Foundalthnnf The East Rircr Brag-c Picrs.—Another typical work exe cuted on the pneumatic system was the founding of the piers for tlie g,reat suspension bridge of 1600 feet span over the East River between NeW York City and Brooklyn (y5/. 48, Jig. 3). This was accomplished by the plenum process with caisson, as will appear from the following description, abridged from Trautwine: The caisson at the bottom was 168 feet long and 102 feet wide (pl. 50, 7). To facilitate the work, as well as to serve as a precaution against accidents, six shafts arranged in pairs were provided. Two of these were water-shafts, 7 by feet in dimensions, for removing with buckets and with hoisting-apparatus the material excavated beneath the caisson; two were air-shafts, 21 inches in diameter, through which air wa; forced from above, to expel the water front the working chamber, so as to allow the laborers to undermine beneath the caisson; and two were supply-shafts, 42 inches in diameter, for admitting laborers, tools, etc. The supply shafts were furnished with air-locks on the principle of that above de scribed. The shafts were of quarter-inch boiler-iron. The foot of the caisson was formed tapering to a sharp edge, the better to facilitate settle ment, and the bottom was shod with cast iron. The wood-work of the interior of the working chamber was made fireproof by a lining of sheet iron. From the bottom np to 14 feet the caisson was built of horizontal layers of timbers of foot-square section, the layers crossing one another at right angles, the timbers being strongly bolted together and the joints made watertight by pitching- and calking. It was then launched and floated to position, and to sink it there were added fifteen courses of tim bers, some one foot square laid one foot apart, the intervals between them being, filled with concrete. When the caisson was sitnk and the water forced out from the working chamber, the enclosed area of the river-bot tom was uniformly excavated, so that the caisson should slowly descend until it reached a solid substratum. The working chamber as well as the shafts was then filled with concrete. A coffer-dant was built from the top of the caisson, and in this the masonry of the great tower was started.
Piers of Ihe Bridge al SI. Louis. portion of the piers of the .,Treat arch briclg-e over the Mississippi at St. Louis was built by the plenum pro cess with the Ilse of the caisson. In this instance the air-locks were not placed at the tops of the shafts above the water-surface, but were located directly over the working, chambers, into which they partly projected. These air-locks were surmounted by shafts open above and communicating directly with the atmosphere. Trantwine refers to this work as affording an excellent illustration of the effective use of the sand-pump in raising sand from cylinders while being sunk in water. By means of a pump-pipe of 31„ inches area, and with a water-jet under a pressure of one hundred and fifty pounds to the square inch, twenty cubic yards of sand per hour were raised 125 feet. In founding this bridge there was attained a depth of t to feet beneath the water-surface (jig. to)—a greater depth than elsewhere had ever been reached by any similar structure.
The arrangement of the air-lock—which has several times been referred to—is shown in its simplest form in Plate 5o ( Jig. r). Its object is to permit those employed in the work to enter and leave the working chamber, and to allow the excavated material to be lifted out of the interior, without suffering the compressed air to escape. It is a small iron chamber securely bolted upon the top of the last section of the cylin der. It is provided at the top and bottom with air-tight doors or flaps, both opening downward, or toward the cylinder.. The upper door affords communication between the air-lock and the outer air; the lower door forms the means of commnnication between the air-lock and the interior of the cylinder or working chamber of the caisson. A valve is placed in
the top-of the lock, communicating with the outer air, and a similar valve is placed in the bottom, communicating with the compressed-air workings. From the illustration it will be understood that the chamber of the air-lock may be placed in communication either with the outer air or with the com pressed air of the working chamber by opening or closing the valves just described.
Operation of the a workman wishes to enter from the outside, he must first open the valve in the top of the.air-lock, which per mits the compressed air in the air-lock chamber to escape, whereupon the flap communicating with its interior may be opened. He thereupon enters the air-lock, closing the flap behind him, and likewise shutting off the valve in the top of the lock-chamber. He then opens the valve in the bot tom of the lock, establishing communication with the compressed-air cham ber. The air escapes from the working chamber into the lock; and when equilibrium between the two has been established, the door between the two falls open of itself, and the man may descend to the floor of the work ing chamber by the ladder: In leaving the working chamber and ascend ing to the outer air, these same manceuvres are performed in the reverse order. The same series of operations is required for the raising and low ering of the buckets conveying the excavated material from the workings to the outer air. In the founding of the Kehl Bridge the removal of the excavated stnff was accomplished by a continuous chain of buckets work ing up and down in an open central shaft (see p. 2S8), and a similar plan is adopted in most works where the pnemnatic method is pursued.
Caisson pneumatic founding the effect of the compressed air upon the workmen does not appear to be notably injurious, so long as the compression does not exceed two atmospheres above atmospheric pres sure, which is equivalent to a depth of about 6S feet below water-surface and to a total pressure of fortv-five pounds to the square inch, or thirty pounds above the normal pressure. I3eyond this the effects are liable to be dangerous, and even fatal. The younger Roebling, who finished the East River Bridge, has suffered seriously from this cause, and the medical pro fession has recognized a distinct type of disease, originating from exposure to compressed air, which is designated as the " caisson disease." Excavating by the Freezing Process.—One of the most original contri butions to this branch of engineering is the method lately devised by Poetsch for sinking shafts and making excavations for the foundations of buildings, bridges, and other structures, and for constructing subaqueous tunnels, bridge foundations, conduits, etc., and, generally speaking, for the execution of all kinds of excavation in quicksand and other unstable and water-bearing strata. The Poetsch system is a radical departure from all methods previously in vogue, and has the merit of great sim plicity. It consists in freezing the material surrounding the space to be excavated, and in thus converting it temporarily into a solid mass or wall which permits the excavation to be made and the permanent work to be executed with the same certainty as similar work in solid ground, and with security against collapse. It dispenses with the pumping plant for keeping the workings free from water while the excavation is going on, and with the elaborate methods of holding the water baek by compressed air, as in the several modifications of tbe plenum process above de scribed, which involves the use of caissons, air-locks, and air-compressing machinery.