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Tunnel vision

Not only are tunnels often the shortest distance between two points, but they can also alleviate the pressures of urban traffic congestion.

    Some 2,500 years ago, architect Eupalinos built a tunnel more than a kilometre long to supply the Greek city of Samos with water from a spring on the other side of Mt Kastron. His builders started at both ends and met in the middle. How Eupalinos managed to avoid creating a big puddle in the mountain remains a mystery, but somehow the tunnel had the right slope and the water flowed from one end to the other.
    Water does not like to climb. Nor do high-speed trains, or, for that matter, motor vehicles. Which is why tunnel-building is undergoing renaissance. As speeds get higher, railways and roads are getting straighter and flatter - and that means tunnels. Tunnels can wipe barriers away. For example, when the new railway from Cologne to Frankfurt is completed in 2002, travellers will be whisked through a few mountains to their destination in less than an hour, rather than the two-plus hours it takes now to meander along the Rhine.
    But speed is not the only consideration. Professor Rolf Katzenbach of the Department of Geotechnology at the Darmstadt University of Technology offers other benefits. "Some parts of the world are suffocating under the burden of traffic," he says. "The towns on the Alpine passes between Switzerland, Austria and Italy are desperate to stop the daily invasion of heavy goods vehicles. There's only one way to stop it, and that's to build more tunnels through the mountains." Hence the new Gotthard and Lötschberg rail tunnels, which will run at base level. For the trains, the mountains will simply not exist.
    Many city streets have also reached their traffic limit, and going underground is still a solution - hence the construction of new underground railways in such cities as Brasilia in Brazil and Chelyabinsk in Russia.

    When Eupalinos built his tunnel, his men hacked their way through solid rock, which was strong enough to hold up the relatively small structure that resulted. However, miners and tunnel builders soon learned to hold their tunnel roofs up in less secure ground with wooden beams and masonry. A breakthrough came in the 19th century when a British engineer, Marc Isambard Brunel, developed the idea of protecting the tunnel builders inside a cylindrical shield as big as the tunnel. The cylinder was pushed forward as they dug, allowing them to remove the earth as they went. Behind them, masons built the tunnel walls. The technique was first used in a tunnel under the Thames, completed in 1843.
    That, in principal, is still what happens, but Brunel would scarcely recognise the machinery that does it now. Huge tunnel boring machines (TBMs) dig their way through the ground while, directly behind the protective shield, tunnel rings made of reinforced concrete segments are fitted into place. A slurry of fine clay particles in water is used to "lubricate" the drilling process and to carry away the crushed rock. The slurry is separated from the rock and returned to the process, while the rock is carried along a conveyor belt to waiting trucks or trains for disposal.
    Such an underground factory is expensive - about 25 million US dollars. It has to be built for each project and precisely matched to the geology. Specific kinds of cutters and grinders have to be used for the conditions, as well as specific methods of producing the pressure differential at the head and of removing rock.

    TBMs are fast. In good conditions they can build tunnels at a rate in excess of a metre an hour. They've been used in many of the most dramatic tunnel building projects of the past few years. The fourth tunnel under the Elbe in Hamburg, Germany, for example, was driven by the world's largest TBM, which dug out space for a two-lane highway in one go. But, because of the cost, such a machine is only worth using for distances of more than two kilometres, so only one of the 30 tunnels for the new railway between Cologne and Frankfurt will be built that way. The others will mostly be dug conventionally and secured using the shotcrete method, with the tunnel being lined with quick-drying concrete as soon as it is dug.
    Water, too, can result in surprises. Brunel's tunnel had to be abandoned several times because water broke in, and many of the rocks in which tunnels are built today are saturated with water. Once it was standard practice simply to pump the water out, but that lowers the water table, and environmental considerations mean that this is now seldom done - typically only as a temporary measure while building is under way.
    Usually water is kept out of the drilling process by the use of excess air pressure matching that of the water, in which case the workers have to enter the tunnel site through an airlock. TBMs solve the problem by isolating the drill head from the rest of the machine, while the tunnel segments seal the tunnel directly behind the shield. In tunnels driven with the shotcrete method, a concrete lining has to be added to provide a water seal, as well as to ensure long-term stability as the shotcrete deteriorates.

But careful research in advance ensures that the risks are minimised. Giovanni Barla, professor of rock mechanics at the Technical University of Turin, says that a project needs to spend 4 to 5 percent of its budget on preconstruction research if it is to avoid surprises on the way. In the Gotthard base tunnel, for example, a 5.5-kilometer-long, 5-metre-wide exploratory adit was driven into the Piora syncline. The research ensured that a difficult geological area with very high water pressures was avoided. "Tunnels are getting longer; this means that the kind of geology you meet is more varied," says Claus Erichsen of Prof. Wittke Consultants for Geotechnical Engineering in Aachen, Germany. "That calls for new methods."
Katzenbach says that the real development over the past 30 years in tunnel building has been the understanding that the mountain will hold itself up. The task of the shotcrete lining or the tunnel rings is just to support the immediate vicinity. "You only need to support three or so metres of rock," says Katzenbach, "The mountain itself will bear most of the load."
This understanding has extended to the building of caverns. Usually in building a cavern, a tunnel is driven first and then widened out to final size. The roof is secured before the structure is extended downwards. Barla says, "Once it was usual to build a concrete arch, but now the securing is done by rock bolting, anchoring and shotcrete." Caverns are mainly used for storage - Barla was recently involved in the building of huge kerosene tanks in Israel - although there are also such buildings as the Gjøvik Ice Cavern, built for the Lillehammer Winter Olympics in 1994 as an ice hockey rink. Cavern builders have it easier in one respect, says Barla. "They can often build a cavern in the best rock in the area, whereas a tunnel usually has to go through whatever is in the way," he says.

Tunnels are among the most demanding of major construction projects and, at up to USD 50,000 per metre of finished tunnel, they're also the most expensive. But the fascination of overcoming the barriers of rivers, mountains and seas will ensure that ambitious tunnels will still be designed. The 35-kilometre Channel Tunnel waited 200 years to be built.