Plenum Process in Caissons.—The pneumatic method of founding sub aqueous structures has of late Years been successfully applied in the erec tion of several great bridges, and a number of instructive modifications have been introduced to adapt the method most advantageously to prevail ing; local conditions. One of these modifications worthy of mention is the plenum process in caissons in which the iron cylinders are enclosed in a pier of solid masonry starting from the roof of a caisson or rectangular box of wood or of wrought iron which rests directly on the firm stratum or rock-bed.
.Fonndation of the Kehl .Bridge Piers.—Th is modification of the plenum process was first adopted in the constniction of the Rhine Bridge at Kehl (pi. 5o, fig. 9). To found the piers at this place by the well-known expe dient of the coffer-dani would have been a very costly and tedious opera tion, on account of the necessity of completing the foundations to the water-line between two floods in the river, the exceedingly rapid current, and the indefinite depth of the mud, sand, and gravel constituting the bed of the river. The first proposition, to use the Triger-plenum method with iron cylinders (above described), was also deemed impracticable because of the time that would necessarily be consumed in removing the great quan tity of material, before the needful depth would be reached, through the contracted area of an air-lock. To overcome these difficulties, M. Fleur St.-Denis proposed the following method with caisson: The caisson was a wrought-iron rectangular box, 23 by 19 feet, I closed at the top. The roof of the caisson was pierced with three cir cular openings, from which proceeded three shafts of sheet iron, the cen- • tral one 5 feet and the side ones 314'. feet in diameter. The two side tubes were each surmounted at the surface by an air-lock similar in arrangement and operation to the air-locks previously deseribed. The arrangement of the central shaft was different. This was left open at the top and bottom; its lower end extended slightly below the lower edge of the working cham ber of the caisson and dipped beneath the surface of a well into which the excavated material was dumped. A chain of dredging-buckets, operating in the central shaft, scooped the deposited material from the well in the centre, and, lifting it to the water-surface, discharged it outside. From
time to time as the caisson sank these three tubes were so lengthened by the addition of sections that their tops always extended above water.
The operation was very simple. When the compressed air was forced into the side tubes and into the working chamber of the caisson with which they communicated, the water in the chamber was forced out to the level of its bottom edge; but, as the central shaft extended somewhat below this level, its open bottom dipped under the level of the water in the centre well, and, being in communication through the loose soil with the water outside of the caisson, the water rose in this shaft to the level of the river. The water was prevented from entering the working chamber by the air pressure, which was a trifle more than sufficient to counterbalance it. The water-column in the central shaft formed a means of communication be tween the working chamber and the open air, the workmen throwing all the material which they excavated from the interior into the well in which the lower end of the shaft was immersed, and from which it was lifted to the surface and dumped outside by the dredges.
In the case of the Kehl Bridge several such caissons were placed side by side, so communicating as to form the support for one pier. From the outer edges of the caissons a wooden casing or box was carried up to and a little above the water-surface, where it was maintained by successive additions as the caisson settled. The interior of this casing was kept filled with beton poured in from time to time. This served not only to form the body of the pier, but also to give the caisson the necessary increment of weight to counterbalance the upward force exerted by the compressed air. When the desired depth was reached, the interior of the caisson and the interiors of the shafts were likewise filled with beton, when the foundation for the pier was complete.
Similar expedients have been adopted in other situations. The piers of the Parnitz Bridge at Stettin are founded upon cylindrical shafts, in sinking which, a round, bell-shaped working chamber was employed. Instead of filling ont the interior of the working chamber and shafts with beton, a filling- of sand may be employed for the same purpose.