PNEUMATIC PILES. Pneumatic piles were once consider ably used for bridge piers, but have now been superseded for that purpose by pneumatic caissons. Since 1894 the compressed-air process has been frequently employed in constructing foundations for tall buildings on Manhattan Island, New York City, and in some cases the pneumatic pile has been used, although it is there usually called a caisson. The following description applies more particularly to the pneumatic piles formerly used for bridge piers,—the practice in New York City being considered later.
The cylinders were made of either wrought or cast iron. The wrought-iron cylinders were composed of plates, about half an inch thick, riveted together and strengthened by angle irons on the inside, and reinforced at the cutting edge by plates on the outside both to increase the stiffness and to make the hole a little larger so as to diminish friction. The cast-iron cylinders were composed of sections, from 6 to 10 feet long and 2 to 8 feet in diameter, bolted together by inside flanges, the lower section being cast with a sharp edge to facilitate penetration. Two of these tubes, braced together, were employed for ordinary bridge piers; and six small ones around a large one for a pivot pier. They were filled with concrete, with a few courses of masonry or a heavy iron cap at the top.
Fig. 91 shows the arrangement of the essential parts of a pneu matic pile. The apparatus as shown is arranged for sinking by the plenum process; for the vacuum process the arrangement differs only in a few obvious particulars. The upper sec tion constitutes the air-lock. The doors A and B both open down wards. To enter the cylinder, the workmen pass into the air-lock, and close the door A. Opening the cock D allows the compressed air to enter the lock; and when the pressure is equal on both sides, the door B is opened and the workmen pass down the cylinder by means of a ladder. To save loss of air, the air-lock should be opened very seldom, or made very small if required to be opened often.
The air-supply pipe connects with a reservoir of compressed air on a barge. If the air were pumped di
rectly into the pile without the in tervention of a storage reservoir, as was done in the early applications of the plenum process, even a momentary stoppage of the engine would endanger the lives of the workmen.
The soil was excavated by ordinary hand tools, elevated to the air-lock by a windlass and bucket, and passed out through the main air-lock. Sometimes a double air-lock with one large and one small compartment was used, the former being opened only to let gangs of workmen pass and the latter to allow the passage of the skip, or bucket, containing the excavated material. Sometimes an auxiliary lock, F G, was employed. The doors F and G are so con nected by parallel bars (not shown) that only one can be opened at a time. The excavated material is thrown into the chute, the door F is closed, which opens G, and the material discharges itself on the outside.
Mud and sand are blown out with the sand-lift (§ 876) or mud pump (§ 877) without the use of any air-lock.
The cylinders were guided in their descent by a framework resting upon piles or upon two barges. One of the chief difficulties in sinking pneumatic piles was to keep them vertical. If the cylinder became inclined, it was righted (1) by placing wooden wedges under the lower side of the cutting edge, or (2) by excavating under the upper side so that the air could escape and loosen the material on that side, or (3) by drilling holes through the uppermost side of the cylinder through which air could escape and loosen the soil, or (4) by straining the top over with props or tackle. If several pneumatic piles are to form a pier, they should be sunk one at a time, for when sunk at the same time they are liable to run together.
After the cylinder had reached the required depth, concrete enough to seal it was laid in compressed air; and when this had set, the remainder was laid in the open air. A short section at the top was usually filled with good masonry, and a heavy iron cap was put over all.