In June 1992 the Norwegian Parliament decided to construct the world’s longest road tunnel. The 24.5km-long stretch of tunnel stretches between Aurland and Laerdal on the new main highway connecting Oslo and Bergen.
The construction was approved to traverse a section of country with relatively poor levels of reliability in road transport due to the mountainous area and narrow roads combined with many fjord crossings.
The Laerdal tunnel is an important part of the extension of a ferry-free, reliable road link between the two largest cities in Norway. The decision to build a tunnel rather than refurbish existing roads was taken to avoid difficult terrain with high risks of rock falls. From an environmental perspective, the tunnel was seen as a justifiable investment to avoid destroying sections of the unspoiled natural landscape.
Operations during the construction of Laerdal tunnel was divided into four main phases: drilling, blasting, loading and transport and excavating and landscaping.
Drilling the tunnel was carried out using computer-controlled drilling jumbos as well as traditional drilling and blasting. To make sure that the tunnel sections met more than 10km into the rock and 1000 metres under the mountain, it was important that the drilling and blasting work were carried out with great precision.
Navigation satellites were used to determine fixed survey points on which other measurements inside the tunnel were based. Inside the tunnel, bearings were indicated using laser beams. A computer on the drilling jumbo captured the laser beams and positioned the drilling equipment automatically, according to a set pattern. Each drilling jumbo contained three automatic hydraulic drills.
Blasting was conducted using approximately 100 holes, 45 – 51mm in diameter and 5.2 metres deep, drilled for each blast. A detonator inside a small stick of Anolit dynamite was places in the bottom of each hole.
Wheel-mounted loaders were used for loading and transport in the tunnel. The excavated materials were transported out of the tunnel using dump trucks. Permanent roads were built in the tunnel, in parallel with the tunnelling work, so that transport vehicles could run on a good, paved road base during the construction period. This improved efficiency and minimised pollution.
The disposal of 2.5 million cubic metres of excavated rock from the tunnel was one of the greatest challenges in planning the tunnel. To avoid major conflicts because of the cultural importance of the landscape and the productive agricultural land in the main valley, the Norwegian Public Roads Administration decided to build more than half of the tunnel from a 2.1km long access tunnel in Tynjadal. Tynjadal is a side valley, opening out about 8km east of the town of Laerdal. Here, the excavated materials were deposited with no visible effect on the main valley, and without any risk of hazardous run-off into the Laerdal watercourses.
The Norwegian Public Roads Administration’s (NPRA) challenge was to design the tunnel so that people did not find the 20-minute-long trip monotonous, thereby losing concentration during the long journey.
A working group led by experienced psychologists at SINTEF (the Industrial and Technological Research Association) worked closely with the NPRA to assess what could be done to make the journey through the tunnel a pleasant experience.
Simulators where used to find the best solutions as regards lighting levels and design and gentle curves and short straight sections make driving through the Laerdal tunnel less monotonous, without breaching the guidelines for safe viewing distance.
At any given point in the tunnel, the safe viewing distance will be 1000 metres or more.
The tunnel has been subdivided into four sections by means of specially widened areas which are large enough to allow coaches and trains to turn without having to reverse.
The tunnel is longitudinally ventilated through only one ventilation air exhaust shaft, 18km from the Aurland end of the tunnel. About 10km from the Aurland end of the tunnel, a cleaning plant for the tunnel ventilation air is installed in a short side tunnel. This plant cleans the passing air for critical polluting components, thus maintaining an acceptable air quality throughout the tunnel even during periods of heavy traffic and limiting the air volume and jet from power consumption which is required in order to ventilate the long tunnel.
The air quality in the tunnel will be continuously monitored, and the air cleaning system will automatically go into operation when required. The electrostatic precipitator will automatically be quenched for de-dusting at regular intervals, while maintenance of the gas filtration catalyst is expected to be required only at intervals of several years.
A great deal of investment was put into safety measures and equipment in the Laerdal tunnel.
Fire safety is the main priority in the construction of any tunnel. The Laerdal tunnel goes through dry rock with little need for sealing water seepage. In the few places where flammable plastic mats have been required for waterproofing, these methods were put to the test in a coach fire in the Ekeberg tunnel in 1996. The heat generation was around 35.000kW, and the temperature below the tunnel roof rose to more than 1000°C with no visible damage occurring to the tunnel cladding.
Even with little risk of accident and safe fire designs, the Laerdal tunnel has a large quantity of emergency equipment. Emergency telephones and fire extinguishers have been installed at closer intervals than is usual in other tunnels.
Several million Norwegian kroner where spent on equipment for monitoring and to check that the ventilation systems, radio connections, lightning systems, traffic lights, emergency equipment, etc., are in working order.
The Norwegian Public Roads Administration’s monitoring centre in Laerdal is responsible for monitoring and checking all tunnel systems in the region.
If any fault should occur in the ventilation system or if any queues should form, creating excessive volumes of exhaust fumes, the tunnel will automatically be closed to traffic.
If a traffic accident or some other situation should occur the tunnel can be closed with immediate effect from the monitoring centre. All drivers who have entered the tunnel will be given information via radio about whether they should wait or possibly turn round and drive out of the tunnel.
Shortly before the opening of the tunnel a fan caught fire in a bus carrying about 50 people though the tunnel to the ceremony in Laerdal.
The bus filled with smoke, forcing passengers to evacuate while 8 miles inside the mountain. No one was hurt. Passengers reboarded and drove on a few minutes later when smoke dispersed.
Interconsult a member of the Cowi Group was involved as electrification and automation consultant to design all electrical installations in the tunnel.
This includes electrical power systems, control systems (PLC) and emergency phone system. Interconsult where also involved in the electrical construction of the tunnel ventilation system.
Interconsult where instrumental in ensuring traffic safety was a key parameter in the design of the technical installations, including lighting designed to give the road user the impression of driving through several shorter tunnels instead of one long by making transition zones in the tunnel with an imitated sky (blue lighting).