Page:St. Nicholas (serial) (IA stnicholasserial402dodg).pdf/90

 640 observed that the shield was larger in diameter than the lining of the tunnel, and that it fitted over the end of the lining like a cap. We saw also how the shield was moved forward. A number of hydraulic jacks were placed all around the shield between the diaphragm and the lining of the tunnel. Then, when all was ready, the water was turned on in the jacks, forcing the plungers out, and pushing the shield bodily forward a distance of about two feet, or just enough to get in another ring of lining. The work required great care because with the jacks the shield was steered and made to move up or down, or to the right or left, so as to follow the course of the tunnel as planned. The tunnel was being pushed through from the other side of the river as well, and, unless the work followed the survey accurately, the headings would not meet properly at the center of the river. Just how accurately the steering was done we learned many, many months later, when the shields of the two headings met. One of the shields was four inches lower than the other, but otherwise the alinement was perfect. Think of it! Only four inches out, after groveling in the dark through a mile of silt!

The way the lining was set in place was interesting, There was an “erector,” or a sort of hydraulic crane, mounted on the face of the shield, with which the lining plates were picked up and placed in position after the shield had been moved forward, These plates were curved to the arc of the tunnel, and had deep flanges on all four sides through which the bolts were passed that fastened them one to the other. The deep flanges made them very strong indeed.

For four hours we toiled steadily. It seemed eight before the gang knocked off for luncheon, I was disappointed to find that the dawn was only just breaking when we emerged from the tunnel. We had n’t thought about eating, and had brought no lunch-pail. The idea of taking lunch at four o’clock in the morning would have seemed ridiculous to us. Needless to say, the idea was far from ridiculous now. Hot coffee was served in the sand-hog house, but we were ravenous for something more substantial. There were no restaurants open in that vicinity at that time of the morning, One of the men took pity on us and gave us a few bites of his luncheon, for which we were truly grateful.

He was a fine fellow, an old hand at the game, and he knew all there was to know about pneumatic work. He it was who explained our problem of the tumbler.

“It’s simple enough,” he said; “the pressure of the water depends on the depth, and so there is more water-pressure at the bottom of the tunnel than at the top; but there is n't any difference worth mentioning in the air-pressure between the top and bottom of the tunnel. If the material out in front of the tunnel was very soft, and we made the air-pressure heavy enough to keep out the water at the bottom of the heading, it would all escape out of the top; and if the air-pressure was just equal to the water-pressure at the top of the tunnel, the water would pour in at the bottom. Just now the material we are going through is clay-like, and we don’t have to bother very much about differences of pressure at the top and bottom of the tunnel; but when we go through quicksand, with very little ‘cover’ between the shield and the bed of the river, then comes trouble. We don’t dare work out in front of the diaphragm, but must open small shutters in the diaphragm and ‘scoop’ out the sand, That ’s when we are apt to have blow-outs. The air will burst through the fluid sand and boil up. Sometimes a burst of air will make the water shoot up like a geyser from the surface of the river.”

“What happens when you strike a rock?” Will inquired.

“We have to blast it out of the way, The worst trouble comes when we strike a ledge at the floor of the tunnel, and have soft silt or quicksand overhead. We had a job like that in the North River once. A shelf, or ‘apron,’ was built out from the shield, half-way up, virtually dividing the front of the shield into an upper and lower chamber. Under protection of this apron, workmen crawled out in front of the shield, drilled holes in the rock for mild charges of explosive, and then crept back within the shield and set off the dynamite. After that they had to crawl out again and haul the broken rock away. It was slow work, because the operations had to be carried on in cramped quarters, and only a little of the rock could be blasted at a time, Fortunately, there was very little rock to pass through. It was merely a reef in the ocean of silt. Before we struck that reef, we found the material so soft that we did n’t bother to dig it away in front of the shield, but merely pushed the shield ahead through the silt with the hydraulic jacks.”

Our friend was in the midst of his explanations when the signal came to resume work, Our half-hour of respite had seemed like only five minutes. We were aching all over. How could we ever endure the three and a half hours of labor before the next shift came on? Luckily for us, the boss did not pay as much attention to us this time as he did before, and we could ease up a bit on our work without having him bawl out at us to “Git busy there!” every two minutes.