The roofs of the caissons for the Municipal bridge over the Miss issippi at St. Louis consisted of :t single. layer of 12-inch timbers with sheeting of 3-inch planks, on both upper and lower surfaces, placed diagonally and well caulked. This acted as a form for the concrete filling, which was reinforced near the lower surface with 1-inch bars, 6 inches apart both longitudinally- and transversely.' In small caissons for foundations of buildings, temporary roofs are sometimes employed, which serve as forms for the concrete filling, and are removed before the working chamber is filled with concrete, in order to make the construction monolithic, with no separation between the bottom concrete and the filling.
In deep caissons, timber cribs are frequently- used upon top of the working eh:uuber, being made with solid end and side walls, braced Ivith cross walls or timbers. Such cribs are usually filled with concrete. but in some instances they are built to carry the whole load of the superstructure, and filling is omitted in order to reduce the weight upon the foundation. As they must not extend above low water, cofferdams are required on top of the crib within which to build the masonry piers.
The shafts connecting the working chambers with the tops of the caissons are steel cylinders. \Vhen the caisson is of sufficient size separate shafts are used for amen and materials. For moderate depths, where ladders are used by the men, the shaft is about 3 feet in diameter, but when elevators are employed, it is made larger. Shafts for transporting materials are generally about 2 feet in diam eter, the shaft casings frequently being made so that they may be reproved before the shaft is filled with concrete, thus eliminating the separation between the concrete used for filling the shaft and that in the shell outside and retaking a practically monolithic job when the working chamber has a concrete roof.'-' In small caissons of moderate depth, the are placed at the top of the shafts, while in large and deep caissons, they may be near the bottom, but far enough above the working chamber to provide a refuge for the men in case of accident at the bottom. The lock is simply a small room with two doors, one leading into the out side air, the other into the compressed-air shaft, connected with the working chamber. The doors are arranged to be held tightly closed when the air pressure is unequal on their two sides. In passing through the lock, the men enter the lock by the upper door, which is then closed and the air pressure in the lock is gradually raised to that in the working chamber, after which the lower door is opened and the men pass into the lower shaft and working chamber.
207. Sinking the Caissons.—The method of constructing and placing a caisson must always be determined by local conditions. When the caisson is to be sunk through a considerable depth of water, it may be constructed on ways built on land and floated to the site where it is to be sunk, and if no suitable location is at hand on shore, it may be built on barges or pontoons, from which it can be launched or lowered into position. In shoal water, a platform on piles is some times built at or around the site, upon which the caisson may be erected and from which it may be placed in position. The working chamber is built and made air-tight, then a sufficient height of crib or coffer (lain added to reach above the water when the caisson is grounded in the position in which it is to be sunk. It is then built up as the sink ing progresses so as to keep the top above water.
For the foundations of buildings, the sinking of the caissons is started in open excavation at about the level of ground water. The working chambers when of small size are constructed at a bridge shop or a wood-working shop and hauled to the site of the building, and are then set up in position and a section of concrete shell constructed on top. The air locks are then placed and the sinking proceeds. In some instances, the whole caisson is built and filled with concrete to the top before sinking begins. As a rule, however, they are built in two or three sections heights of 30 or 40 feet being sunk at once. When the working chamber is of concrete, it is built in position for sinking upon cutting edges previously placed and held in position by the concrete forms.
In large caissons sunk through water, the concrete in the cribs provides sufficient weight to cause sinking to take place as the material is removed from beneath the caisson, without the use of temporal loadings. Sometimes water jets are used to reduce friction upon sides. As in the smaller caissons used in building foundations, temporary loadings—frequently pig iron—are required to force the caissons down, some means for handling such loadings easily must be provided. It is CI minion practice to employ derricks, tvhicli handle the loads in blocks Neighing 2000 to 5000 pounds, 100 to 500 tuns total weight being generally needed.