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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.

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 stationss 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".

The St. Gotthard Tunnel opened in Switzerland on September 5, 1980 as the world's longest highway tunnel at 10.14 miles (16.32 km) stretching from Goschenen to Airolo.

Table of contents
1 Construction
2 Wartime tunnels
3 Examples of tunnels


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.

Wartime tunnels

Examples of tunnels

See also: List of tunnels, Wind tunnel, Underground city.

"Tunnel" was also the name of a large New York City nightclub with multiple rooms themed after subway tunnels. It was perhaps best known for hosting Kurfew, its gay-oriented Saturday night parties with resident DJ Johnny Vicious. Tunnel closed its doors soon after the demise of nearby Twilo, the victim of the owner's debts and of Rudy Giuliani's quality-of-life campaign.