220. Open Caissons. A caisson is literally a box; and an open caisson is virtually a huge box which is built on shore and launched in very much the same way as a vessel, and which is sunk on the site of the proposed pier. (See Fig. 61.) The box is made somewhat larger than the proposed pier, which is started on the bottom of the box. The sinking of the box is usually accomplished by the building of the pier inside of the box, the weight of the pier lowering it until it reaches the bed prepared for it on the subsoil. The preparation of this bed involves the same difficulties and the same objections as those already referred to in the adoption of cribs. The bottom of the box is essentially a large platform made of heavy timbers and planking. The sides of the caissons have sometimes been made so that they are merely tied to the bottom by means of numerous tie rods extending from the top down to the extended platform at the bottom, where they are hooked into large iron rings. When the pier is complete above the water line so that the caisson is no longer needed, the tie-rods may be loosened by unscrewing nuts at the top The rods may then be unhooked, and nearly all the timber in the sides of the caisson will be loosened and may be recovered.
221. Hollow Cribs or Caissons. The foundation for a pier is sometimes made in the form of a box with walls several feet in thickness, but with a large opening or well through the center. Such piers may be sunk in situations where there is a soft soil of con siderable depth through which the pier must pass before it can reach the firm subsoil. In such a case, the crib or caisson, which is usually made of timber, may be built on shore and towed to the site of the proposed pier. The masonry work may be immedia telystarted ; and as the pier sinks into the mud, the masonry work is added so that it is al ways considerably above the water line. (See Fig. 62.) The deeper the pier sinks, the greater will be the re sistance of the subsoil, un til, finally, the weight of the uncompleted pier is of it self insufficient to cause it to sink further. At this stage, or even earlier, dredg ing may be commenced by means of a clam-shell or orange-peel dredging buck et,through the interior well. The removal of the earth from the center of the subsoil on which the pier is resting, will cause the mud and soft soil to flow toward the center, where it is within reach of the dredge. The pressure of the pier accomplishes this. The deeper the pier sinks, the greater is its weight and the 'greater its pressure on the subsoil, although this is somewhat counteracted by the constantly increasing friction of the soil around the outside of the pier. Finally the pier will reach such a depth, and the subsoil will be so firm, that even the pressure of the pier is not sufficient to force any more loose soil toward the central well. The interior
well may then be filled solidly with concrete, and thus the entire area of the base of the pier is resting on the subsoil, and the unit pressure is probably reduced to a safe figure for the subsoil at that depth.
This principle was adopted in the Hawkeshury bridge in Aus tralia, which was sunk to a depth of 1S5 feet below high water— a depth which would have been impracticable for the pneumatic caisson method described later. In this case, the caissons were made of iron, elliptical in shape, and about 4S feet by 20 feet. There were three tubes S feet in diameter through each caisson. At the bottom, these tubes flared out in bell-shaped extensions which formed sharp cutting edges with the outside line of the caisson. These bell mouthed extensions thus forced the soil toward the center of the wells until the material was within reach of the dredging buckets.
This method of dredging through an opening is very readily applicable to the sinking of a comparatively small iron cylinder. As it sinks, new sections of the cylinder can be added; while the dredge, working through the cylinder, readily removes the earth until the subsoil becomes so firm that the dredge will not readily excavate it. Under such conditions the subsoil is firm enough for a foundation, and it is thin only necessary to fill the cylinder with concrete to ob tain a solid pier on a good and firm foundation.
One practical difficulty which applies to all of these methods of cribs and caissons, is the fact that the action of a heavy current in a river, or the meeting of some large obstruction such as a boulder or large sunken log, may deflect the pier somewhat out of its intended position. When such a deflection takes place, it is difficult if not impossible to force the pier back to its intended position. It there fore becomes necessary to make the pier somewhat larger than the strict requirements of the superstructure would demand, so that the superstructure may have its intended alignment, even though the pier is six inches or even a foot out of its intended position.
222. Pneumatic Caissons. 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 tum bler 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 exem plified. It is, of course, necessary to have a means of communi cation 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.