Based on the simplified structure of rock mass, FLAC3D numerical modeling technique is used to systematically analyze the distribution features of the secondary stress field, strain field and plastic zones in the surrounding rock mass of the underground cavities after the excavation. And the variation characteristics of stress field, strain field and plastic failure zones in the surrounding rock mass of the underground cavities are summarized. The results show that the underground workshop building caverns are whole stable, but parts of them are unstable and the unstable parts are shallow. The results provide basic information and reference for the evaluation of underground cavities stability and project construction.

1 Preface

Yantan hydropower station lies in Hongshuihe in Yantan town, Dahua County, Guangxi. Four generating units of the first phase project of hydropower station had been built in 1995, the reservoir's normal water level is 223m, the maximum height of the concrete gravity dam is 110m, the powerhouse at the dam toe in right bank, and the installed capacity is 1210MW. The underground powerhouse of second phase extension project locates in the right mountain of the first phase powerhouse and dam project, including diversion tunnel, main powerhouse tunnel, main transformer cavern tunnel and tailrace tunnel etc., installed capacity is 2×300MW. The type of underground powerhouse is one unit with one tailrace tunnel in pressure, the maximum excavation width of over crane beam of main powerhouse is 30.8m and the length is 129m, the highest height difference of the powerhouse is 76.67m[1]. The water diversion and power generation system of extension project is placed in the mountain in the right bank, the thickness of overlying rocks is 75m~120m in the underground powerhouse area, and the surrounding rock is hard diabase. The diversion tunnel and underground powerhouse cavern group located in F48 fault footwall, the rock mass is relatively integrated and belongs to II class surrounding rock. The stress-deformation field of cavern surrounding rock will be changed by excavation of underground cavern group. On the basic of the excavated geological model, to simulate excavation of underground powerhouse cavern group in whole process by FLAC3D and analyze the change of stress field and deformation field. Many users at home and abroad have used the software[2~6], the results show that it has enough reliability and rationality. Particularly, its function is more powerful in deformation calculation and rock and soil mass stability analysis than finite element method.

2 Geological Conditions
2.1 Topographical condition

The direction of valley is N160W. The altitude of low water level is 148m~150m, width of river level is 100m and water depth is 18m~20m. The lowest altitude of riverbed is 129m, both sides of riverbed are stone floodplains, the altitude floodplain face is 155m~168m. The two sides of riverbed is flood plain, flood plain altitude is 155m~168m. The first terrace over flood plain whose altitude is 185m ~200m. Landform of two banks is middle-low mountain whose altitude of crest is 500m, and its hillside gradient is 27°~36°.

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