Cut and cover

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 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".

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

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

• The Lincoln Tunnel between New Jersey and New York is one of the busiest vehicular tunnels in the world.
• The Sapperton Tunnel in the Thames & Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km long and allowed ship transport of coal.
• Box Tunnel in England, which opened in 1841, is one of the oldest railway tunnels in the world. It is dug and has a length of 2900 m.
• The Thames Tunnel built by Marc Isambard Brunel and his son Isambard Kingdom Brunel which opened in 1843 - the first underwater tunnel and the first to use a tunnelling shield.
• The Atlantic Avenue Tunnel, Brooklyn, New York is the world's oldest underground railway tunnel in a street, built in 1844 by the cut-and-cover method for the Long Island Rail Road. It is 800 m long.
• The North Cape Tunnel in northern Norway, connecting the island of Magerøya with the mainland, is the world's longest undersea road tunnel at a length of about 7 km. It reaches a depth of 212 m below sea level and was constructed from 1995 to 1999.

See also

• List of tunnels
• Wind tunnel
• Underground city
• Tunnel (nightclub)