Spillway slope is building from the mountain excavation. Excavation is a rock mass unloading process, that is to say, rock mass excavation is a mechanical condition of unloading. The mechanical characteristics of rock mass is essentially different between the unloading rock mass and loading rock mass (different stress-strain paths, different mechanical parameters, different yield criteria, different analytical methods). Based on actual excavation condition of Shuibuya spillway slope: rock stress field was changed, rock quality was cracked and damaged, and rock excavation unloading mechanics theory was used to analyze the slope stability. Stability analysis includes natural condition, the rock was excavated by design, rock unloading condition. Without regard to unloading, traditional theory was used to analyze the rock slope excavation, and the results showed that the displacement which considering excavation unloading is more obvious than the displacement without considering unloading. This indicate that the excavation unloading rock mass should be analyzed by the unloading mechanics theory.
Shuibuya multi-purpose project is the first stage project in the three cascade development scheme on the main stream of the middle and lower research of Qing River. It is a large-scale water conservancy project which has huge comprehensive benefits including power generation, flood control, shipping, tourism, etc. This project located in Badong County Shuibuya Township of Hubei Province, the distance from the dam site to Enshi Tujia and Miao autonomous prefecture is 117 km, the distance from the dam site to Geheyan multi-purpose project is 92 km, and the distance from the dam site to the converging point (Yidu City) where Qing River flows into Yangtze River is 153 km. This dam is 233m high, and it was the highest face rockfill dam in the world. The total installed capacity of this power station is 1860 Mw, and the annual energy production is 4.1 billion Kwh. This project was officially announced to commence in 2000, and has closured the river in October 2002. The first power generation was planted in 2007, and this project will complete in 2009. Shuibuya multi-purpose project is made up of dam, power station constructions, spillway, emptying tunnel, etc. Bank-run spillway located on the left bank, and it is made up of diversion canal, control segment, discharge trough, energy dissipater, scour protection project, etc. The diversion canal has a compound trapezoidal cross-section, and the bottom width is 101m, the length of centerline is 896m, the elevation of channel bottom is 350m.
Engineering rock mass, especially the excavation unloading rock mass, the history of loading condition presents an unloading process and the physical and mechanical parameters change with the unloading process. During the process of long-term geological tectonism, structural plane which is different in size and direction was formed. That is to say, current rock has suffered from variety of action, and the stress in the rock mass is residual stress now.