TUNNEL, a more or less horizontal underground passage made without removing the overlying rock or soil. In former times any long tube-like passage, however constructed, was called a tunnel. In 1928 the word was sometimes popularly applied to an underground passage constructed by trenching down from the surface, building the arching or other form of structure and then refilling over the top with soil; but a passage so constructed, although indistinguishable from a tunnel when completed, is more correctly termed an "aqueduct," a "covered way" or "subway" (depending on its use) and the operations "cutting" and "cover ing," instead of tunnelling. Making a small tunnel, afterwards to be converted into a larger one, is called driving a heading, and in mining operations small tunnels are termed galleries, drifts and adits. If the passage leading underground is vertical it is called a shaft ; if inclined from the vertical it is called a slope, or inclined shaft ; if the shaft or slope is begun at the surface the operations are known as sinking ; and if worked upwards from a previously constructed heading or gallery the operations are called risings or stopes. (See COAL AND COAL MINING; MINING, MET3LLIFEROUS.) Tunnels may be driven through earth, which usually requires timbering during excavation (and there are many systems of timbering) or the use of a shield ; or in rock, which may or may not require timbering, depending on the firmness. (See Plate, figs. 2 and 3.) In earth, and often in rock some form of permanent lining is necessary and it is sometimes formed of closely spaced heavy timbers but more often of some type of masonry. Tunnels may be excavated either above or below water level, and in the former case excavated in free air, but, in the latter, where in earth, modern practice usually requires the use of compressed air.
Tunnelling has been effected by natural forces to a far greater extent than by man. In limestone districts innumerable swallow holes, or shafts, have been created by sinking rain water follow ing joints and dissolving the rock, and from the bottom of these shafts tunnels have been made by the water to the sides of hills in a manner strictly analogous to the ordinary method of executing a tunnel by sinking shafts at intervals and driving headings there from. Many rivers thus find a course underground. In Asia
Minor one of the rivers on the route of the Mersina railway extension pierces a hill by means of a natural tunnel. The Mam moth Cave of Kentucky, the caverns at Carlsbad, New Mexico, and the Peak caves of Derbyshire are due to natural tunnelling.
Mineral springs bring up vast quantities of matter in solution. It has been estimated that the Old Well spring at Bath has dis charged since the beginning of the 19th century solids equivalent to the excavation of a 6 ft. by 3 ft. heading, 9 m. long; and yet the water is perfectly clear and the daily flow is only the 150th part of that pumped out of the great railway tunnel under the Severn. Tunnelling is also carried on to an enormous extent by the action of the sea.
Subaqueous Demolitions.—The most gigantic subaqueous demolition hitherto carried out by man was the blowing up in 1885 of Flood rock, a mass about 9 ac. in extent, near Long Island sound, N.Y. To effect this gigantic work by a single in stantaneous blast a shaft was sunk 64 ft. below sea-level, from the bottom of which 4 m. of tunnels or galleries were driven so as completely to honeycomb the rock. The roof rock ranged from 10 to 24 ft. in thickness, and was supported by 467 pillars 15 ft. square; 13,286 holes, averaging 9 ft. in length and 3 in. in diameter, were drilled in the pillars and roof. About 8o,000 cu. yd. of rock were excavated in the galleries and 275,000 remained to be blasted away. The holes were charged with 110 tons of "rackarock," a more powerful explosive than gunpowder, which was fired by electricity. The sea was lifted roo ft. over the whole area of the rock.