The diameter of the air pipe should be determined by a study of the saving of the cost and operation of the air equipment compared to the increased cost of a larger pipe line. Other factors affecting the size of the pipe line to be used are: the available space in the tunnel, the temporary character of the installation, the use of the exhaust from high-pressure air machines for the purpose of ventilation, etc. Cast-iron, spiral-riveted galvanized sheet iron, and canvas pipes have been used for conducting low-pressure ventilating air.
Ventilation in tunnels working under air pressure is supplied from the compressors, and the air is delivered near the face of the heading, except that being used in the locks. In tunnels using air drills, the air for the drills is conducted through a separate pipe as it is not economical to compress the ventilating air to the pressure necessary to operate the drills.
170. Compressed Air.—Compressed air is used in tunnel work to prevent the entrance of water into the tunnel and to keep the work dry. The pressure of air used is closely that of the pressure of the ground water but in a large tunnel or a tunnel with a weak roof the pressure may be somewhat lower on account of the danger of blowing through the roof. It is evident that the water pressure cannot be balanced at the top and the bottom of the tunnel. To balance it at the bottom makes a blow out near the top more probable. To balance the pressure at the top may leave the bottom wet. Judgment and care must be exercised during construction and if the pressure is balanced at or near the bottom the roof must be carefully guarded by grouting and puddling with clay, or the surface, particularly if under water, may be covered with a clay bank. If the cavities in the tunnel lining are large, sawdust can be mixed with the grout to advantage, the mixture being pumped through holes in the roof by hand or power operated force pumps. “Blows” must be carefully guarded against as they endanger the lives of the workmen and threaten the loss of the tunnel. The pressure and volume of air supplied for some large subaqueous tunnels is shown in Table 61.
Labor under compressed air is arduous and dangerous with the best of safeguards.[[96]] Pressure more than about 43 pounds per square inch cannot be used and at this high pressure men cannot work more than four hours at a time. Little or no distress is noted at pressures less than 15 pounds.
| TABLE 61 | |||||
|---|---|---|---|---|---|
| Volume and Pressure of Compressed Air in Tunnels | |||||
| (American Civil Engineers Pocket Book) | |||||
| Tunnel | Maximum Distance High Water to Invert, Feet | Minimum Cover in Feet | Maximum Air Pressure, Pounds per Square Inch | Average Air Pressure, Pounds per Square Inch | Conditions and Cubic Feet of Free Air per Minute |
| City and South London | 34 | 42 | 15 | In water bearing-sand. 1660 cubic feet per minute per face. When grouted 1000 to 1300 cubic feet per minute per face | |
| Blackwall | 80 | 5 | 37 | 35 | 10,000 cubic feet per minute per face in open ballast for some time |
| Baker St. and Waterloo | 70 | 18 | 35 | 28 | In gravel, 3300 cubic feet of air per minute per face. Parallel tunnel 1650 cubic feet per min. per face |
| Greenwich | 70 | 30 | 28 | 20 | Average 83.5 per man per minute. Never less than 66.7 |
| Battery, East River. N. Y. | 94 | 12 | 42 | 26 | In sand. Two working faces. Maximum 32,000 |
| East River, N. Y., Penn. R.R. | 93 | 8 | 42 | 27 | Maximum for one face 25,000 cubic feet per minute for 24 hours. Capacity of plant for 8 faces, 80,400 cubic feet per minute |
| North River, N. Y., Penn. R.R. | 98 | 20 | 37 | 26 | Maximum in gravel 10,000 cubic feet per man per hour. Generally ranged 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.
Explosives and Blasting[[97]]
171. Requirements.—The desirable features in an explosive to be used in trenching and tunneling in rock are: (1) stability in make up so as not to deteriorate in strength or to become dangerous during storage, (2) imperviousness to ordinary variations in temperature and moisture, (3) insensibility to ordinary shocks received in transportation and handling, (4) not too difficult of detonation, (5) convenient form for transportation and loading and for making up charges of different weights, (6) the non-formation of poisonous gases when fired, (7) imperviousness to water and usefulness in wet holes, (8) power without bulk, etc.
172. Types of Explosives.—Explosives which fill some or all these requirements can be divided into two classes, deflagrating and detonating. A deflagration is an explosion transmitted progressively from grain to grain. A detonation is a sudden disruption caused by synchronous vibrations of a wave-like character. The deflagrating explosives are represented by gun-powders and contractors’ powders. They must be carefully tamped in the hole to develop their full power and they must be ignited by a fuse or flame. They are valueless in water or moist holes. These powders are used mainly for loosening frozen earth, soft sandstone, cemented gravels and similar materials where a thrusting action rather than a disruption is desired. The detonating explosives are most commonly represented by the dynamites. These are exploded by a shock usually caused by another explosive which has been ignited by a fuse or electric spark, and which is known as the “detonator.” Detonating explosives are more powerful than deflagrating explosives and are used in all but the softest materials.
Gunpowder.—This is a mechanical mixture of sulphur, charcoal, and saltpeter generally in the proportions of 10 parts sulphur, 15 parts charcoal, and 75 parts saltpeter (sodium nitrate). It weighs about 62½ pounds per cubic foot and produces about 280 times its own volume in gas at a pressure of 4.68 tons per square inch at a temperature of 32 degrees F., which amounts to a pressure of approximately 38 tons per square inch at the temperature of explosion of 4,000 degrees F.