Study on Instantaneous and long-term Mechanical Properties of Phyllite under Loading and Unloading Conditions at Wudongde Hydropower Station

As a giant hydropower hub on the Jinsha River in China, the underground powerhouse and diversion tunnel of the Wudongde Hydropower Station (WHS) pass through phyllite strata. During tunnel excavation, the unloading stress path has a significant impact on the changes in the mechanical properties of the surrounding rock, while the deformation and failure of the rock mass accumulate continuously, making it urgent to conduct an in-depth study on the time-dependent characteristics of mechanical parameters. In view of this, phyllite samples from the engineering site were selected to carry out a comparative study on the instantaneous and long-term mechanical properties under loading-unloading conditions. To simulate the instantaneous effect of unloading during tunnel excavation, triaxial loading-unloading tests on phyllite were performed using the MTS815.03 test system, with synchronous acoustic emission (AE) monitoring. The test results revealed the evolution law of rock strength parameters under different unloading paths and the differences in the micromechanical mechanisms of unloading-induced failure. To simulate the evolution of mechanical behavior during the long-term operation of the tunnel, triaxial creep tests under loading-unloading stress paths were conducted using a TLW-2000 rock triaxial creep testing system. A creep constitutive model was established to describe the evolution relationship between the deformation and failure of the surrounding rock and time, and the degradation characteristics of its long-term strength parameters were clarified. The study indicates that for phyllite tunnels under long-term operation, the long-term degradation effect of mechanical parameters should be taken into account, and it is more reasonable to adopt the obtained long-term triaxial rheological strength parameters to assess the long-term operational stability of the tunnel. The research results provide a technical basis for the stability analysis and excavation support design of the tunnel.