The JP-II hydropower tunnels project is the deepest underground engineering in china, each tunnel approximately 16.7 km in length, 13m in diameter and maximum 2525m in overburden. The hollow bolts make the grouting process much more reliable, but rebar's elongation rate only 6% which may cause some risk in deep tunnel construction. In this paper we discuss how to use rock mechanic and construction experience from transportation tunnels of JP-II project to assess the applicability of hollow core bolts in deep hydropower tunnels. Firstly, in-situ stress along the tunnels is discussed based on measurement, site brittle failure position around the tunnel and core discing position at deep site. Then the constitutive of deep brittle rockmass and related property determine method is carried out to discuss. The boundary condition and rockmass property of numerical model are calibrated using site geophysical data in transportation tunnels. At last the elongation of hollow core botls after installed are predicted base on numerical model in different condition, i.e. excavation method, and these result used to support the decision making of hollow core bolts' application.

1.1 JP-II project and hydropower tunnels

Jinping Second Stage Hydropower Station (JP-II) is located at Yalongjing River in Liangshan Yi Autonomous Region, Sichuan province. The sluice gate is situated at Maomaotan in the west of Jinping Mountain and the plant at Dahuigou in the east. The four hydropower tunnel, each of approximately 16.7 km in length running parallel to each other and crossing the Jinping Mountain, an alpine karst zone, will connect the sluice gate and the plant units. The four hydropower tunnels are designated 1 to 4, respectively. Tunnels 1 and 3 will be made by Drill & Blast method (D&B) and tunnels 2 and 4 will be mainly constructed by TBM and partially by Drill and Blast method. The average overburden is 1500 ~ 2000 m, the maximum overburden is 2525m. The TBM bore diameter has been defined as 12.4m, while the D&B diameter is 13m.

The two transportation tunnels lay parallel to hydropower tunnels which construction works started in November 2003 and breakthrough on 8 August 2008. Figure 1 show the layout of the tunnels, sluice gate, shaft and plant. The transportation tunnels will not only serve as traffic and transport access to both JP-I and JP-II 2 hydropower projects, but also be the exploratory adit, test adit and construction adit to the hydropower tunnel of JP-II. The construction works of hydropower tunnel 2 and 4 start in December 2006, and TBM tunnel, hydropower tunnel 1 and 3, start in October 2008.

The tunnels are characterized by long alignment, high overburden, high ground water pressure and complex geological conditions. Therefore, proper and effective methods should be taken for solving the key technical problems such as high rock stresses leading to rock burst and squeezing, possible adverse ground conditions in large fault zones. It has been emphasized during the design and in the contract that proper excavation methods and treatment are essential factors for safe.

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