CAISSON is also used for a wooden frame, or chest, used in laying the foun dations of the piers of a bridge.
The practice in building in caissons is a method sometimes adopted in laying the foundation of bridges in very deep or rapid rivers. There are large hollow vessels, framed of strong timbers, and made water tight, which being launched and floated to a proper position in the river, where the ground has been previ ously excavated and levelled, are there sunk. The piers of the bridge are then built within them, and carried up above, or nearly to the level of the water, when the sides of the caisson are detached from the bottom, and removed ; the bottom, composed of a strong grating of timber, remaining and serving for a foundation to the pier. The most considerable work, where caissons have been used, was in the building of Westminster bridge ; of these, therefore, a particular account may be acceptable. Each of the caissons contained 150 loads of fir timber, and was of 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, which were 10 feet . in height, were formed of timbers laid horizontally over one another, pinned with oak trunnels, and framed together at all the corners, except the salient an gles, 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 thick ness of the sides was 18 inches at bottom, and 15 inches at top ; and in order to strengthen them the more, every angle, except the two points, had three oaken knee ti mhe rs,properly bolted and secured. These sides, when finished, were fasten td to the bottom, or grating, by 28 pieces of timber on the outside, and 18 within, called straps, about 8 inches broad, and about 3 inches thick, reaching and lapping over the tops of the sides ; the lower part of these straps were dove-tailed to the outer curb of the grating, and kept in their places by iron wedges. The pur pose of these straps and wedges was, that when the pier was built up suffici ently high above low-water mark, to ren der the caisson no longer necessary for the masons to work in, the wedges be ing drawn up gave liberty to clear the straps from the mortices, in consequence of which the sides rose by their own buoy ancy, leaving the grating under the foun dation 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 tim bers 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 labour ers stood to hoist the stones out of the lighters, and to Iower them into the cais son. The 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 of the cais son, and about five 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 finally fixed, the masons worked in the usual methods of tide-werk. 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 lowest ebb of the tide, for the masons to set and cramp the stonework of the succeeding courses. Then, when the tide had risen to a con siderable height, the sluice was opened again, and the water admitted ; and as the caisson was purposely built but 16 feet high, to save useless expence, the high tides flowed some feet above the sides, but without any damage or incon venience to the works. In this manner the work proceeded till the pier rose to the surface of the caisson, when the sides were floated away, to serve the same purpose at another pier.