A pneumatic caisson is essentially a large inverted box on which a pier is built, and inside of which work may be done because the water is forced out of the box by compressed air. If an inverted tumbler is forced down into a bowl of water, the large air space within the tumbler gives some idea of the possibilities of working within the caisson. If the tumbler is forced to the bottom of the bowl, the possibilities of working on a river bottom are somewhat exemplified. It is, of course, necessary to have a means of communication between this working chamber and the surface; and it is likewise necessary to have an air-lock through which workmen (and perhaps materials) may pass.

The process of sinking resembles in many points that described in the previous section. The caisson is built on shore, is launched, and is towed to its position. Sometimes, for the sake of economy (provided timber is cheap), that portion of the pier from the top of the working chamber to within a few feet below the low-water line, may be built as a timber crib and filled with loose stone or gravel merely to weight it down. This method is usually cheaper than masonry; and the timber, being always under water, is durable. As in the previous section, the caisson sinks as the material is removed from the base, the sinking being assisted by the additional weight on the top. The only essential difference between the two processes consists in the method of removing the material from under the caisson. The greatest depth to which such a caisson has ever been sunk is about 110 feet below the water line. This depth was reached in sinking one of the piers for the St. Louis bridge. At such depths the air pressure per square inch is about 48 pounds, which is between three and four times the normal atmospheric pressure. Elaborate precautions are necessary to prevent leakage of air at such a pressure. Only men with strong constitutions and in perfect health can work in such an air pressure, and even then four hours work per day in two shifts of two hours each is considered a good day's work at these depths. The workmen are liable to a form of paralysis which is called caisson disease, and which, especially in those of weak constitution or intemperate habits, will result in partial or permanent disablement and even death.

In Fig. 63 is shown an outline, with but few details, of the pneumatic caisson used for a large bridge over the Missouri River near Blair, Nebraska. The caisson was constructed entirely of timber, which was framed in a fashion somewhat similar to that shown in greater detail in Fig. 62. The soil was very soft, consisting chiefly of sand and mud, which was raised to the surface by the operation of mud pumps that would force a stream of liquid mud and sand through the smaller pipes, which are shown passing through the pier. The larger pipes near each side of the pier, were kept closed during the process of sinking the caisson, and were opened only after the pier had been sunk to the bottom, and the working chamber was being filled with concrete, as described below. These extra openings facilitated the filling of the working chamber with concrete. Near the center of the pier, is an air-lock, with the shafts extending down to the working chamber and up to the surface. The ends of three trusses, which were made part of the construction of the caisson in order to resist any tendency to collapse, are also shown.

Fig. 63. Outline of Pneumatic Caisson.

Fig. 63. Outline of Pneumatic Caisson.

A caisson is necessarily constructed in a very rigid manner, the timbers being generally 12 by 12-inch, and laid crosswise in alternate layers, which are thoroughly interlocked. An irregularity in the settling may often be counteracted by increasing the rate of excavation under one side or the other of the caisson, so that the caisson will be guided in its descent in that direction.

A great economy in the operation of the compressed-air locks is afforded by combining the pneumatic process with the open-well process described in the previous section, by maintaining a pit in the center of the caisson. A draft tube which is as low as the cutting edge of the caisson prevents a blow-out of air into the central well. The material dug by the workmen in the caisson is thrown loosely into the central well or sump, from which it is promptly raised by the dredging machinery (see Fig. 64). By the adoption of this plan, the air-lock needs to be used only for the entrance and exit of the workmen to and from the working chamber.

When the caisson has sunk to a satisfactory subsoil, and the bottom has been satisfactorily cleaned and leveled off, the working chamber is at once filled with concrete. As soon as sufficient concrete has been placed to seal the chamber effectively against the entrance of water, the air-locks may be removed, and then the completion of the filling of the chamber and of the central shaft is merely open-air work.