Read the text and make a list of tunneling terminology

A tunnel is a long, completely enclosed on all sides man-made horizontal passage built under the ground with openings at each end. Tunnel entrance and exit are tunnel portals. The top half of the tunnel or the "roof" is the crown, and the bottom half is the invert. As tunnels must withstand tremendous pressure from all sides, thecontinuous arch is the ideal shape, especially for railway or motorway tunnels passing through a hill, under a building, road and river or under the sea (fig.17.1).

In general, tunnels must be at least twice as long as they are wide. Normally civic planners speak of a tunnel if it is at least as long as 0.16 km. A shorter structure is a chute. Relatively small-diameter tunnels serve as utility lines or pipelines.

Tunnels for transporting people by rail or by motor vehicles are rather complex structures accommodating two or more parallel passages for opposite-direction traffic, service machinery, and emergency exits.

a – Tunnel Portal	b – Underwater Tunnel
c – Railway Tunnel	d – General Tunnel Structure

Figure 17.1 Underground and Underwater Transport Tunnels

Underwater tunnels refer to immersed tunnels because they are commonly made of long, prefabricated tube sections, towed out to the construction site, and sunk in a dredged canal in the sea or riverbed (fig.17.2a). Sand or other backfill material cover immersed tunnels. These tunnels allow travel beneath bodies of water but they may obstruct or influence the moving of fish and create obstacles to water wildlife. Moreover, the immersion operations in waterways are rather difficult due to heavy currents, and in case of failure, this operation may result in serious cost escalation.

Many tunnels are technological masterpieces, for instance, the Seikan Tunnel that passes under the Tsugaru Strait. It is a 53-km long, 9.7-m diameter railway connection between the northern part of Japan’s main island of Honshu and the island of Hokkaido. Completed in 1988, it is the world’s longest person-carrying submarine tunnel. Its construction involved a 24-year struggle facing many challenges due to hard and soft rock excavation 100 m below the seabed where the sea is up to 140 m in depth.

a - Prefabricated tube sections for underwater (submerged) tunnels	b – A single tube section is ready for sinking
c – An entrance to an underwater tunnel	d - An underwater tunnel entrance with opposing traffic

Figure 17.2 Underwater Transport Tunnels

The earliest tunnelling belonged to prehistoric people who had to enlarge their caves, and their tunnels were hand-dug. Then several ancient civilizations in the Indian and Mediterranean regions developed tunneling methods. About 2160 BC the inhabitants of Babylonia constructed the 900-m long brick-lined pedestrian passage under the Euphrates River. Roman engineers built an extensive network of tunnels to carry water from mountain springs to their densely populated cities and to drain sewage away. These tunnels were part of the well-known aqueduct systems, which included sloping bridge-like structures supported by arches, and carrying water to the city of Rome.

The earliest canals, as man-made waterways used for travel, shipping or irrigation, inspired the construction of transportation tunnels through hills or mountains. By the 20^th century, railways and roads had replaced the use of canals, and the notable tunnels appeared all over the world because the developing transport system needed them badly. For instance, the Holland Tunnel between New York City and New Jersey, completed in 1927, was one of the first roadway tunnels. It is one of the world’s greatest engineering projects. Our country also constructed the world famous Severomuiskiy Tunnel that is 15.3 km long. It was open to traffic in 2003 on the Baikal-Amur railway in the Russian republic of Buryatia.

Tunnels serve for transportation including metro, road vehicles and trains. They are also necessary for ore mining, for conducting water and sewage, for pumping stations, for underground hydroelectric-power plants and for canals. Subways, pedestrian passages, etc. also use tunnels.

Tunnels are the most complicated and costly engineering structures because underground work deals with various difficulties worsened by flammable gas, the weaknesses of the ground, the water inflow, etc. It took $21 billion to complete the Channel Tunnel (fig. 17.3) that appeared to be the most expensive construction project being 700 times more expensive than the construction costs of the Golden Gate Bridge.

All tunnels can be classified into one of the following criteria:

Criterion 1. (Tunnel function):

1.1 Transportation tunnels. (for travelling and carrying goods by any type of vehicle.)

1.2 Mining tunnels. (for extracting coal or other minerals.)

1.3 Utility tunnels. (for heat supply, gas, electricity, vision cable and phones in large cities.)

1.4 Waterpower tunnels (for hydroelectric stations) or hydraulic tunnels (to supply water for consumption and for sewage)

1.5 Secret tunnels for military purposes.

Transportation tunnels may be classified in the following way:

Criterion 1.1 (Mode of transport):

5.3.5 Railway tunnels.

The Channel Tunnel, also called Euro tunnel, serves for freight and passenger traffic. It is 50 km long and runs underneath the English Channel between Folkestone, England, and Sangatte (near Calais), France. It consists of three tubes: two for rail traffic and a central one for services and security. Passengers can travel by ordinary rail coach or can stay within their own motor vehicles in special rail cars. The trip takes 35 minutes as trains travel at speeds as high as 160 km per hour.

1.1.2 Motorway tunnels.

1.1.3 Pedestrian tunnels.

1.1.4 Metro tunnels.

1.1.5 Navigation tunnels.

Criterion 2. (Tunnel location):

2.1 Flat ground tunnels.

2.2 Mountain tunnels. (for railways running through maintain ranges or watershed)

2.3 Underwater (submerged) tunnels or immersed tunnels.

The Mersey Tunnel in Great Britain links Liverpool (north) with Birkenhead (south). The construction began from both banks and the breakthrough took place beneath the riverbed in the middle of the river. Currently several tunnels beneath the Thames provide railway links (the twin Blackwall Tunnels – Southbound and Northbound), pedestrian walkways – the Greenwich Tunnel, and the first sub aqueous tunnel in the world, the Wapping-Rotherhithe Tunnel that was built in 1825, and refurbished in the late 1990s. This pedestrian horseshoe shaped cross-section tunnel is brick-lined. For many years, it was the largest soft-ground tunnel. Since 1913, the tunnel has been a part of the Tube.

Criterion 3. (Construction method):

3.1 Tunnels built by the cut-and-cover method.

3.2 Rock tunnels, built by the cutting method.

3.3 Shield driven tunnels.

Tunnelling under rivers was impossible until M. Brunel developed the protective shield in England. Jacks propel a shield forward and the workers put permanent lining segments in place under protection of the shield tail.

Criterion 4. (Tunnel laying depth):

4.1 Shallow-lying tunnels (up to 10 m deep).

4.2 Deep-laid tunnels (over 10 m deep).

Criterion 5. (Shape of the tunnel cross-section):

5.1 Rectangular section tunnels.

5.2 Circular section tunnels (the strongest shape).

5.3 Horseshoe section tunnels (flat bottom provides roadway).

a – A double deck motor tunnel	b - A double deck road-cum-rail tunnel

Figure 17.3 Double Deck Tunnels

Some tunnels are double-decked (fig.17.3), for example, the Eastern Harbour Crossing in Hong Kong where the lanes for motor vehicles and rails (the MTR metro) occupy different decks. Drivers of private cars, public buses and special purpose vehicles use either the manual tollbooths or auto toll service to pay tolls for crossing the tunnel.