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A tunnel is an underground passage. When designed for use by traffic, it may be called an underpass.

It may be for pedestrians and/or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some tunnels are constructed purely for carrying water (for consumption, for hydroelectric purposes or as sewers), while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species.

Tunnels are dug in various types of materials, from soft clays to hard rocks. Depending on the type of soil, a method of excavation is chosen. When digging soft clays, the New Austrian Tunnelling method or NATM is used. When digging in weak rocks a tunnel boring machine or TBM is used. In hard rocks blasting is usually the fastest method, as in the Norwegian tunnelling method .

Various combinations of these methods are also possible. The central part of a metro network is usually built in tunnels. To allow non-level crossings, some lines are in deeper tunnels than others. At metro stations there are often also pedestrian tunnels to walk from one platform to another.

Ground-level railway stations often have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks.

In the UK a pedestrian tunnel or other underpass beneath a road is called a "subway". This term was also used in the past in the US.

The Lærdal Tunnel in Norway stretching from Lærdal to Aurland is the world's longest road tunnel at 24.5 km, but the St. Gotthard Tunnel that opened in Switzerland on September 5, 1980 is the world's longest highway tunnel at 16.32 km (10.14 miles) stretching from Göschenen to Airolo. By contrast, rail tunnels like the Seikan Tunnel in Japan and the Channel tunnel between England and France both exceed 50 km (30 miles). A new St. Gotthard Rail Tunnel with a length of 57 km (36 mi) is currently under construction.



Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing.

Shallow tunnels are of the cut-and-cover type (if under water of the immersed-tube type), deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible.

Tunnel boring machines can be used to automate the entire tunneling process.

Examples of tunnels

See also

Wikimedia Commons has multimedia related to Tunnel .

External links

  • Directory of the world's longest tunnels by category

Last updated: 02-07-2005 07:13:16
Last updated: 02-27-2005 18:53:34