Unloading deformation during layered excavation for the underground powerhouse of Jinping I Hydropower Station, southwest China

The paper discusses the strong asymmetric deformation and failure of the rock surfaces which have taken place in the 40–75 MPa marble during the excavation for the underground powerhouse and transformer chamber of the Jinping I Hydropower Station, southwest China. The deformation and damage of the rock mass closely relate to the magnitude of the geo-stresses, boundary conditions of the underground caverns, rock properties and the method and sequence of excavation. Ground stress test results show that the maximum principal stress reaches 35.7 MPa in the slope, and the average σ₁ intersects with the axis of underground powerhouse at an angle of 28°. Strong displacements had taken place at the sidewalls of the caverns and significant instantaneous uplift deformation of the floor occurred, particularly when the first layer was excavated.