A kind of chest-framed or flat-bottomed boat, sometimes used in laying the piers of bridges in deep or rapid rivers. The caissons for this purpose consist of a very strong platform of timber, to which are attached the two sides, in such a manner as to admit of their removal when no longer required. These sides, which are also very strongly framed of timber, to resist the great pressure of the water, are curved towards their extremities, so as to meet each other, and form a salient angle up and down the stream, and inclosing a space somewhat wider than the foundation of the pier of the bridge. The site of the pier being levelled by dredging or otherwise, the caisson is brought over the spot, and moored in the proper position; two or three of the lower courses of masonry are then built upon the platform of the caisson, and the water is then slowly admitted by a sluice in the caisson, so as to cause the caisson to settle into its place; and to prevent the effects of too great a pressure of water when the rise and fall of the tide is considerable, the water is generally admitted some time before high water, and pumped out again after the ebb tide has commenced, so that the workmen may resume their labours before low water.

When the masonry is brought up as high as the level of the water, the sides of the caisson are detached from the bottom, and removed. Westminster and Black-friars bridges were built on caissons, but the preference is now generally given to coffer dams. A patent was obtained by Mr. Deeble, for constructing jetties, piers, quays, etc. by means of what he terms "metallic caissons," or a peculiar description of cast iron boxes, variously combined by dovetails; these boxes, when fixed in their places to form a pier, or quay, etc. are filled with liquid lime and rubble, which soon sets hard, and forms a solid mass girt with metal. For the better elucidation of the plan, a few of the more simple forms of caissons, and their mode of uniting, as exemplified in the construction of a pier, are engraved herewith. The caisson is open generally both at top and bottom; the thickness of the sides proportioned to the strength and gravity required. It is proposed that each caisson be 7 feet in length, 5 feet in height, and from 2 to 5 feet in width, according to the nature of the work in which it is to be used.

Caissons constituting foundations should be closed at bottom, and in raising one tier above another, each layer would become united to those immediately above and below it, by commencing the alternate vertical courses with a half caisson. Fig. 1 is the plain oblong square, with dovetails at the ends, only applicable to straight lines, either in banking exposed to the water, or to the interior of heavy works, as cross forts, or in bracings and buttresses to be buried in the earth. This form admits but little variation in its application, and none in its strength or gravity beyond what may be gained by increasing the thickness of its sides. Fig. 2 is the most universal form; it may be multiplied to any extent, yet is perfect in itself, requiring no change of form on the side to finish a work, and the ends may be conveniently completed by filling up the dovetail groove with a portable half dovetail. Fig. 3 is the radiated form, which may be used in a waved line along the coast, where great strength is required; it also applies to piers and bastions. The dotted projection is the half dovetail, which would be required to attach it to the cross fort; or the radiated caisson, should it be considered necessary to add another waved line, would give the effect of arch and counterarch.

Fig. 4 is a radiated caisson, having extra dovetails for uniting the main line to the bastion. Any angle may thus be gained, simply by mooring this caisson in the bastion, in the direction required. Fig. 5 is the termination of a pier with a bastion: the external dotted line shows the boundary of the sloped bank; the cross forts are introduced at suitable distances to insure great stability; and the inner dovetailed grooves being left in the inner lines, will enable the engineer to add cross forts and buttresses to any extent that may be required.

Caisson 288

Fig. 5.

Caisson 289