Note 107 — CHAPTER THE FOURTH

Cofferdams are embankments which surround the site so as to exclude water from it. “They are formed in general by driving two rows of piles round the site so as to enclose between them a watertight wall of clay puddle; in depths of less than three or four feet, where there is little current, a simple clay dam may be used. In greater depths, the timber walls consist of guide piles at intervals, with some form of sheet piling between them; in extreme depths the timber walls may be composed of stout piles driven in side by side all round. The dam must be sufficiently strong to bear the pressure of the water against the outside when the space enclosed has been pumped dry.... The ‘Cours de Ponts,’ at the School of the Ponts et Chaussées, states that a cofferdam need never be made of greater thickness than from four to six feet, as the interior can always be sufficiently stayed inside. This method of founding is now seldom practised; it is costly and causes great obstruction in the stream.”—Professor Fleeming Jenkin.