This article is about underground passages. For other uses of the word tunnel, see tunnel (disambiguation).

A tunnel is an underground passage. When designed for use by traffic, it may be called an underpass. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels have a ratio of the length of the passage to the width of at least 2 to 1. In addition, they should be completely enclosed on all sides (save the openings) for the length of the covered area.

A tunnel may be for pedestrians and/or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some 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.

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, but is now used to refer to underground mass transit systems, generally called metros outside the US.

The central part of a metro or subway network is usually built in tunnels. To allow non-level crossings, some lines are in deeper tunnels than others. At metro stations there are usually also pedestrian tunnels from one platform to another. Often, ground-level railway stations also have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks.

Types of tunnels

Tunnels are dug in various types of materials, from soft clays to hard rocks, and the method of excavation depends on the type of soil. When digging soft clays the New Austrian Tunneling method might be applied. When digging in weak rocks a tunnel boring machine (TBM) is often used. In hard rocks, blasting may be deemed most suitable, as in the Norwegian tunneling method. Various combinations of these methods and many others are also possible.

There are also several approaches to underwater tunnels, for instance an immersed tube as in Sydney Harbour.

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

Tunnel-boring machines (TBMs) can be used to automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions. One type of TBM, called an earth-pressure balance machine, can be used deep below the water table. This pressurizes the cutter head with either fluid or air in order to balance the water pressure. As a result operators of the TBM must go through decompression chambers, much like divers.
The biggest TBM built was operated to drill the tunnel as part of the High Speed Raillink South in the Netherlands. Its diameter is 14.85 m. Tunnel Groene Hart (the Netherlands)

Very effective is the New Austrian Tunneling Method (NATM), which was developed in the 1960s. The main idea of this important method is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself.
Based on geotechnical measurements, an optimal cross section is computed. The excavation is immediately protected by thin shotcrete, just behind the TBM. This creates a natural load-bearing ring, which minimizes the rock's deformation.

By special monitoring the NATM method is very flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate tools for tunnel strengthening. In the last decades also soft ground excavations up to 10 km became usual.

Other tunneling methods are:

• Drill and blast
• Lunardi Method
• Slurry-shield machine
• Wall-cover construction method.

Choice of tunnels vs. bridges

For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans when intersecting with shipping channels at some locations, necessitating use of a tunnel. Additionally, bridges usually require a larger footprint on each shore than tunnels; in areas with particularly expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in tunnels' favor. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia and the Westerscheldetunnel.

Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link.

Very short tunnels

A short tunnel can be a convenient way to build an overbridge. Very short tunnels resemble overbridge. For example, the Croom Tunnel on the Nowra, New South Wales railway line.

Artificial Tunnels

Overbridges can sometimes be built by covering a road or river or railway with brick or still arches, and then levelling the surface with earth. In railway parlence, a surface-level track which has been built or covered over is normally called a covered way.

Snowsheds are a kind of artificial tunnel built to protect a railway from avalanches of snow. Similarly the Coal Cliff, New South Wales steel tunnel, on the Nowra, New South Wales railway line.

Examples of tunnels

In history

• Eupalinos Tunnel on the island Samos (Greece). Built 520 BC by the greek engineer Eupalinos. Eupalinos organised the work so that the tunnel was begun from both sides of the hill and the two teams met in the middle. The estimates for the time required range from 5 to 15 years: the mountain is solid limestone and one has to suppose that many of the slaves doing the work died. The tunnel's existence was recorded by Herodotus (as was the mole and harbour, and the third wonder of the island, the great temple to Hera, thought by many to be the largest in the Greek world). The precise location of the tunnel was only re-established in the 19th century by German archaeologists. The tunnel proper is 1030 metres - 3432 feet - long and visitors can still enter it Eupalinos tunnel.
• Sapperton Tunnel on the Thames & Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km long and allowed ship transport of coal. Above it runs the Sapperton Long Tunnel which carries the "Golden Valley" railway line between Swindon and Gloucester.
• Box Tunnel in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug and has a length of 2.9 km.
• The Thames Tunnel, built by Marc Isambard Brunel and his son Isambard Kingdom Brunel and opened in 1843, was the first underwater tunnel and the first to use a tunnelling shield.
• The Cobble Hill Tunnel and Murray Hill Tunnel in New York City are the world's oldest railway tunnels lying below streets, roofed over in 1850 and the 1850s, respectively.
• The oldest sections of the London Underground were built using the cut-and-cover method in the 1860s. The Metropolitan, Hammersmith & City, Circle and District lines were the first to prove the success of a metro or subway system.

See also the History of Rapid transit.

The longest

• A new St. Gotthard Rail Tunnel with a length of 57 km (36 miles) is currently under construction.
• The Seikan Tunnel in Japan is the longest rail tunnel in the world at 53.9 km (33.4 miles), of which 23.3 km (14.5 miles) is under the sea.
• The Channel Tunnel between France and England under the English Channel is the second-longest, with a total length of 50 km (31 miles), of which 39 km (24 miles) is under the sea.
• The Lærdal Tunnel in Norway from Lærdal to Aurland is the world's longest road tunnel, intended for cars and similar vehicles, at 24.5 km.
• The St. Gotthard Tunnel from Göschenen to Airolo in Switzerland, opened on September 5, 1980, is the world's longest highway tunnel at 16.32 km (10.14 miles).
• The North Cape Tunnel in northern Norway, connecting the island of Magerøya with the mainland, was the world's longest undersea road tunnel when opened in 1999, at about 7 km. It reaches a depth of 212 m below sea level.

Notable

• The Lincoln Tunnel between New Jersey and New York is one of the busiest vehicular tunnels in the world.
• Williamson's tunnels in Liverpool are probably the largest underground folly in the world.
• New York City Water Tunnel No. 3[1], started in 1970, has an expected completion date of 2020.

Other uses

Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with military use during armed conflict, or civilian responses to threat of attack, or a means of protection from attackers.

See also

• List of tunnels
• Underground city
• Urban exploration
• Roof and tunnel hacking
• List of rathole tunnels
• World's longest tunnels

External links

• Directory of the world's longest tunnels by categorycs:Tunel