VOLUME AND PRESSURE OF COMPRESSED AIR IN TUNNELS
(American Civil Engineers Pocket Book) ──────────┬────────┬───────┬─────────┬─────────┬─────────────────────── Tunnel │ │ │ Maximum │ Average │ Conditions and Cubic │Maximum │ │ Air │ Air │ Feet of Free Air per │Distance│ │Pressure,│Pressure,│ Minute │ High │ │ Pounds │ Pounds │ │Water to│Minimum│ per │ per │ │Invert, │ Cover │ Square │ Square │ │ Feet │in Feet│ Inch │ Inch │ ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── City and │ │ │ │ │In water bearing-sand. South │ │ │ │ │ 1660 cubic feet per London │ │ │ │ │ minute per face. When │ │ │ │ │ grouted 1000 to 1300 │ │ │ │ │ cubic feet per minute │ 34│ 42│ 15│ │ per face ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── Blackwall │ │ │ │ │10,000 cubic feet per │ │ │ │ │ minute per face in │ │ │ │ │ open ballast for some │ 80│ 5│ 37│ 35│ time ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── Baker St. │ │ │ │ │In gravel, 3300 cubic and │ │ │ │ │ feet of air per Waterloo│ │ │ │ │ minute per face. │ │ │ │ │ Parallel tunnel 1650 │ │ │ │ │ cubic feet per min. │ 70│ 18│ 35│ 28│ per face ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── Greenwich │ │ │ │ │Average 83.5 per man │ │ │ │ │ per minute. Never │ 70│ 30│ 28│ 20│ less than 66.7 ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── Battery, │ │ │ │ │In sand. Two working East │ │ │ │ │ faces. Maximum 32,000 River. │ │ │ │ │ N. Y. │ 94│ 12│ 42│ 26│ ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── East │ │ │ │ │Maximum for one face River, │ │ │ │ │ 25,000 cubic feet per N. Y., │ │ │ │ │ minute for 24 hours. Penn. │ │ │ │ │ Capacity of plant for R.R. │ │ │ │ │ 8 faces, 80,400 cubic │ 93│ 8│ 42│ 27│ feet per minute ──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── North │ │ │ │ │Maximum in gravel River, │ │ │ │ │ 10,000 cubic feet per N. Y., │ │ │ │ │ man per hour. Penn. │ │ │ │ │ Generally ranged R.R. │ 98│ 20│ 37│ 26│ between 1500 and 5000 ──────────┴────────┴───────┴─────────┴─────────┴───────────────────────
Entrance and exit to the tunnel are gained through air locks. These are sheet iron cylinders concreted into the lining of the tunnel or shaft. Air-tight iron doors are provided at both ends, which open inwards towards the tunnel. On entering the lock from the outside the door to the tunnel is found tightly closed. The outside door is then closed by hand, the air valve is opened and air is admitted to the lock until the pressure on the lock side of the tunnel door equalizes that on the tunnel side and the tunnel door is swung open by hand. When the lock is open to the tunnel the pressure in the tunnel keeps the outside door closed. In order to leave the tunnel the process is reversed. Materials are passed through the lock by the lock tender or tenders who pass through the lock with the material if the pressure is low, or who manipulate the air outside of the lock if the pressure is high. If pressures of 30 to 40 pounds are being used, two or even three locks may be necessary.