Tunnels

A tunnel is an underground passage. 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. more...

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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 or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, 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. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels.

In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a subway. This term was also used in the past in the United States, but is now used to refer to underground rapid transit systems.

The longest canal tunnel is the Standedge Tunnel in the United Kingdom, which stretches over three miles.

The central part of a rapid transit 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. However, in the United Kingdom bridges are an equally popular method of pedestrian access between two or more different railway station platforms.

Construction

Tunnels are dug in various types of materials, from soft clays to hard rocks, and the method of excavation heavily depends on the ground conditions.

Cut-and-cover

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.

Boring machine

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 air or another fluid 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 used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. Its diameter is 14.85 m.

Read more at Wikipedia.org