The employment of compressed air under such conditions is really a rather simple thing in itself, and means merely that the pressure of air in the chamber where men are working is maintained at a point sufficient to offset the pressure of the hydrostatic head of water and thereby prevent its inflow. A crude comparison may be made by saying that if the ceiling of a room was weak and threatening to fall—if we filled the room with sufficient pressure of air, it would support the ceiling and prevent it falling in. In tunnel work, air is supplied under compression from the mechanical construction plant located on the surface, and the pressure of air maintained in the working chamber is determined by the depth of the work below tide level, as the hydrostatic head increases with the depth.
Control of air pressure is never entrusted to any but the most reliable, competent and experienced man, as it is of the utmost importance that air pressure be maintained properly. The first impulse of an inexperienced man, should he notice an inrush of water, would be to increase the air pressure, which might be a very dangerous thing to do. An experienced man, however, would very likely first lower his pressure in such an emergency, and then put up with the nuisance and difficulty of having a good deal of water in his working chamber. By doing this, he would permit the greater external pressure to squeeze the soil into the leaking pockets and thereby choke the leak.
Apron in Front of Shield, Five Minutes Before Shoving
To improperly or inopportunely raise the air pressure would be quite likely to result in the air blowing a hole through the roof of the tunnel heading, allowing all air pressure to escape, and permitting an uncontrollable volume of water to rush in and flood the work.
The outer shell of the tunnel shield is composed of two- or three-ply boiler plates, and the interior is braced with a system of steel girders. The shields used weighed approximately sixty-seven tons each. Sixteen or eighteen were used. To move the shield forward, each shield was equipped with sixteen hydraulic jacks, arranged around the shield circumferentially. These jacks were controlled by a series of valves, which were so designed that any one jack or any set of jacks desired could be operated. This was necessary as the direction of the shield was, as it were, guided by the pressure of the jacks. When it was desired to alter the direction of the shield, either upwards or downwards, or to the right or left, the jacks on the opposite side to which the shield was to point, were operated. The hydraulic pressure operating these jacks was 5,000 pounds per square inch, and the total energy, when all jacks were employed at the same time, was equivalent to 2,500 tons, which was equal to eleven tons per square foot of heading.
Cutting Edge of Shield in North Tunnel
Air pressure used to prevent the inflow of water and soft dirt varied from nothing up to forty-two pounds, although a fair average throughout was thirty-two pounds. It varied, of course, according to the condition encountered.
The working chamber is the space between the tunnel heading where work is in progress and the air-lock. The air-lock is a device used for the purpose of enabling workmen and materials to pass from the portion of the tunnel where the atmospheric pressure is normal into the portion where the air pressure is greater than normal; that is, the working chamber. The air-lock is a cylinder, usually about six feet in diameter and twenty feet in length, with a heavily constructed iron door at each end. This lock is placed horizontally in the tunnel at such a level as the conditions of the work necessitate, but usually near the bottom, and around this cylinder, and completely filling the cross-section of the tunnel, a concrete bulkhead is built and is known as the lock bulkhead. The two doors open in the same direction; the one at the normal pressure end opening into the cylinder, and the one at the heading end opening away from the cylinder. One door is always closed, and both doors are closed during the operation of entering or leaving the air-pressure section.