How Was The World Longest Tunnel Was Constructed.

IF YOU want to see impressive mountains , come to western Norway! You will be awestruck! In addition, the narrow, winding roads and the many tunnels attest to man’s ingenuity. Recently a new tunnel was completed—a feat of engineering that surpasses any of its kind. It is the Laerdal Tunnel, the world’s longest road tunnel—a highway 15.2 miles [24.5 km] long, bored through solid rock! Imagine driving into the mouth of the tunnel, knowing that after just a few minutes, there will be more than 3,000 feet [1,000 m]of mountain overhead!

Why was such a long tunnel needed? It is an important part of the main link between the two biggest cities of Norway, namely Oslo (the capital, in the east) and Bergen (on the west coast). Other mountain roads between these cities are difficult during winter because of snow and wind. Therefore, a new route, one that would be reliable in bad weather, was greatly needed. In 1992 the Norwegian parliament decided that the new highway would include a tunnel between the small communes of Aurland and Laerdal. After five years of construction, the tunnel was officially opened in November 2000. How was this feat of engineering accomplished? How safe is the tunnel? What is it like to drive through it? Let’s take a look.

The Problems of Construction

The tunnel connects Laerdal and Aurland, but the workers actually started in three places at the same time. A team started from each end, and a third group began work on a ventilation tunnel 1.6 miles [2 km] long, which was to meet the main tunnel 4 miles [6.5 km] from the Laerdal entrance. How could the three teams coordinate the drilling so as to be sure to meet, deep inside the mountain? To determine the exact starting point of each team, satellite navigation systems were used, and the direction of the drilling was guided by laser beams. These laser beams controlled the movement of the drilling rigs to ensure that holes drilled for explosives would be accurately positioned.

For each blast, about 100 holes were drilled, each 17 feet [5.2 meters] in depth. These holes were packed with some 1,000 pounds [500 kg] of explosives, which resulted in about 20,000 cubic feet [500 cu m] of crushed rock. This rubble was then hauled out by truck. Before the drilling operation could start up again, the walls and the roof of the tunnel had to be secured. Long steel bolts were used, and the surfaces were sprayed with fiber-reinforced concrete, known as shotcrete. Each team advanced some 200 to 230 feet [60-70 m] weekly. In September 1999, the breakthrough took place when the two teams working on the main tunnel met, with a deviation of about 20 inches [50 cm]! Fourteen months later the tunnel was opened on schedule. At this point, the cost had reached $120 million.

How Was Ventilation Provided?

Ensuring good air quality always presents a challenge to tunnel engineers. Since it takes approximately 20 minutes to drive through the Laerdal Tunnel, it is especially important that the air be clean enough to breathe. How was this achieved?

The 1.6-mile [2 km] ventilation tunnel, which is 4 miles [6.5 km] from the Laerdal opening, reaches out to a nearby valley and serves as a chimney, or outlet channel. Fresh air is drawn into the tunnel from both ends, and polluted air exits through the ventilation tunnel. Two powerful fans installed in the ventilation tunnel—with a combined top capacity of 60 million cubic feet [1.7 million cu m] an hour—can be used to increase the airflow when the air is highly polluted. This system supplies adequate fresh air at the Laerdal side of the tunnel; yet, something more had to be done for the Aurland side, which is longer. Therefore, 32 smaller fans, impulse fans, were installed in the tunnel roof to increase the airflow toward the ventilation tunnel. However, as this air flows the long distance from the Aurland side to the mouth of the ventilation tunnel, it becomes more and more polluted. How could this problem be solved?

The solution was to build a cleaning plant in a 300-foot-long [100 m] parallel tunnel six miles [9.5 km] from the Aurland opening. At both ends, this tunnel is connected with the main tunnel. The air in the main tunnel is diverted into this parallel tunnel, where up to 90 percent of the dust and nitrogen dioxide is extracted.

With this ventilation system and cleaning process, the Laerdal Tunnel can satisfactorily handle up to 400 cars an hour. Sensors inside the tunnel monitor air quality and regulate the effect of the ventilation system. If the pollution level becomes too high, the tunnel will be closed to traffic, but so far this has not been necessary.

How Safe Is It?

Some people are apprehensive about driving through a tunnel. This fact along with recent serious accidents and fires in several European tunnels has made safety a matter of high priority in the Laerdal Tunnel. What has been done to make the tunnel safe?

A control center in Laerdal constantly monitors the various security systems in the tunnel, and if safety is compromised, the tunnel will be closed. Many precautions have been taken in order to enable quick closing and rapid evacuation of the tunnel. Also, emergency telephones have been installed every 820 feet [250 meters], and two fire extinguishers every 410 feet [125 meters]. The control center automatically registers the location of any extinguisher removed from its place. If one is removed, red traffic lights warn drivers not to enter the tunnel, and signs and lights inside the tunnel instruct drivers to drive out of the tunnel in the safe direction, away from the danger. The drivers are able to turn their vehicles around because there are turnarounds for cars every 1,640 feet [500 meters] and 15 locations that allow larger vehicles to turn around. The tunnel is also equipped with a radio antenna system that makes it possible to inform drivers by means of their car radio. Counting and photo systems monitor all traffic in and out of the tunnel. The road authorities consider this to be a very good level of security in view of the relatively low traffic density.

How Is This Tunnel Different?

What is it like to drive through the tunnel? An important aim of the engineers was to make driving through the tunnel a positive experience so that drivers would feel safe and would also drive safely. To accomplish this, the interior of the tunnel was designed with the help of, among others, traffic psychologists at a research institute and professional light designers, as well as a driving simulator.

The result? Well, the tunnel is not quite straight. Gentle curves help prevent drivers from becoming drowsy, yet they can see 3,000 feet [1,000 meters] ahead. Curves also make it easier to determine the distance to oncoming traffic. Three big cavelike mountain halls serve to break up the monotonous journey. This creates an illusion of driving through four shorter tunnels instead of one long one. Special lighting in these halls, with yellow or green light at the floor and blue light above, gives an illusion of daylight coming in and a sunrise. These effects together with good light along the length of the tunnel make most drivers feel comfortable and safe.

Now travelers can enjoy the unique experience of passing through the world’s longest road tunnel. This feat of modern engineering has resulted in a reliable link between the east and the west of Norway. It is powerful evidence of what human can do when he uses his skill and ingenuity in a constructive way.

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