Foundations are frequently constructed through shallow bodies of water by means of cofferdams. These are essentially walls of clay confined between wooden frames, the walls being sufficiently impervious to water so that all water and mud within the walled space may be pumped out and the soil excavated to the desired depth. It is seldom expected that a cofferdam can be constructed which will be so impervious to water that no pumping will be required to keep it clear; but when a cofferdam can be kept clear with a moderate amount of pumping, the advantages are so great that its use becomes advisable. A dry cofferdam is most easily obtained when there is a firm soil, preferably of clay, at a moderate depth (say 5 feet to 10 feet), into which sheet piling may be driven. The sheet piles are driven as closely together as possible. The bottom of each pile (when made of wood) is beveled so as to form a wedge which tends to force it against the pile previously driven (see Fig. 49). In this way a fairly tight joint between adjacent piles is obtained. Larger piles (see Fig. 60, a) made of squared timber are first driven to act as guide-piles. These are connected by waling strips (Fig. 60, b), which are bolted to the guide-piles and which serve as guides for the sheet piling (Fig. 60, c). The space between the two rows of sheet piling is filled with puddle, which ordinarily consists chiefly of clay. It is found that if the puddling material contains some gravel, there is less danger that a serious leak will form and enlarge. Numerous cross-braces or tie-rods (Fig. 60, d) must be used to prevent the walls of sheet piling from spreading when the puddle is being packed between them. The width of the puddle wall is usually made to vary between three feet and ten feet, depending upon the depth of the water. When the sheet piling obtains a firm footing in the subsoil, it is comparatively easy to make the cofferdam watertight; but when the soil is very porous so that the water soaks up from under the lower edge of the cofferdam, or when, on the other hand, the cofferdam is to be placed on a bare ledge of rock, or when the rock has only a thin layer of soil over it, it becomes exceedingly difficult to obtain a water-tight joint at the bottom of the dam. Excessive leakage is sometimes reduced by a layer of canvas or tarpaulin which is placed around the outside of the base of the cofferdam, and which is held in place by stones laid on top of it. Brush, straw, and similar fibrous materials are used in connection with earth for stopping the cracks on the outside of the dam, and are usually effective, provided they are not washed away by a swift current.

Fig. 60. Plan and Cross Section of a Cofferdam.

Fig. 60. Plan and Cross-Section of a Cofferdam.

Although cofferdams can readily be used at depths of 10 feet, and have been used in some cases at considerably greater depth, the difficulty of preventing leakage, on account of the great water pressure at the greater depths, usually renders some other method preferable when the depth is much, if any, greater than 10 feet.