When used in this Act, unless the context indicates otherwise, the term
     (a) “Caisson” means a wood, steel, concrete or reenforced concrete air-tight and water-tight chamber in which it is possible for men to work to excavate material.
     (b) “Lock” means a chamber designed to facilitate the passage of men and materials from an air pressure greater than normal, as in a compartment, caisson or tunnel, to the ground or water level or normal air pressure.
     (c) “Pressure” means gauge air pressure in pounds per square inch.
     (d) “Tunnel” means a subterranean passage or chamber.