Soft Rocks in Underground Hydroelectric Schemes

The purpose of this chapter is to analyze the behavior of underground works associated to hydroelectric schemes when soft rocks formations are present. The work starts with a brief introduction to the different types of underground hydroelectric schemes with a reference to the specific works related to the hydraulic circuit, to the powerhouse complex and to surge chambers. Considerations about risk analysis and risk management are presented taking into consideration soft rock masses. Examples of application of underground hydroelectric schemes are illustrated. The first case is related to the Socorridos hydroelectric scheme in Madeira Island, Portugal, in volcanic formations. Emphasis is made to the existence of weak formations and to the development of a new empirical system for volcanic rock formations. The Mingtan underground pumped station in Taiwan presented complex geological formations. Reference is made for the LFJ in situ tests performed for deformability evaluation. Another case is related to a wedge failure occurred at Cahora Bassa, Mozambique due to the existence of a low strength surface. The last case is related to the mega-project Jinping II, in China. Deep tunnels intersected by chlorite schists are studied in detail. A conclusion is drawn.