tunnel

Introduction

tunnel, underground passage usually made without removing the overlying rock or soil. Although tunnels are approximately horizontal, they must be built with sufficient gradient for proper drainage. Tunnels may be ventilated by shafts leading to the surface or by exhaust fans at the ends.

Significant Historic and Modern Tunnels

The origin of tunnel building is disputed. The Egyptians built tunnels as entrances to tombs. The Babylonians built (c.2180 b.c.) a tunnel under the Euphrates using what is now called the “cut-and-cover” method; the river was diverted, a wide trench was dug across its bed, and a brick tube was constructed in it and covered up. The ancient Greeks and Romans built tunnels for carrying water and for mining purposes; some of the Roman tunnels are still in use. One of the first notable tunnels in Great Britain was part of the Grand Trunk Canal. It was nearly 2 mi (3.2 km) long and was completed in 1777. The Mont Cénis Tunnel, a railroad tunnel in the French Alps that opened in 1871 and is now 8.5 mi (13.7 km) long, was probably the first tunnel built using compressed-air drills.

The first tunnel of importance in the United States was the tunnel through the Hoosac Range in Massachusetts. There are hundreds of miles of tunnels in New York City and its vicinity, e.g., for subways, roads, water systems, and railroads. The Delaware Aqueduct, which provides part of New York City's water supply, is at 105 mi (168 km) the longest continuous tunnel in the world. Road tunnels include the Holland Tunnel and the Lincoln Tunnel, which connect New York City's Manhattan Island with New Jersey, and the Hugh L. Carey (formerly Brooklyn-Battery) Tunnel, which connects Manhattan Island with Brooklyn and is the longest vehicular tunnel (1.7 mi/2.7 km) in the United States. The Anton Anderson Memorial Tunnel, also known as the Whittier Tunnel (2.5 mi/4 km), which opened in 1943 to rail traffic and in 2000 to vehicular traffic, connects Whittier, Alaska, to Anchorage and other cities; the unique single-lane tunnel allows rail or road traffic in one direction only at a time. The Chesapeake Bay Bridge-Tunnel in Virginia, opened in 1964, has a length of 17.6 mi (28.2 km) and includes two tunnel segments over a mile long.

The Simplon Tunnel (opened 1906; see under Simplon) through the Alps was for many years the longest railway tunnel (12.3 mi/19.8 km) in the world. The Gotthard Base Tunnel (2016; see under Saint Gotthard), also in the Alps, is now the world's longest tunnel (35.4 mi/57 km), and the Seikan Tunnel (1988), connecting Honshu and Hokkaido, Japan, is the world's longest underwater tunnel (33.5 mi/53.6 km). The Channel Tunnel (1994; 31 mi/50 km) under the English Channel, however, has the longest underwater section. The world's longest vehicular tunnel, the Lærdal Tunnel (15.2 mi/24.5 km long), connects Lærdal and Aurland, Norway, and is an important overland link between Oslo and Bergen. The St. Gotthard Tunnel (10.2 mi/16.4 km long), in the Swiss Alps, was formerly the longest vehicular tunnel.

Design and Construction Techniques

Methods of tunneling vary with the nature of the material to be cut through. When soft earth is encountered, the excavation is timbered for support as the work advances; the timbers are sometimes left as a permanent lining for the tunnel. Another method is to cut two parallel excavations in which the side walls are constructed first. Arches connecting them are then built as the material between them is extracted. Portions of the unexcavated center, left temporarily for support, may be removed later. A tunnel cut through rock frequently requires no lining. Hard rock is removed by blasting.

In constructing tunnels under rivers, the ordinary methods can be used as long as a stratum of impermeable material lies between the tunnel and the riverbed. In all cases, however, pumping equipment must be installed. Where mud, quicksand, or permeable earth is present in underwater tunneling, it becomes necessary to provide some means of holding back the water while the enclosing sections of the tunnel are placed in position. For this purpose the shield was devised and first used in 1825 by the French-born engineer Sir Marc I. Brunel when boring between Wapping and Rotherhithe, in England. Considered unsuccessful, the device was not employed again until 1869, when the British engineer James H. Greathead and the American inventor Alfred E. Beach developed improvements at about the same time. Their shields were metal cylinders fitting around the outside of the tunnel, the forward end closed by a diaphragm plate. As the rock or earth was cut away, the shield was shoved forward into the earth by hydraulic rams, compressed air being used to keep seepage to a minimum. The use of the pneumatic shield is now universal in tunneling under rivers. The actual cutting is performed by huge rotating cutter heads, each with up to fifty separate cutters, capable of penetrating 10 mm (1/2 in.) per revolution.

River-crossing tunnels are also constructed by dredging a trench in the riverbed and then lowering prefabricated tunnel sections through the water into the trench, where they are connected to each other. The trench and tunnel are then covered over. In 1969, a tunnel was constructed across the Schelde River in Belgium, using sections 330 ft (100 m) long. Often, to speed construction, work is started at both ends. This poses no problem with the cut-and-cover method, but when the tunnel is bored from within, it must be assured that the tubes will actually meet in the center. Modern methods accomplish this with high precision.

Bibliography

See T. M. Megaw and J. V. Bartlett, Tunnels (1981–82); B. Stack, Handbook of Mining and Tunnelling Machinery (1982); Approaching the 21st Century (1987).

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