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Read the following text and fill in the gaps with the words from the boxes below.

A (1) _________ is an underground passageway. The definition of what constitutes a tunnel is not universally agreed upon. Tunnels in general, however, are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, safe for the openings at each end. Some civic planners (2) _________ a tunnel as 0.1 miles (0.16 km) in length or longer, while anything shorter than this should be called an underpass. For example, the (3) _________ under Yahata Station in Kitakyushu, Japan, is only 0.08 mi long (420 ft; 0.13 km) and therefore should not be considered a tunnel.

A tunnel may be for pedestrians (= people who are walking in the street) or cyclists; for general road traffic; for motor (4) _________ only; for (5) __________ traffic; or for a canal. Some are aqueducts, constructed for carrying water — for consumption, for hydroelectric purposes or as sewers. The others may carry other services such as (6) __________ cables. There are even tunnels designed as wildlife crossings for European badgers (= small animals with black and white lines on its head that lives in holes in the ground and comes out at night) 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 or the tunnels connecting the Gaza Strip to Egypt. Some tunnels are not for (7) __________ at all, but they are more like fortifications (= walls or towers to protect a place from attack), for example Mittelwerk and Cheyenne Mountain.

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

The central part of a rapid transit network is usually (8) __________ in tunnels. To allow non-level crossings, some lines run in deeper tunnels than others. Rail stations with much traffic usually provide pedestrian tunnels from one platform to another, though others use bridges.

It is essential that any tunnel (9) __________ starts with a comprehensive investigation of ground conditions. The results of the investigation will allow proper choice of (10) __________ and methods for excavation (= digging) and ground support. And this will reduce the (11) __________ of unforeseen problems that may occur.

Tunnels are dug in various types of (12) __________ , from soft clay to hard rock. So, the method of excavation depends on the ground conditions. Cut-and-cover is a (13) __________ method of (14) __________ for shallow (= not deep) tunnels where a trench (= a long narrow hole that is dug in the ground for water to flow along) is excavated and roofed over. Two basic forms of cut-and-cover tunnelling are available - bottom-up method and top-down method.

Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube (= Underground) station in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and ventilation, smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral. This contrasts with most (15) __________ stations of London Underground, where bored (= excavated) tunnels were used for stations and passenger access.

The (16) __________ of tunnels may also suffer different types of hazards. One of them, and the most (17) __________ one, is fire. That happens because of the enclosed space of a tunnel. The main dangers are gas and (18) __________ production, with low concentrations of carbon monoxide being highly (19) __________ . Fires killed 11 people in the Gotthard tunnel fire of 2001, for example. Over 400 passengers died in the Balvano train disaster in Italy in 1944, when the locomotive broke down in a (20) __________ tunnel. Carbon monoxide poisoning has been the main cause of the horrifying death rate. Fires have also occurred in the Channel Tunnel, leading to great delays for users.