The main tailrace tunnel of the Xiangjiaba right bank underground power station has the biggest designed cross-section dimension of its kind in the world. Due to this fact, its construction is complex. The principle of "top arch" lining first, then "excavation down" was adopted based on the location conditions during construction and thus addressed the issue of super-big cross-section tunneling works. The safety monitoring data of the project indicate that the stress strain develops in line with the project development law and demonstrate that the parameters designed for the excavation and support are reasonable. Construction safety meets the design requirements and the tunnel remains safe and stable. Therefore, the method for this tailrace tunnel can be used as a reference for other similar projects.

1 Summary of the Project

The Xiangjiaba Hydropower Station is developed at the last cascade of the Jinsha River. Its main function is power generation, combined with the auxiliary functions of improving the ship channel, flood control, irrigation, trapping sediment, etc. The total installed capacity of the power station is 6400MW, which includes four units that are each capable of 800MW on the right bank underground power station.

1.1 Design of the project

The tailrace system of the right bank underground power station is composed of a diffusion section, a tailrace adit and the main tailrace tunnel (the top elevation varies). Every two tailrace adits converge into one main tailrace tunnel and the two main tailrace tunnels are deployed. Please refer to Figures 1 and 2 for the profile and plan arrangement of the tailrace system.

The two main tailrace tunnels are quite large. They are 263.78m and 199.78m long and have a longitudinal grade of 3% and 4%, respectively. The tunnels are D-shaped, their excavation height is between 35.10m and 38.20m and their excavation width is between 22.40m and 24.40 m. A conventional support with rockbolts, anchors and lining concrete was adopted. In addition, certain safety supports were utilized during the construction period. During the later stages, the tunnels were lined with C25 concrete 1.0 m, 1.5m and 2.0m thick, respectively based on the type of crosssection. The lined tunnel has a net width of 20.0m and a net height of 34.0 m. Additionally, backfill grouting for the crown at 120° and full-section consolidation were carried out.

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