CAISSON, in water-building, a large chest of strong timber, made watertight, and used in large and rapid rivers for building the pier of a bridge. The bottom consists of a grating of timber, so contrived as to be detached from the sides when necessary. The ground under the intended pier is first levelled, and the caisson being launched and floated to a proper position, is sunk, and the pier built as high as the level of the water, or nearly so; then the sides are detached, and the bottom remains as a foundation for the pier.
The most considerable work that has come to our know ledge, where caissons have been used, is Westminster Bridge ; of this, therefore, a particular account may be acceptable. Each of the caissons contained 150 loads of fir timber, and more tonnage than a man-of-war of 40 guns ; their size was nearly 80 feet from point to point, and 30 feet in breadth ; the sides, 10 feet in height, were funned of timbers, laid horizontally over each other, pinned with oak trunnels, and framed together at all corners, except the salient angles, where they were secured by proper iron work, which being unscrewed, would permit the sides of the caisson, had it been found necessary, to divide into two parts. These sides were planked across the timbers, inside and outside, with 3-inch planks, in a vertical position. The thickness of the sides was 18 inches at the bottom and 15 inches at the top; and in order to strengthen them the more, every angle, except the two points, had three oaken knee-timbers, properly bolted and secured. These sides, when finished, were fastened to the bottom, or grating, by twenty-eight pieces of timber on the outside, and eighteen within, called straps, about S inches broad and 3 inches thick, reaching and lapping over the tops of the sides ; the lower parts of these straps were dovetailed to the outer kirb of the grating, and kept to their places by iron wedges. The purpose of these straps and wedges was, that when the pier was built up sufficiently high above low-water mark, to render the caisson no longer necessary fbr the masons to work in, the wedges being drawn up, gave liberty to clear the straps from the mortises, in consequence of which the sides rose by their own buoyancy, leaving the grating under the foundation of the pier.
The pressure of the water upon the sides of the caisson was resisted by means of a ground timber, or ribbon, 14 inches wide, and 7 inches thick, pinned upon the upper row of timbers of the grating; and the top of the sides was secured by a sufficient number of beams laid across, which also served to support a floor on which the labourers stood, to hoist the stones out of the lighters, and to lower them into the caisson.
caisson was also provided with a sluice to admit the water. The method of working was as follows: a pit being dug and levelled in the proper situation for the pier, of the same shape as the•caisson, and about 5 feet wider all round ; the caisson was brought to its position, a few of the lower courses of the pier built in it, and sunk once or twice, to prove the level of the foundation ; then, being filially fixed, the masons worked in the usual method of tide-work. About two hours before low-water, the sluice of the caisson, kept open till then, lest the water, flowing to the height of many more feet on the outside than the inside, should float the caisson and all the stone-work out of its true place, was shut down, and the water pumped low enough, without waiting for the low ebb of the tide, tin. the masons to set and cramp the stone-work of the succeeding courses. Then when the tide had risen to a considerable height, the sluice was opened again, and the water admitted ; and as the caisson was pur posely built but 1(1 feet high, to save useless expense, the high tides flowed some feet above the sides, but without any damage or inconvenience to the works. In this manner the work proceeded till the pier rose to the surflice of the caisson ; when the sides were floated away, to serve at another pier. (Lahelye's Description of Westminster Bridge.)