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Going Below

As cities become overcrowded and traffic more congested, public authorities and developers worldwide are turning to underground construction as a means of providing more environment-friendly infrastructure in urban areas. Today, churches, sports facilities, production halls and offices are being built below ground, and underground and subsea road tunnels are becoming increasingly popular around the globe. A rise in the construction of long-distance rail tunnels is also anticipated in the near future.


Those in the know seek out Norwegian rock engineering expertise. On the growing international market, Norwegian companies and R&D institutes are among the leaders in drilling, blasting and boring technology and geotechnical investigations. Dozens of Norwegian companies are involved in building below-ground tunnels and facilities worldwide, taking with them expertise based on decades of domestic underground construction. The Norwegian Tunnelling Society (NFF) represents the Norwegian rock engineering industry, acting as a liaison for its member companies' services, and providing information on Norwegian tunnelling expertise.


Exceptionally Flexible

The "Norwegian Tunnelling Method" (NMT) is a system based on expertise, cooperation, flexibility and risk sharing. The NMT includes specific techniques such as the addition of steel-fibre reinforcement to wet-process shotcrete, which has completely redefined Norwegian tunnelling technology. Instead of the traditional method of building the same support structure along the entire length of the tunnel, the Norwegian shotcreting method employs operator-guided robots to apply steel-fibre reinforced shotcrete adapted to the specific needs of each excavated area. Today, with the use of additives, it is possible to apply one layer of shotcrete that is comparable in thickness and strength to conventional in-situ concrete, and which offers exceptional flexibility and savings of up to two-thirds of the normal cost of a cast concrete arch.


The Q-system for rock mass classification is a Norwegian innovation which helps engineers to determine the types and quantities of rock support techniques appropriate for any given type of project and existing geological conditions. The system was developed by the Norwegian Geotechnical Institute in 1974 and is based on data from more than 1,000 road tunnels and caverns constructed during the past two and a half decades. It is now one of the most widely-used rock classification system worldwide. Together, the Q-system and steel-fibre reinforced shotcrete have been successfully applied to all kinds of tunnel sections, even those bored in rock of extremely poor quality, such as sections of the 2.2-km-long Oslo Tunnel built beneath the city of Oslo in the early 1990s.


Environment-friendly Facilities

Since the 1950s, man-made caverns have been used increasingly for shelter and storage. They were first used for hydropower plants and military facilities, where they provided the advantages of increased safety and reduced energy and maintenance costs. Such caverns are ideal structures as the high cost of underground construction is more than offset by the savings. The Olympic Hall at Gjøvik, Norway, built for the 1994 Winter Olympic Games, is an example of how such a facility can save energy. The world's biggest cavern for civilian use, this eight-storey sports and concert arena lies beneath the city, with dimensions of 61 x 91 x 25 m and a capacity of 6,000 spectators. The cavern's record-breaking 61-m crown span was secured by Norwegian steel-fibre reinforced shotcrete and anchor bolts. The entire hall and entrance tunnel were excavated in nine months by two drilling rigs and one wet-process shotcreting rig. In addition to conferences, exhibitions, concerts and team handball matches, the Gjøvik Olympic Hall is used year round for ice skating and for hockey practice and matches. Normally, maintaining an ice rink in the summertime is prohibitively expensive, but ice production in the Gjøvik Olympic Hall actually reduces the total energy bill because the hall retains all the heat created by the refrigeration equipment. On a yearly basis it uses an estimated 50 per cent less energy than normally required by a comparable above-ground facility.


Subsea Specialists

Subsea road tunnels provide an excellent way of reducing travel time around Norway's many fjords and ensuring safe travel during snowy, icy winter months. They also offer significant savings on road maintenance costs. Norwegian contractors have carried out a number record-breaking projects, among them the world's deepest subsea road tunnel located at Hitra, off Norway's western coast. This 5.6-km-long tunnel lies 264 m below sea level, and has a 10 per cent gradient at both ends. The country is also home to the world's longest subsea road tunnel - the Freifjord Tunnel (5.1 km). It had broken three records by the time it was completed in the early 1990s. It was constructed at the lowest cost (approximately USD 4,933/m), with the fastest construction time: a total of 310,000 m3 of rock mass was excavated in the course of only 14 months, with an average excavation rate of 100 m/week. Another large project involving subsea tunnelling technology is the Oslo Fjord Crossing, due to be completed in 2000. Finished in November 1999, the Crossing's 7.2-km-long subsea road tunnel lies near the Oslo Fault, one of the areas with the most seismic activity in Norway, so freezing technology had to be employed to permit excavation in one fault zone.


Leading Oil & Gas Ashore

With the advent of the offshore petroleum era, Norwegian engineering and tunnelling experts faced the challenge of safely transporting oil and gas to the mainland. The deep trench along the Norwegian shoreline had to be crossed to gain access to the shallow seabed of the North Sea. First-generation shore approach consisted of subsea bridges constructed from prefabricated concrete elements. These bridges were replaced by subsea tunnels, which were refined for the Troll project. In order to bring gas from the Troll field to an onshore processing plant in Norway, two parallel tunnels situated 250 m below sea level were constructed, with shafts 7 m wide and 3.6 km long. The tunnels have a 70-year lifespan thanks to permanent rock support consisting of 13-cm-thick steel-fibre reinforced shotcrete, rock bolts and steel straps. Subsea tunnels constructed for oil pipelines include the Kårstø offshore tunnels.


These 12-km-long tunnels feature a 24-m2 cross section constructed and secured by shotcrete which serves as both temporary support and permanent lining.


Constructive Construction

Norwegian companies have devised a special system to tailor bids to suit customer needs and expectations, and to account for all contingencies. Developed over the last 20 years, the Norwegian Tunnelling Contract System aims to minimize costs and avoid disputes by stating specifications and unit prices for the entire range of potential modifications necessary to adapt to actual rock conditions. Such flexibility allows both contractor and client to make sound economic decisions on the spot - whether construction is taking place kilometres into a mountainside or many metres below the ocean floor. Based on the contract, the parties can work constructively as a team during all phases of construction.


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