Text 1. TUNNEL

Tunnel is an underground passageway. 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. Tunnels were known in ancient times. They were, for instance, driven into the rock under the Pyramids of Egypt, and the Romans built one in Rome for their chief drain, parts of which still remain. One of the earliest tunnels known was made in Babylon. It passed under the Euphrates river and was built of arched brickwork being 12 feet high and 15 feet wide. Other ancient tunnels were built for water supply and for drainage. Tunnels are dug through hills and mountains, and under cities and waterways. They provide highways, subways, and railroads with convenient routes past natural and artificial obstacles. Miners use tunnels to reach valuable minerals deep within the earth. Tunnels also carry large volumes of water for hydroelectric power plants. Some tunnels provide fresh water for irrigation or drinking, and oth­ers transport wastes in sewer systems. In addition, tun­nels provide underground space for cold storage. A tunnel may be for pedestrians and/or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. 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. 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 rapid transit systems. 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.

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

Дата добавления: 2015-08-27