Cofferdam, a watertight structure used in engineering for excluding the water from the foundations of bridges, quay walls, &c., so as to allow of their being built dry. Cofferdams are generally formed of timber piles driven close together (called sheeting) in two or more rows, according to the depth of water and the nature of the bottom; the space between the rows, which may vary from 4 to 10 feet, being spooned out, down to the solid and impervious bottom, and filled up with clay puddle. Sometimes they are made of only one row of piles of the full height, caulked above low-water, with a low or dwarf row outside to confine the puddle up to that level, or, where there is no wave or current, with a mere bank of clay thrown against the outside; and occasionally the upper work is formed of horizontal planking, fixed on open main piles, and caulked in the joints. When the bottom is rock, so as to prevent piles being driven, and is not much below low-water, cofferdams are occasionally formed of two stone walls, with a space between filled with clay. Cofferdams require to be strongly shored within, to prevent their being forced inwards by the pressure of the external water; and the rows of piles require to be strongly bolted together, to resist the pressure of the clay puddle, which otherwise would burst them. This method of founding is now seldom practised; it is costly and obstructive to the stream.
In the construction of the Victoria Embankment on the river Thames, between Blackfriars and Westminster Bridges, mile in length, the coffer- dams were constructed of wrought-iron Caissons (q.v.) or boxes, in substitution for ordinary timber dains; on the suggestion of Sir Joseph Bazalgette, in order to effect a saving by using the same caissons two or three times successively in different parts of the dam. The caissons were oval, 12½ feet long, 7 feet wide, joined end to end, and made watertight at the joints by felting. In about two-thirds of the length of the embankment, 195 caissons were employed.