=167. Tunnel Machines.=—Tunnel machines have been used successfully on sewer tunnels in soft materials, but not in rock.[95] The machines are of different types, but in general consist of a revolving cutting head, equipped with knives, and driven by an electric motor. The bearing on which the shaft for the cutting head rests is supported against the sides of the tunnel. The muck is carried away by means of a conveyor and dumped into muck cars without rehandling. Rapid progress can be made with these machines in suitable conditions.
FIG. 119.—Method of Drilling and Loading Rock Tunnel Face.
=168. Rock Tunnels.=—Tunnels in rock are advanced by drilling into the face as shown in diagrammatic form in Fig. 119. The holes near the center are driven in at an angle towards the center and to depths from 6 to 15 feet. The harder the rock the greater the angle with the tunnel. This is called the center cut. Other holes are driven near the outer edge of the tunnel and parallel to its axis. When fired, the wedge of rock between the center cut holes is thrown back into the tunnel and a delayed explosion then throws the sides into the hole thus made. A final delay thrusting shot throws the muck so formed away from the face of the tunnel. For tunnels up to 6 or 8 feet in height the entire bore is cut out in this fashion. For larger tunnels, the upper portion called the heading, is taken out in this way, and the remainder, called the bench, is taken out by drilling and blowing holes normal to the axis of the tunnel. The amount of powder necessary in the bench holes is much less than that required in the heading.
=169. Ventilation.=—No tunnel more than 50 feet long should be built without ventilation. A fair amount of air for ordinary conditions is 75 cubic feet of free air per minute per person in the tunnel, and double this amount for each animal. Where explosive gases are met, or under conditions where the tunnel is hot, five or six times as much air may be needed in order to cool the tunnel or to dilute the gases. In order that the air may be fresh and cool at the face of the tunnel where work is going on it should be conducted to the tunnel face in a pipe and blown out into the tunnel. Immediately following a blast at the face the current should be reversed so as to draw the poisonous gases out of the tunnel through the duct. The high pressure air line leading to the drills should be opened at the same time to create a current towards the face in order to accelerate the clearing of the air at the heading. The capacity of the air machines should be sufficient to exhaust four times the volume of the gases created by the explosion, in 15 minutes. This will ordinarily call for a capacity of about 4,000 cubic feet of free air per minute. If the same blower is to be used for exhausting the gases as for ventilation while work is going on, it should have a high overload capacity to care for this situation. The air line should be arranged to allow for reversal of flow.