Page:St. Nicholas, vol. 40.1 (1912-1913).djvu/768

 534

Will explained that we were anxious to know how foundations are sunk.

“Simply a case of dig, dig, dig,’ said Mr. Squires, “until we get to rock; when we get down to water, we keep it out of the excavation with compressed air.”

“How do you do that?”

“On the same principle as the diving-bell. You know, if you plunge a tumbler mouth down into a basin of water, the air trapped in the tumbler will keep the water from filling it completely. If enough air were pumped into the tumbler, it would be possible to keep out every drop of water. We do that very thing in building foundation piers. First we make a big diving-bell, called a caisson. It is a large box of wood, or steel, or concrete, with the top and bottom open. At the bottom, the box has fairly sharp cutting edges; about seven feet up from this cutting edge, there is a horizontal partition called the ‘deck.’ This is made very strong, because it has to carry the weight of the whole concrete column while the digging is going on. ‘Sand hogs,’ as we call them, get into and out of the working chamber under this deck through a tube, or ‘shafting, as it is called. They dig away the soil and gravel below, constantly undermining the caisson, so that it gradually sinks into the earth. As the caisson is sunk, the concrete pier is built up on its deck, and its weight helps to force the cutting edges into the ground. As the work progresses, new caisson sections are added on top, and the shafting is extended for the sand hogs and excavating material.”

“But where does the water come from?” I asked.

“The lower end of New York is built over a deep layer of sand and porous soil that is saturated with water from the river. About fifty feet below the curb, in this part of New York, we come to water; then we have to put on the air-pressure to keep it out. The first thing we must do is to put an “air-lock” on the shafting, so as to let the men in and out without losing the