Tunnel Tunnel Special cities of Korea Category="Bridges"Category="Subterranea"Category="Tunnels"
This article is about underground passages. For other uses of the word tunnel, see Tunnel (disambiguation).

[image] 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

[image] 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 fourth tube of the New Elbe Tunnel in Hamburg. Its diameter is 48 ft (14.2 m).

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:

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 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.

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.

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

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See also the History of Rapid transit.

The longest

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Notable

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

External links