Tunnel, an underground or underwater passageway in a roughly horizontal direction. Some of the earliest man-made tunnels may have been dug in a quest for better stone or ore for making implements. Others were dug for burial purposes, temples, or pedestrian passageways. When the cities of ancient civilizations were established, tunnels were constructed for water supply and sewers. In medieval times some tunnels were built for military purposes, as in attacking castles.
With the rise of commerce and industry many canal and railway tunnels were built through mountains and under rivers, shortening the travel time for freight and passengers. As the urban population grew, a need for subway tunnels arose. The next great impetus to tunnel building was brought on by automobiles and highway building.
Since then, there has been an increasing need for tunnels, especially for water at hydroelectric plants, water-supply and sewage systems for cities, subways and rapid-transit systems, and underwater passageways for motor vehicles. One major consideration in the expanding use of tunnels is that they do not disturb the web of plant, animal, and human life on the surface of the earth.
How Tunnels Are Built
After the general direction for a tunnel has been determined, the next steps are a geological survey of the site and a series of borings to obtain specific information on the strata through which the tunnel may pass. The length and cross section of a tunnel generally are governed by the use for which it is intended, but its shape must be designed to provide the best resistance to internal and external forces. Generally, a circular or nearly circular shape is chosen.
In very hard rock, excavation usually is accomplished by drilling and blasting. In soft to medium-hard rock, a tunnel-boring machine typically does the excavating work. In soft ground, excavation usually is accomplished by digging or by advancing a shield and squeezing the soft material into the tunnel. In all cases the muck (excavated rock or earth) is collected and transported out of the tunnel. In underwater tunneling a shield is used to advance the work. Another method of building an underwater tunnel is to sink tubular sections into a trench dug at the bottom of a river or other body of water.
Hard-Rock Tunneling. Short tunnels through hard rock are driven only from the portals, but longer ones usually are driven also from one or more intermediate shafts. Some long tunnels have been built with the aid of a small pilot tunnel driven parallel to the main tunnel and connected with it by crosscuts at intervals. The pilot tunnel furnishes not only additional points of access but also a route for removing muck and for ventilation ducts and drainage lines.
Another method is the heading-and-bench system, formerly used on most large tunnels because it required smaller amounts of powder and permitted simultaneous drilling and mucking (removal of excavated material). The heading (upper portion of the tunnel) is driven ahead of the bench (lower part). A separate crew is thus able to muck in the lower portion of the tunnel while the upper portion is being drilled.
Soft-Ground Tunneling. Some tunnels are driven wholly or mostly through soft material. In very soft ground, little or no blasting is necessary because the material is easily excavated.
At first, forepoling was the only method for building tunnels through very soft ground. Forepoles are heavy planks about 5 feet (1.5 meters) long and sharpened to a point. They were inserted over the top horizontal bar of the bracing at the face of the tunnel. The forepoles were then driven into the ground of the face with an outward inclination. After all the roof poles were driven for about half of their length, a timber was laid across their exposed ends to counter any strain on the outer ends. The forepoles thus provided an extension of the tunnel support, and the face was extended under them. When the ends of the forepoles were reached, new timbering support was added, and the forepoles were driven into the ground for the next advance of the tunneling.
The use of compressed air simplified working in soft ground. An airlock was built, through which men and equipment passed, and sufficient air pressure was maintained at the tunnel face to hold the ground firm during excavation until timbering or other support was erected.
Underwater Tunneling. The most difficult tunneling is that undertaken at considerable depths below a river or other body of water. In such cases, water seeps through porous material or crevices, subjecting the work in progress to the pressure of the water above the tunneling path. When the tunnel is driven through stiff clay, the flow of water may be small enough to be removed by pumping. In more porous ground, compressed air must be used to exclude water. The amount of air pressure that is needed increases as the depth of the tunnel increases below the surface.
E. E. Halmost, Jr., J. R. Owen,
and Leo J. Ritter, Jr.
Engineering
News-Record
Source: "Tunnel." Encyclopedia Americana. Grolier Online http://ea.grolier.com/cgi-bin/article?assetid=0394980-00 (accessed August 13, 2007).