A Treatise On The Principles And Practice Of Dock Engineering

104 DOCK ENGINEERING. In nearly every case, clay is the material mainly relied upon for the staunchness of a dam. It must be judiciously selected : free from stones, roots, and soil; not of a marly or brittle nature, but tenacious and adhesive; well tempered, watered, and worked to a proper consistency. When these points are carefully attended to, the resultant clay puddle, as it is termed, is capable of forming a thoroughly impervious barrier. If clay of an inferior quality be used, there is sure to be trouble with leaks and inbursts. Temporary dams may be classified according to their composition, as follows :— Earth dams. Timber dams. Stone dams. Concrète dams. Iron dams. Earth Dams are peculiarly appropriate to situations where there is ample space and where a very slight elevation is required, as in shallow water They simply consist of a mound of clay, or of a hearting of earth, covered with an outer layer of clay, deposited by tipping from waggons, skips, or hopper barges. Under the action of tipping, the mound has a decided tendency to subside, and this is still further accentuated by the softening effect of water upon the material, so that, in any case, long flat slopes are inévitable, and hence plenty of room is an absolute necessity for this class of dam. It is advisable where the natural surface of the ground is mud or silt, to excavate the site of the dam down to a solid stratum, better able to support an imposed load and to make a watertight joint with it. This last is an important point, as, if the stratum below a dam be pervious water may be forced through it under external hydrostatic pressure. An example of an earth dam is given in fig. 186. r Timber Dams are frames of woodwork with or without an enclosure of clay puddle. They are subdivisible into (a) Skin dams or sheeting dams. (b) Cofferdams. Skin Dams consist of a single row of sheeting piles, of whole or half timber, retained by tiers of horizontal walings. Lacking sufficient stiffness in themselves, they have to be supported by perpendicular or raking shores abuttmg upon a firm surface. Skin dams are very suitable for adoption in front of quay walls which it is desired to underpin, reface, or repair. In such cases the wall forms a convenient surface for the shore abutments, and the outer hydrostatic pressure is transmitted to the wall through the medium of the shores. The walings should be spaced at intervals” corre- sponding as nearly as possible with the extent of zones of equal hydrostatic pressure. The amount and distribution of this pressure is calculable upon the same principles as those formulated in Chapter vin., for dock gates.