AIR-LOCK, a steel-plated chamber provided in a tunnelling shield or a caisson (q.v.), to prevent the escape of compressed air during the passage of workers. The pressure in the caisson may be 25 to 3o lb. per sq.in., though higher pressures have been used. A steel door at each exit of the lock provides for the safe passage of men or material, one being opened while the other is shut. There will be several locks in a tunnel shield and in some big caissons. The diagram indicates the shape of a caisson, sunk to make the foundation for a bridge pier, two shafts for ma terial and one for men being fitted. The shaft down which concrete is shot has its air-lock placed lower than those of the other shafts, because the con crete-mixer happens to be located on the lower platform.
Air-lock also means the trap ping of air in a bend ; e.g., of a water or petrol supply, stopping the flow.