Apart from the design methodology required for excavation and support, common for all types of tunnels, the specific methodology for design of water pressure tunnels is presented. From definition of the objectives of the design, the necessary data to obtain from investigations are listed. Prevention of hydrojacking, management of seepage and knowledge of the specific characteristics of the rock masses where the tunnel is to be excavated will dictate the layout, as well as support/lining or treatment to be applied for the different sections of the works. Emphasis is put on proper application of the different methods, empirical, analytical and numerical, for achievement of a safe design, with the help of some case stories.
Hydropower projects, with always higher heads within comparatively less favourable geological environment than in previously achieved plants, result in more and more challenging conditions for design of water pressure tunnels. Aspecific methodology is presented here for such tunnels, providing as well some important guidelines for a safe design.
It is important to quote here that, since design approaches for "classic" tunnelling has been already largely discussed, problems linked to excavation of water tunnels will not be treated in the present paper, which is to focus on design issues specific to pressure tunnels. Hence, and except if necessary, we will not refer to the design part related to tunnel excavation, even if the often acute excavation problems have also a great influence on the final design.
The specific design objectives assigned to the engineer for pressure tunnels (apart from finding the best layout to limit construction problems, as already emphasised), can be summarised in determining the best alternative, technically but also economically, for the layout and characteristics (arrangement, lining, etc.) to conveywater under pressure (of up to several hundreds meters or more) to the power house. It shall be able to withstand the different loads cases to occur during construction and operation, including filling and unfilling, during the whole life of the scheme. This shall also be achieved in compliance with requirements over leakage amount, since less water reaching the power house means less energy produced.
Other requirements, such as limitation of the headlosses in the circuit (and, of course, excavation conditions) will also influence the design.