Tunnelling research

Tunneling technology has been a key component in development of the vast Norwegian hydropower system. Today, more than 95% of all electricity in Norway is produced from hydropower. Norway is famous for deep fjords and mountains, and development of this huge hydropower capacity would not have been possible without tunneling. There exist 4000 km of hydropower tunnels in Norway, an impressive number, which equals the distance from London and Baghdad. In most of the tunnels only 2-4% of the total tunnel length is concrete lined, while the rest is unlined with only individually placed rock bolts as protection (Broch, 2013).

The development of hydropower in Norway peaked in the 1980’s and has declined since. An important research question in Norway now is the expected lifetime of hydropower tunnels. Do they last forever, or do they have an expiration date? More frequent load changes in the hydropower plant’s affect the tunnels in different manners, ending up with a different strain than what they are designed for. Several hydropower plants are also due for refurbishment, and several companies use this as an opportunity to increase the installed capacity of the power plants. This is most often done without an expansion or upgrading of the tunnel system, which results in the need to research the effects and consequences for the tunnel system.

HydroCen

In 2015, NTNU established the Norwegian Center for Hydropower (NVKS) as an umbrella covering the hydropower education and research at NTNU. NVKS became converted to a research center, HydroCen, the following year. HydroCen is a hub for research on renewable energy with a current budget of €50 million in an eight year period. The vision of the research center is to double the value creation in the Norwegian hydropower system within 2050. An ambitious goal, considering the already very high turnover in this industry, and the incoming competition from wind and solar power. Much of the value creation is however expected to be based on system services to allow a higher ratio of exactly wind and solar power to penetrate the energy marked.

HydroCen is a cooperation between the Norwegian University of Science and Technology (NTNU), the research organisations SINTEF and NINA, and the hydropower industry in Norway. The industry is represented by the largest hydropower companies, consultants, and manufacturers of hydropower equipment. The work is carried out by the reserachers at NTNU and SINTEF with involment of the industry through case-studies of existing and planned hydropower projects. HydroCen is a successor of the similar and successful research center CEDREN.

Tunneling research in HydroCen

HydroCen has four work packages; (1) hydropower structures, (2) electromechnical equipment, (3) market and strategy, and (4) environmental design. Research in tunnel technology occure only in workpackage 1 and is fully run by NTNU. The main activity is carried out by several PhD projects. HydroCen has a strong focus on tunneling research, which is conducted as a part of the work package on hydropower structures. Researchers from the engineering geology group and the hydraulic engineering groups at NTNU are cooperating together with hydropower owners to conduct widespread mapping and data collection from existing hydropower tunnels. In the period since the start of the research center over 50km of hydropower tunnel in five different power plants has been inspected after dewatering, and several field measurement campaigns have been conducted in existing hydropower plants.

The tunneling research composes the following main topics;

• Lifetime of rock protection in unlined tunnels
• Swelling materials and related collapse of tunnels
• Methods for placing of concrecte plugs
• Impact on pore pressure from peaking operation of hydropower plants
• Design of pressure tunnels and surge tanks for converting HPPs to PSPs
• Invert cleaning and design of sandtraps for unlined tunnels
• Some examples of the work carried out are shown below:
• Swelling materials and tunnel collapses

Shortly after the startup of the research in HydroCen, an example of the importance of the rearch was highligthed when the newly commisioned Matre-Haugsdal power plant (180MW, 525m) in the western part of Norway experienced a tunnel collaps. The tunnel was inspected by consultants and the researchers in HydroCen and the reason was found to be swelling material in a crossing weaknes zone. The weakness zone considered and supported with shotcrete and rock bolts in the construction phase, but not sufficiently. The collapse resulted in several months outage to construct a bypass tunnel and close of the collapse, and the need to clean the turbine seals and valves. The tunnel collapse is now used for studies of swelling materials in HydroCen.

Invert cleaning and sand traps in unlined hydropower tunnels

Unlined tunneling requires handeling of the remaining rock material on the tunnel invert after construction. For long tunnels, complete cleaning of the invert is very time-consuming and will often delay the commisioning of the power plant. This problem is in Norway most commonly solved with the use of sand traps, which is the most time and construction cost effective solution, as this allows the materials to be left in the tunnel without removal, and instead empty the sandtrap when necessary. However, for upgrading of existing hydropower plants, the sand traps have been found to be a limitation. Research in HydroCen is now conducted to find cost-effective solutions to upgrade existing sandtraps to utilize the full potential for upgrading of existing hydropower plants.

In parallell, research at HydroCen is conducted to reconsider different methods of tunnel invert cleaning for unlined tunnels for construction of new hydropower plants. Asphalt lining is one such example, and at least three hydropower plants in Norway has been constructed with this solution. Roskrepp power plant (50 MW, H0 = 83 m) in the south of Norway is used as a case study to further investigate the use of asphalt lining of hydropower tunnels. This power plant was commisioned in 1979 and the tunnel invert was lined with concrete asphalt for three reasons; conserve the sand and rock material after excavation to avoid time-consuming cleaning and flushing, reduce headloss, and preserve a driveway for later inspections and repair work. As a part of the research in HydroCen, the complete tunnel system at Roskrepp has been 3D scanned and the result can be seen in Figure 1 and Figure 2.

Upgrading of existing hydropower plants to peaking and pumped storage plants

About 75% of the hydropower potential in Norway is developed (The Norwegian Government, 2015) and much of the remaining is protected, so the main focus for further development is found in upgrading of existing hydropower schemes. The tunnel systems are seen to be the main limitation, as headloss, sand transport and hydraulic transients increases with increasing flow. Upgrading of the tunnel system itself is normally too expensive to be feasible. This results in the need for research to determine the limits for safe upgrading, and to develop new methods to improve critical components such as the sand traps and the surge tanks to tacle increased flow. Upgrading of existing sand traps and surge tanks is very different from construction of new ones, and there is a large potential for development of innovative solution that will minimize construction time and outage, limiting the construction costs, and maximizing the effect. The Ulla-Førre hydropower scheme (2100 MW) in the western part of Norway, and the Sira-Kvina hydropower scheme (1760 MW) in the south of Norway are used as case-studies for this research. Fig. 3 presents the Ulla-Førre hydropower scheme with key information about the power plants and the tunnel system.

Lifetime of unlined hydropower tunnels

The expected lifetime of an unlined hydropower tunnel is one of the questions the reserachers at HydroCen want to investigate. To adress this question, six pore pressure sensors have been installed inside the rockmass adjecent to the headrace tunnel in Roskrepp power plant. These sensors are placed at different depths into the rock and will record the power pressure as a function of the power plant operation. The theory is that the pore pressure is a destabilizing force that may cause failure of the rock mass, and that the lifetime is dependent on the frequency and amplitude of starts and stops of the power plants. The pressure sensors will measure the effect on the pore pressure during one year of power plant operation.

Tunnel roughness

In paralell with the HydroCen there are associated research projects, mainly started before the establishement of the center. The TunnelRoughness-project aim to develop better methods for estimation of headloss from unlined hydropower tunnels, based on recent scanning technology. Several hydropower tunnels have been scanned and the data are converted to scaled roughness and reproduced in scale models for laboratory testing at NTNU. With available information from laser scanning during the tunnel construction, estimates of the head loss will be produced through algorithms and measures may be taken.

The authors are: Leif Lia, Prof. NTNU, leif.lia@ntnu.no; and Kaspar Vereide, Adj. Ass. Prof. NTNU, kaspar.vereide@ntnu.no

References

1. Broch E. (2013). Underground hydropower projects . In: Norwegian Hydropower Tunneling II. Norwegian Tunneling Society.
2. The Norwegian Government (2015). Facts – Energy and water resources in Norway. Norwegian Ministry of Petroleum and Energy.
3. Solvik Ø., Tesaker E. (1997). Floor paving in unlined hydropower tunnels. In: Hydropower’97. Balkema.