COFFER, a casket for keeping jewels or other valuables. Caskets and chests were sometimes made of iron, but more frequently of wood.
' a water-tight structure used in engineering for excluding the water. from the foundations of bridges, quay-walls, etc., so as to allow of their being built dry.. Coffer-dams 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 four to ten 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, calked 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 calked in the joints. When the bottom is rock, so as to prevent piles being driven, and is not much below low-water, coffer-clams are occasionally formed of two stone-walls, with a space between filled with clay.
The coffer-dams before spoken of are all what are called high-water dams, and exclude the water at all states of the tide. They require to be provided with sluices, to allow of the water, when first to he excluded, getting out during the ebb, and to shut against it during the flood. The remainder of the water, and all leakages, must be got rid of by pumps, generally worked by a steam-engine. For moderately shallow foundations, and more especially where there is a great rise and fall of tide, tidal-dams are often used. These are sometimes made of sheeting piles, but are often boxes formed of planking or of iron, weighted and sunk into the ground by digging inside in the same way that wells are sunk. These dams can only be used for a couple of hours or thereabouts at low-water, and, of course, require to be pumped out every tide. All coffer-dams 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 over come the pressure of the clay puddle, which otherwise would burst them.