The Forth Bridge/Generally about Cofferdams
Generally about Cofferdams.
In the description of the pier foundations, frequent use is made of the terms caisson and cofferdam, and to those not conversant with foundation work in water a short explanation may be acceptable. A cofferdam or caisson may be described as an enclosure in water for the purpose of laying dry the space enclosed, or, at any rate, of preventing a flow of water through it. In soft ground this is done by driving a double row of piles at a distance of from 2 ft. to 4 ft. from each other, and by continuing to drive piles between those already placed until a double timber wall exists all round. Sluice doors or valves are placed so as to allow the tide to flow in and out. The single timber piles are held together by longitudinal timbers being placed on each side and bolted through, and stays or struts are placed between the walls and across the inner space in all directions to give stiffness and resistance to the water pressure from without. The space between the double walls is now cleaned out as well as can be done, and clay puddle is filled into this space and trodden down hard or pressed down by other means, and this is carried on until the whole space is filled up to and beyond half tide or full tide as the case may be. The sluices can then be closed and the water pumped out from the enclosure, and the bottom upon which the foundation is to be placed can be examined, and all necessary excavation made. It depends partly upon the hold the piles have taken of the ground, partly upon the external forces acting upon the timber-walls—namely wind and waves—whether or not the construction of a whole tide dam is advisable. In a whole tide dam, if tight at bottom, when once the water has been pumped out, the work of excavation—of laying the foundation and building a pier or wall—can be carried on without interruption to the end. In the case of a half-tide dam, the space enclosed can only be pumped dry after the tide outside has fallen below the level of the clay puddle wall, and the water requires to be admitted so soon as the tide has turned and is rising again up to that level. Work in a half-tide dam is termed tidal work. When working on rock this mode of forming an enclosed space by driving piles is not admissible, and other means must be found to keep the water out, one of which will be described below. In some cases, whether working upon soft ground or rock, it is not absolutely necessary to lay the bottom dry in order to excavate or to inspect the ground the latter being done by means of divers and the former by dredging or otherwise. The object of such a dam is to arrest the flow of any current through the caisson while the foundation is being laid, and to deposit the material of which the foundation is to consist most generally concrete by lowering it through the water in boxes or skips, the bottoms of which are provided with hinged doors.
Finally, if instead of a caisson open at top, the caisson is covered in like a bell or gasholder, and the water is forced out by forcing air in, thereby allowing the workmen to enter and excavate in the dry, it is called a caisson worked by the pneumatic process. In this manner the deep-water foundations of the circular piers were executed, as will hereafter be described, in the case of the two south piers, Inchgarvie, and the four Queensferry circular piers.