Abstract: Based on the analysis of physical and normal deformation characteristics, it is considered that the mechanical properties of clastic rock in dam foundation in hydropower station are extremely complex. Therefore, triaxial creep tests are conducted to investigate the creep behaviors of clastic rock by using an automatic triaxial servo-instrument. Firstly, the axial, lateral and volume creep properties and strain rate are discussed. Secondly, the effects of creep properties on the stress-strain curve are analyzed, and then the scanning electron microscope (SEM) tests are performed on the fractured rock specimens. Finally, the long-term strength is determined by isochronous curves. Based on the volumetric expansion of specimen, a new method is proposed to determine long-term strength by assuming that the fast creep failure occurs at the critical point where lateral volumetric expansion rate exceeds the axial compression rate. The results show that the clastic rock has a significant creep property, and the creep curve generally have two stages including transient creep and steady creep. However, the creep accelerates after applying the last level of stress. Thus the creep behavior aggravates until the specimen finally shows the characteristics of large axial compression, obvious volumetric expansion and large steady creep rate. In addition, the relationship between creep rate and the deviatoric stress can be described by an exponential function. The long-term strength of clastic rock, which is 54% to 80% of conventional strength, is basically the same as the intersection of the steady-state creep rate and the stress threshold of volumetric dilation. Therefore, the experimental results in this study provide dependable reference for the establishment of rock creep model and analysis of long-term stability.

Key words: rock mechanics, clastic rock, triaxial creep test, creep rate, long-term strength, microscopic failure mechanism