The #3 and #4 diversion tunnels of Wudongde Hydropower Station pass through a long surrounding rock of adverse geological condition. The rock formation of the adverse geological tunnel sections is thin-layer marbleized dolomite with steep dip angle, with existing interlayer cleavages and microfissures. The tunnel orientation and the strata layer's strike intersect at small angle. During the excavation of the upper layer sections, local collapses and fall-blocks were frequently observed, most of which are in the left crown. This brought serious instability concerns. A numerical model was established using finite element method to analyze the mechanism and characteristics of such instability. Thin layer and steep dip angle rock formations with lower dip angle joints and fissures generally has lower strength; therefore, affected by excavation, unstable failures occur at the left crown. A series of actions have been taken to treat the collapse areas, including shotcrete-rock bolt support, steel arch, concrete backfill, and consolidation grouting without concrete covering. The treatment results are satisfactory per construction requirements.

1 Introduction
1.1 Project overview

Wudongde Hydropower Station is located in lower reaches of Jinsha River, where Sichuan Province marches with Yunnan Province. As a large-sized hydropower station, it has a 406,100 km2 watershed area, a 7.408 billion m3 total storage capacity, and a 10,200MW total installed capacity. The under construction hydropower station will be completed by 2020, and will be the fourth-largest hydropower station in China.

There are five diversion tunnels in both left and right banks. The #3, #4, and #5 diversion tunnels are in the right bank. The spacing distance between the tunnel axes is 38 to 50 m, the thickness of the middle wall between the tunnels is 28.1 to 30.1 m. The maximum embedment depth of the tunnels is 560 m. #3 and #4 division tunnels are a bit lower than #5 tunnel. The elevation of #3 and #4 tunnels' entrance is 812.0 m, and the exit is 800.0m high. The length of #3 division tunnel is 1472.4m (stake number is 0–000 to 1–472.4) and 1613.6m for the #4 division tunnel (stake number is 0–000 to 1–613.6). #3 and 4 # division tunnel both have a horseshoe-shaped tunnel portal, and the lined tunnel dimension is 16.5m×24.0m (width×height).

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