CRIB AND OPEN-CAISSON PROCESS.
Unfortunately there is an ambiguity in the use of the word caisson. Formerly it always meant a strong, water tight box having vertical sides and a bottom of heavy timbers, in which the pier is built and which sinks, as the masonry is added, until its bottom rests upon the bed prepared for it. With the intro duction of the compressed-air process, the term caisson was applied to a strong, water-tight box—open at the bottom and closed at the top—upon which the pier is built, and which sinks to the bottom as the masonry is added. At present, the word caisson generally has the latter meaning. In the pneumatic process, a water-tight box—open at the top—is usually constructed on the roof of the working chamber ("pneumatic chamber"), inside of which the masonry is built; this box also is called a caisson. The caisson open at the bottom is sometimes called an inverted caisson, and the one open at the top an erect caisson. The latter when built over an inverted, or pneumatic, caisson, is sometimes called a coffer-dam. For greater clearness the term caisson will be used for the or pneumatic, caisson; and the erect caisson, which is built over pneumatic caisson, will be called a A caisson employed in other than pneumatic work will be called an open caisson.
This method of constructing the foundation consists in building the pier in the interior of an open caisson, which sinks as the masonry is added and finally rests upon the bed prepared for it. The masonry usually extends only a foot or two below extreme low water, the lower part of the structure being composed of timber crib-work, called simply a crib. The open caisson is built on the top of the crib, which is practically only a thick bottom for the box. The timber is employed because of the greater facility with which it may be put into place, as will appear presently. Timber, when always wet, is as durable as masonry; and ordinarily there is not much difference in cost between timber and stone.
If the soil at the bottom is soft and unreliable, or if there is danger of scour in case the crib were to rest directly upon the bottom, the bed is prepared by dredging away the mud (§ 839) to a sufficient depth or by driving piles which are afterwards sawed oft (˘ 791) to a horizontal plane.