Excavation-induced relaxation effects and hydraulic conductivity variations in the surrounding rocks of a large-scale underground powerhouse cavern system

Located in the middle reach of Yalong River in China, the Jinping-I Hydropower Station consists of a large-scale cavern system for water conveyance and power generation. Compared to other typical large-scale underground powerhouse cavern systems in Southwestern China, the construction site is characteristic of higher in situ geostresses, lower uniaxial compressive strength (UCS) and poorer quality of the surrounding rocks, resulting in greater depth of the disturbed zone. In this study, the excavation-induced relaxation effects and their impacts on the hydraulic conductivity variations and seepage behaviors in the surrounding rocks of the Jinping-I underground powerhouse caverns were assessed with site characterization data and numerical simulations. The excavation-induced disturbance zones around the caverns were modeled using the plastic yield zone predicted with an equivalent elasto-plastic model and a constant deviatoric stress criterion based on the Hoek-Brown parameters of the surrounding rocks, respectively. The predicted results agree rather well with the disturbed zones detected by the in situ acoustic wave velocity measurements and borehole TV images. The excavation-induced permeability changes in the surrounding rocks were characterized with a strain-dependent hydraulic conductivity model that accounts for the development patterns and deformation behaviors of the critically-oriented fractures. The seepage behaviors with consideration of the permeability changes in the surrounding rocks were modeled with a variational inequality method at a steady state, and the numerical results imply the significance of proper characterizations of the excavation-induced disturbance effects and permeability changes in better understanding the groundwater flow and its controlled effect in the surrounding rocks.