CONSTRUCTION OF TEE CAISSON. Wood Caisson. The con struction of the caisson differs materially with its depth. The simplest form is made by erecting studding by toe-nailing or tenoning them into the top course of the crib and spiking planks on the outside. For a caisson 6 or 8 feet deep, which is about as deep as it is wise to try with this simple construction, it is sufficient to use studding 6 inches wide, 3 inches thick, and 6 to 8 feet long, spaced 3 feet apart, mortised and tenoned into the deck course of the crib. The sides and floor (the upper course of the crib) shoWd be thoroughly calked with oakum. The sides may be braced from the masonry as the sinking proceeds. When the crib is grounded and the masonry is above the water, the sides of the box or caisson are knocked off.
When the depth of water is more than 6 to 8 feet, the caisson is constructed somewhat after the general method shown in Fig. 89. The sides are formed of timbers framed together and a covering of thick planks on the outside. The joints are carefully calked to make the caisson water-tight. In deep caissons, the sides can be built up as the masonry progresses, and thus not be in the way of the masons. The sides and bottom are held together only by the heavy vertical rods; and after the caisson has come to a bearing upon the soil, and after the masonry is above the water, the rods are detached and the sides removed, the bottom only remaining as a part of the permanent structure.
For an illustration of the form of caisson employed in sinking a foundation by the compressed-air process, see Fig. 92 and 93, page
432 and 434.
The caisson should be so contrived that it can be grounded, and afterwards raised in case the bed is found not to be accurately leveled. To effect this, a small sliding gate is sometimes placed in the side of the caisson for the purpose of filling it with water at pleasure. By means of this gate, the caisson can be filled and grounded; and by cloy ig the gate and pumping out the water, it can be set afloat. The same result can be accomplished by putting on and taking off stone.
Since the caisson is a heavy, unwieldy mass, it is not possible to control the exact position in which it is sunk; and hence it should be larger than the base of the proposed pier, to allow for a little adjust ment to bring the pier to the desired location. The margin to be allowed will depend upon the depth of water, size of caisson, facil ities, etc. A foot all round is probably none too much under favor able conditions, and generally a greater margin should be allowed.