New design concepts, including the application of high pressure epoxy grouting techniques has been successfully employed for the construction of concrete plugs for water pressures up to 965 metres at Norwegian hydropower projects. In addition to hydropower plants, the developed technology may also be adapted to other projects with extreme demands for effective fluid or gas containment such as rock storages for natural gas (LNG) and radioactive waste repesitories.
In Norway more than 80 unlined pressure shafts and pressure tunnels are presently in operation, with water pressures ranging from 150 m up to 965 m.
Critical construction elements at these plants are the plugs linking the unlined conduit to the steel penstock, and access port plugs for inspection of the tunnel system.
Traditionally, such plugs were often constructed with a plug length equal to 3–5% of the water pressure, and with cement grouting for sealing. Leaks primarily occured at the concrete-rock interface, and often it was necessary to perform several rounds of post grouting to bring the leakages down to an acceptable level.
During the last 10 years, the Norwegian hydropower industry has pushed the limit in water head on unlined rock from 500 m up to the present world record of 965 m static head in an unlined pressure shaft.
This has called for new design and construction concepts for concrete plugs to ensure safe operation and watertightness at these extreme pressures.
It involves systematic high-pressure leakage testing and pregrouting of the rock in the plug area, use of specially developed grouting hoses for high-pressure epoxy contact grouting of the plug, and high-pressure post grouting through drillholes for verification of plug integrity.
Detailed planning and systematic quality control routines through all phases of the concreting and grouting operations are vital for the successful completion of a high-pressure plug.
The author has recently been responsible for design and construction control for this new type of high-pressure plugs at Eikelandsosen power plant (450 m head) and Nyset-Steggje power project (965 m head on unlined rock), and is presently designing a plug for 740 m head at Mel power plant in Western Norway.
Some main experiences from these projects are summarized below.
The two main types of concrete plugs used in hydropower projects are illustrated in Fig. 1.
The penstock plug is located at the upstream end of the steel penstock, at the transition to the unlined headrace or pressure tunnel. 261 CONCRETEPLUG WATER STEEL PRESSURE PENSTOCK 1000 M PENSTOCK PLUG ~~~.// Access to the unlined tunnel system is usually provided by an access gate plug located in the construction tunnel adjacent to the pressure tunnel.
PLUG LENGTHS illustrates the relation between plug length and water pressure at a number of Norwegian hydropower projects. As can be seen, plug lengths vary from 2,5% up to 11% of the water pressure, with recent high-pressure design in the range of 3–5%.
The design of a high-pressure plug involves both structural, hydraulic and rock mechanic considerations, combined with practical experience concerning concreting and grouting technology.