Jan Bergh-Christensen described Norway as a country with a striking geography of high mountains and deep fjords. Although the fourth largest country in Europe, the population is only about 4.5 million or a mere 14 persons per km². The country has 2,650km of coastline much of it around Norway’s surrounding 50,000 islands, many of them inhabited.
The bedrock is Precambrian and in the south and west predominantly gneiss. Sedimentary and volcanic rocks of the Carboniferous and Cretaceous periods predominate near Oslo. “Some people just think of Norway as a large lump of good quality granite. Not so, we have quite a wide range of rocks,” said Christensen.
Norwegians think of themselves as a nation of tunnellers. Christensen elaborated, “We already have a lot of underground infrastructure – 170 hydropower stations with 3,000km of tunnels, 750 railway tunnels, 900 road tunnels and 6 million cubic metres of storage caverns”. Norway’s pressing need for tunnels is two-fold. Firstly the existing surface road network suffers from extreme exposure to frequent snow avalanches and rock falls and secondly because many of the country’s fjords or straits are too wide or too deep for a bridged solution.
The first sub sea tunnel was completed in 1976 to carry petroleum products. Since then a further seven have been completed to bring oil ashore from the North Sea Gas Fields. Since 1976 Norway has also completed eight sub sea tunnels for water supply and sewerage systems. The first sub sea road tunnel opened in 1982 in northern Norway and was the 2.6km long Vardo Tunnel. Since then 22 more have been completed. The main design criteria used for these types of tunnel in Norway, all of which have an impact on costs, are:
Previous sub sea road tunnels have used rock cover as low as 20m but present regulation insists on a minimum 50m cover. This does not mean less site investigation. The exact cover to the tunnel and the quality of the rock must still be determined. These investigations generally start with feasibility studies that include alternative corridor and technical/economical studies followed by conceptual design to determine the layout and cost estimates, culminating in a go/no go for the project. Finally comes detailed design and the production of tender documents.
The site investigation methods range from geological mapping to seismic surveys to diamond core drilling. Acoustic profiling is most common, with some refraction seismics followed by minimal diamond core drilling. Costs per km range from US$240 to US$720 for the acoustics; US$12,800 to US$25,600 for refraction; up to US$48,000 to US$96,000 for core drilling. These investigations are always supplemented by more investigations during construction including geological mapping of the tunnel and exploratory drilling.
Design and safety standards
Morten Knudsmoen started his presentation by stating there were more than 900 road tunnels in Norway with a total length of 840km. Production in the last 10 years had seen 20 to 40km constructed every year.
With the country’s population scattered across wide areas, connecting them together had to be done as cheaply as possible. Tunnel design standards therefore had to produce economically viable tunnels, while making them as safe, if not safer, than the adjoining road network.
The longest and deepest sub-sea tunnels are close to 8km long and up to 260m below sea level. Norway’s 23 sub sea tunnels mostly replace ferry connections. Some 80% of these tunnels are designed for traffic volumes of less than 5,000 vehicles per day. Greater lengths and depths had been developed as experience had been gained in construction. Design standards are issued by the Norwegian Public Roads Administration (NPRA). The latest edition of ‘Tunnel guidelines’ was published in June 2002 and any deviation from these need approval by the Road Directorate.
The EU’s much talked about new Directive on Tunnel Safety sets out minimum requirements for tunnel safety. Existing Norwegian standards are in some cases even stricter than this EU Directive. However the NPRA will start reviewing the consequences of the directive for some 130 of it’s existing tunnels.
Norwegian tunnels are divided into six classes and labelled from ‘A’ to ‘F’ according to the Annual Average Daily Traffic (AADT) volume. Generally for 7,500 vehicles per day, a tunnel width of 8.5m is required. For higher volumes of traffic the width must increase to 9.5m. The maximum gradients in tunnels are between 6% and 8% depending on AADT and the traffic arrangements must be set for expected traffic volumes 20 years ahead.
Safety equipment depends upon the class of tunnel and to some degree on the tunnel location. Most tunnels are now equipped for continuous cellular phone operation. Fire safety is regarded as particularly important, and tunnels longer than 500m are regarded as special objects with a particular Emergency Plan and Risk Analysis prepared according to independent guidelines. The design Fire Load should be 5MW for AADT<10,000 vehicles and 20MW for AADT> 10,000 vehicles
There have been 41 fires in tunnels to date, only three inside sub sea tunnels, with no personal injury reported in a seven year period.
Lining and ventilation
Lining systems are really only there to shed water and debris away from the roadway. The rock is generally self supporting and needs only a few rock bolts and shotcrete for extra stability. In order to prevent ice build up in the tunnels, a free standing lining has been developed consisting of pre-cast units and a 50-60mm thick polyethylene sheet covered by shotcrete for fire protection. The pre-cast units also insulate against frost and ice on the rock side and are designed to take ice pressure.
Sometimes polyethylene sheets are used in conjunction with pre-cast concrete wall panels to produce a lining that is lighter to handle and can accommodate changes in tunnel shape or sharp curves in the road.
Water and frost shielding in tunnels has been a major development over the last 15 to 20 years with advances in:
The more modern types of lining, such as pre-cast concrete panels, can be erected at rates of up to 150m per week. Special rigs then install the PE roof structure to form the finished lining.
Tunnel ventilation is usually longitudinal, using jet fans in the prevailing direction. The designed capacity is based on traffic volumes 10 years ahead. Ventilation speeds are set at less than 10m per second for one-way tunnels and less than 7m per second for two-way tunnels. Ventilation in the case of fires is reduced to 2m – 3.5m per second.
Financing
The majority of the tunnels in Norway cost between US$5,600 and US$9,600 per metre. Financing is generally partly by tolls (40%) and partly by government funding (60%). Prepayments have been financed through raising ferry prices before construction started. The payback period is usually 15 years.
Operation and Maintenance costs vary according to the tunnel length, traffic volume etc. and are typically about 1% of the initial investment costs. Generally the annual O and M reinvestment costs are in the order of US$56-$112 per metre of tunnel.
Knudsmoen concluded by mentioning two planned projects that would extend the limits of sub sea tunnelling even further. The Eiksund Link would connect four islands with the mainland. Construction of the 7.8km long, 287m below sea level tunnel would start in 2003. The second project being considered is the Boknafjord link on the Coastal Highway E39 between Stavanger and Haugesund. This link will include a 24km long sub sea tunnel alternative to the ferries. It is under consideration at present with a possible construction start date of 2006.
Related Files
Combined lining construction sequence
Location map
