Code 6251 applies to specialist contractors who perform tunnel construction.  This classification applies to all operations including lining.  It does not include subway construction.  The operations include earth, rock, and water removal by mechanical equipment, explosives, and pumps and drains piped to the outside of the bore which will vary in size depending upon the ultimate use of the tunnel.  Shaped steel I-beams are placed as the bore progresses, to which horizontal beams or solid bar stock are welded or bolted for support.  Shielding the bore, between the I-beams, requires bolting wood bulkheads or welding steel plates to the beams followed by filling gaps between the face of the bore and shielding with sand or concrete grout.  The tunnel will then be lined with concrete, steel or tile, if required, and lighting, ventilation, and drains installed.

This classification also applies to employers engaged in pneumatic tunneling operations which includes lining, all employees working under air pressure, and all others working in the tunnel or in work on the apparatus connected therewith.  Steel caissons, equipped with a shield and air locks for workers and material, are sunk to the required depth at the portals (also equipped with locks) at each end of the tunnel to be constructed.  The work proceeds to the center (until breakthrough) by pushing the shield (a long steel tube) through the inside of the caisson. Material is hauled back through the caisson and lifted to the surface for removal.  As the shield moves forward, a steel lining is constructed in the tunnel bore. Concrete is then forced between the lining and the walls of the hole. At breakthrough, the portals are opened to the ground connections.