloading lateral stress. The results showed that when the specimens fail, the corresponding smallest stress difference is found on the specimen with the angle of 30°, the middle one is specimen with the angle of 60 °, and the largest one is the specimen with the angle of 90°. Overall, shear failure occurs in the intact specimen and the specimen with the angle of 90°, while the cracks initiate from internal tips of pre-existing flaw of specimens with angles of 30° and 60°. The Burgers model is also employed to describe deformation characteristics of all intact rock and fractured rock without the accelerated creep stage. The values of parameters are used to distinguish the deformation characteristics of specimens with different fissures and under different stress conditions. The obtained results are consistent with the views of Lemaitre equivalent strain and Sidoroff energy equivalent principle on the correlations between damaged materials and undamaged materials. According to above results, damage creep constitutive models are put forward for the fractured rock mass. Moreover, the relationship between the intact rock and fractured rock masses is established using the proposed models. Therefore, this study can provide references for further study on time-dependent deformation characteristics of fractured rock mass.

Key words: rock mechanics, fractured rock, unloading triaxial creep tests, damage creep model