On one occasion, when for some reason the shield was stopped with the shove only partly done, and the exhaust valves had not been shut off, the platforms continued to slide and allowed the face to collapse; the shield platforms and doorways, however, caught the falling sand and gravel and the flow choked itself.
As soon as the rock surface was penetrated and the sand and gravel were met, which happened almost at the same time in the two Weehawken Tunnels, the escape of air increased enormously, and it at once became clear that it was impossible to keep enough air in the two tunnels by the methods then in use, even when working the three compressors, each capable of compressing 4,400 cu. ft. of free air per min. at top speed. When the shields just entered the sand and gravel, the face had been held by light breasting, without any special effort to prevent the escape of air, but when it was found impossible to supply enough air, a large amount of straw and clay was used in front of the boards.
This cut down the escape, but, as much air was escaping through the joints of the iron lining, these were plastered with Portland cement. Even then, the loss was too great, therefore one tunnel was shut down entirely and all the air was sent to the other. This allowed a pressure of 10 lb. to be kept up in the working tunnel, and this, though less than the head, was enough to allow progress to be made. In order to use one tunnel as a drain for the other, the two faces were always kept within 150 ft. of each other by working them alternately. The timbered face was never grouted, though this would have reduced the loss of air, as at the same time it would have decreased the progress very much, and any one who saw the racing engines in the power-house, and realized that a breakdown of one of them would mean the loss of the faces, was ready to admit that the quicker this particular period was cut short, the better.
Above the sand and gravel lay the silt, and, when it showed in the roof, the escape of air was immediately reduced and the two faces could be worked simultaneously. Almost at the same time the piles supporting the large warehouse, known as the Fowler Building, were met. Although the face now took much less timber, the same system of breast boards as had been used in the gravel was kept up, but in skeleton form. They were set 2 ft. 6 in. ahead of the shield, however, instead of 1 ft. 3 in., and the transverse roof poling boards were replaced by longitudinals resting on the shield. The more piles in the face the less timbering was done. The piles were cut into handy lengths with axes and chisels.
All timbering was light compared with the weight of the ground, but, as the shove took place as soon as the set was made, it served its purpose. When a face was closed down the whole system was greatly reinforced by braces from the shield, the face of which was closed by the doors.
In driving through such a face the typical 8-hour shift gang was about as follows:
| General: | ||||||
| ⅓ | Tunnel superintendent | @ | $300.00 | per | month. | |
| 1 | Assistant tunnel superintendent | " | 5.00 | per | day. | |
| 1 | General foreman | " | 5.00 | " | " | |
| ½ | Pipefitter | " | 3.25 | " | " | |
| ½ | Pipefitter's helper | " | 2.75 | " | " | |
| ½ | Electrician | " | 3.00 | " | " | |
| ½ | Electrician's helper | " | 2.75 | " | " | |
| Timbering: | ||||||
| 3 | Timbermen | " | 2.50 | " | " | |
| 3 | Timbermen's helpers | " | 2.00 | " | " | |
| Mucking: | ||||||
| 1 | Foreman | " | 3.50 | " | " | |
| 6 | Muckers | " | 2.75 | " | " | |
| Erecting Iron and Driving Shield: | ||||||
| 1 | Erector runner | " | 3.25 | " | " | |
| 1 | Foreman | " | 4.00 | " | " | |
| 4 | Iron workers | " | 3.00 | " | " | |
The drillers were not kept on after the rock disappeared; a foreman was added who divided his time between iron erection and mucking.