ADIT

horizontal opening, or entrance, made from the Earth's surface, usually to intersect a seam of coal or a mineral vein. An adit is frequently called a tunnel, a term more properly reserved for a working that is open at both ends, such as through a mountain or a ridge. Adits are often driven during tunnel construction to drain water from the workings. Adits can be driven only in hilly country where the portal will be at such an elevation as to provide an adequate slope for water to flow out and to favour removal of coal or ore. Where either a vertical shaft or an adit can be used to reach a mineral deposit, both the generally lower cost of driving an adit and the saving in the cost of pumping water and hoisting the coal or ore through the shaft dictate in favour of the adit. Consequently adits as long as one or two miles are often economically feasible. The cross section of adits is square, round, elliptical, or-more commonly-horseshoe-shaped. The diameter is usually just enough to allow a person to stand erect or to allow passage of haulage equipment. The walls may be of the natural rough rock or may be lined with concrete, masonry, wood, or steel.