Numerical Analysis On Surrounding Rock Stability During Construction Of Large Underground Cavern Transition Section

As the stability problem of surrounding rock during the excavating process of the large and complex underground cavern is connected with the construction cost and management benefit of the whole power station project, it is always a concern for the academic and engineer circles. Transition section is at the entry (or exit) of underground caverns, with large excavation section and thin overburden. Design and construction of transition section is complicated because it's affected by many factors and no codes to follow. The safety of the transition section is very important in the tunnel construction process.Therefore,ensuring stability and safety of transition section in the excavating process is a significant subject.In this paper, a systematical study is made on the example of 2# diversion tunnel entry transition section of Nuozhadu Hydropower Station. First, the 3D model of the project field is built according to the geological condition and design scheme. Second, numerical simulation analysis is made on the whole construction process of the transion section. According to the results, the stability of surrounding rock is evaluated. Third, a study is made by sensitivity analysis on the influence that field geotechnical parameter variation of surrounding rock in different locations has caused to the stability of the surrounding rock. At last, an evaluation is made on the advantages and disadvantages of flattening excavation technique. A preliminary discussion is carried on the application of flattening excavation technique to different classes of surrounding rock, whereas the economic and social benefits of the technique.