Tunnels are used for transportation, mining, and drainage purposes, as well as for installation of power sources. Although tunneling is difficult and dangerous, the benefits are often worth the effort. Early civilizations relied on tunnels to transport water for drinking and irrigation. The Egyptians tunneled into cliffs to construct temples, and in 2100 B.C., the Babylonians went so far as to build a pedestrian tunnel under the Euphrates River by diverting the river during the tunnel's construction. The Greeks and Romans practiced tunneling extensively, the Romans constructing aqueduct tunnels through mountains.
Tunnel technology went through a long hiatus until about the 1700s, when tunnels were included in the development of canal networks in Europe and North America. The use of gunpowder blasting in excavations of solid rock was a major advance; different explosives are used today, although the blasting method is still similar. As railroads gained prominence beginning in the 1830s, tunnels became crucial elements of the rail system. Since trains had to operate on the lowest grade slope possible, tunnels were built in such great numbers that tunnels and trains became closely associated with each other. The father-and-son team of Marc Isambard Brunel and Isambard K. Brunel engineered the Thames River Tunnel, the world's first underwater tunnel, in the early 1800s. Several cave-ins led them to develop a cast-iron tunneling shield to protect workers in 1825. Peter Barlow improved this shield in 1869, and James Greathead (l844-l896) used compressed air to reduce seepage of water and mud into the work space.
Compressed air was also used in construction of the London subway in 1886. In 1855, the 4.5 mi. (7.2 km) Hoosac Tunnel, begun in the Berkshire Mountains of Massachussetts, became the first railroad tunnel in the United States. It took eighteen years to complete, but many new procedures were developed in the process. While hand-drilling had been standard practice before the Hoosac project, dynamite and compressed-air drills were used for the first time during its construction.
The twentieth century has seen the development of several tunneling machines. The rotary excavator, or mole , forces its way forward through the bedrock. The loosened rock is then carried back through the tunnel in muck cars. Rock mechanics has become a major part of tunnel planning, since a number of disasters have occurred because the rock structure was either not evaluated or was misjudged. Accordingly, the mass of the rock must be analyzed and geological stress predicted, since stress from earthquakes and other past geologic events can affect the tunnel. Another important consideration in tunnel construction is stand-up time--the amount of time, whether seconds or hours, that an excavated stretch will stand without support. The tunneling crew has to reinforce the tunnel at its heading or excavation face.
Most modern tunnels have been built to accommodate automobile traffic. Since the grade of the roadbed is less critical a factor than it is with railroads, road cuts can suffice for a highway where a tunnel might have been necessary for railroad tracks. Many major automobile tunnels run under rivers and harbors, and trench tunnels are preferred for these situations. In this method a trench is dug at the bottom of the body of water, and the tunnel sections, often double-barreled for opposite traffic flow, are lowered into the trench, then buried and drained of water. Pipe-jacking is used for smaller tunnels; this involves lowering pipe segments into a vertical shaft, then moving them into place as the tunnel boring progresses. Mining shafts and tunnel networks can extend miles into the earth, both vertically and horizontally.
Despite all precautions, underground mining is quite dangerous. Gases can accumulate in pockets of a mine, causing explosions; the noise from the machinery can damage hearing and make communication difficult; and cave-ins are a constant threat. Construction on the English Channel Tunnel between England and France, an engineer's dream for centuries that was envisioned and encouraged by Napoleon, was begun in 1987. Referred to as the Chunnel , it was completed in 1994 at a cost of $13 billion. The two rail tunnels (one for northbound and one for southbound traffic) and one service tunnel are each 38 km (24 mi) in length and have an average depth of 40 m (131 m) under the seabed. It is the first physical link between Britain and the European Continent. Passenger rail service is provided, as well as the ferrying of automobiles and trucks. Travel times from London to Paris have been reduced from more than five hours (over sea) to three hours via the Chunnel.
The Seikan Tunnel in Japan was placed in service in 1988. The 53.85 km (33.44 mi)-long tunnel connects the northern tip of Japan's main island of Honshu with the island of Hokkaido, passing under the Tsugaru Strait. The Seikan Tunnel is the world's longest submarine tunnel, involving excavation 100 m (330 ft) below the seabed across a strait where the sea is up to 140 m (460 ft) in depth.
Other such monumental tunneling projects will likely be attempted. It may be more realistic to consider a tunnel or submerged conduit for the Strait of Gibraltar, for example, than to attempt to bridge it.
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