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. 51.) 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.

Fig. 61. Section of Open Caisson.

Fig. 61. Section of Open Caisson.