TUNNEL

horizontal or nearly horizontal subsurface passageway that occurs naturally or is made by man. Man-made tunnels are used for various purposes, including finding and exploiting mineral deposits; providing passageways for trains and motor vehicles; conducting water for domestic supplies, power, and irrigation; diverting rivers around damsites; and housing such underground installations as power plants. The earliest tunnels were probably extensions of prehistoric cave dwellings. Ancient civilizations used tunnels to carry water for irrigation and for drinking, and in the early 22nd century BC a tunnel for pedestrian traffic was built in Babylonia under the Euphrates River. The Egyptians excavated temple rooms inside cliffs, while the Romans built aqueduct tunnels through mountains. By the 17th century tunnels were being constructed for canals. During the 19th and 20th centuries the development of railroad and later motor-vehicle transportation led to a tremendous expansion in the number of tunnels and in their length. (See the Table.) Early tunnel-building techniques varied. The Egyptians used copper saws that were capable of cutting soft rock, while the Babylonians constructed masonry tunnels. The Romans tunneled through solid rock by heating the rock face with fire and then rapidly cooling it with water, causing the rock to crack. Tunnel building has always been hazardous, and often hundreds or even thousands of workers died constructing ancient tunnels. Besides greatly increasing the rate at which tunnels can be constructed, the development of modern tunneling technology has also made possible vast improvements in worker safety. Tunnels can be divided into four general categories, depending on the material through which they pass: soft ground, solid rock, soft rock, and subaqueous. Before starting to excavate a tunnel, investigation of the geologic conditions of the site is always necessary and is accomplished by conducting air surveys, analyzing surface samples, and drilling test holes. Excavation of a drift, or horizontal shaft, can begin from a hill or mountain slope (in which case the entrance is called a portal) or from a vertical shaft, down which equipment is lowered and out of which rubble or muck is removed. All tunnels need some form of ventilation to supply air to workers and, later, to traffic and also to draw out potentially dangerous fumes from blasting or from gas deposits. Soft-ground tunnels are generally shallow and are often built for use as subways, water-supply systems, and sewers. Excavation is much easier than it is in solid rock, but the stand-up time (the time an excavated section will safely stand up without support) is very short. In order to prevent the tunnel from collapsing, a support structure is continously built around the heading, or excavation face. A circular or arch-shaped design has been found to be the best at bearing the ground load from above. Brick and stone were used for support in early tunnels, but in modern tunneling steel is generally used to provide temporary support until a concrete lining can be installed. Soft-soil excavation can be accomplished by a number of methods, from simple hand mining with shovel and pick-ax to full-face boring with sophisticated machinery. One such device, the tunneling mole, utilizes a rotating wheel set with teeth that continuously excavates material and loads it onto a conveyor belt. In some machines the cutter is pressed against the face, forcing earth through sievelike holes and onto the belt. Although tunnels through solid rock can be excavated at only about half the rate of tunnels through soft earth, rock bores have the advantage of much longer stand-up times. If a tunnel is pushed through unfractured blocks, it may need little or no additional support. Tunnelers often encounter areas of defective rock, however, and must be able to quickly change their method of tunneling to suit the conditions. One of the greatest advances in solid-rock excavation was the introduction of gunpowder blasting in the 17th century. The basic method has remained the same, although different explosives are used. Conventional blasting done with dynamite is a cyclical process in which pilot holes are first drilled for the explosives, the blast is detonated, fumes are ventilated from the shaft, and rubble is removed. In order to preserve the strength of the rock, Swedish engineers have perfected a technique called sound-wall blasting, in which numerous small charges are placed according to complicated plans. Another method that cuts a smooth surface in the tunnel is the rock mole, which dislodges rock with disk cutters or uses drill bits similar to those used in oil wells. The problem of water inflow, in which water from a subterranean source fills up the shaft, can occur in any type of tunneling operation; it is a constant danger during the construction of subaqueous tunnels. An early solution involved using a pressurized excavation chamber that held back incoming water; but the danger, expense, and slow progress of this method made it prohibitive. Alternate methods include the construction of parallel drainage tunnels and the use of prefabricated sections that can be floated into position, sunk, and attached to other sections. Tunnel building has become increasingly efficient and safe, especially with the use of innovations that stabilize the heading. In soft soils concretelike substances called grout are often injected into the tunnel area to stabilize the soil and to check water seepage. This helps prevent soil loss (the settling of earth from above), which is a major concern when tunnels are built under city buildings. A method of tunnel support, called shotcreting, was developed in Sweden. It involves the spraying of a cement mixture (shotcrete) onto the tunnel crown with a long robot arm shortly after the excavation. Shotcreting provides immediate support and minimizes soil loss. A permanent shield can then be built by thickening the concrete lining; steel ribs can be used for additional support. This method can also be used in rock tunnels.