This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The foundation for a pier is sometimes made in the form of a box with walls several feet in thickness, but with a large opening or well through the center. Such piers may be sunk in situations where there is a soft soil of considerable depth through which the pier must pass before it can reach the firm subsoil. In such a case, the crib or caisson, which is usually made of timber, may be built on shore and towed to the site of the proposed pier. The masonry work may be immediately started; and as the pier sinks into the mud, the masonry work is added so that it is always considerably above the water line. (See Fig. 62.) The deeper the pier sinks, the greater will be the resistance of the subsoil, until, finally, the weight of the uncompleted pier is of itself insufficient to cause it to sink further. At this stage, or even earlier, dredging may be commenced by means of a clam-shell or orange-peel dredging bucket, through the interior well. The removal of the earth from the center of the subsoil on which the pier is resting, will cause the mud and soft soil to flow toward the center, where it is within reach of the dredge. The pressure of the pier accomplishes this. The deeper the pier sinks, the greater is its weight and the greater its pressure on the subsoil, although this is somewhat counteracted by the constantly increasing friction of the soil around the outside of the pier. Finally the pier will reach such a depth, and the subsoil will be so firm, that even the pressure of the pier is not sufficient to force any more loose soil toward the central well. The interior well may then be filled solidly with concrete, and thus the entire area of the base of the pier is resting on the subsoil, and the unit-pressure is probably reduced to a safe figure for the subsoil at that depth.
Fig. 62. Hollow Crib Material.
This principle was adopted in the Hawkesbury bridge in Australia, which was sunk to a depth of 185 feet below high water - a depth which would have been impracticable for the pneumatic caisson method described later. In this case, the caissons were made of iron, elliptical in shape, and about 48 feet by 20 feet. There were three tubes 8 feet in diameter through each caisson. At the bottom, these tubes flared out in bell-shaped extensions which formed sharp cutting edges with the outside line of the caisson. These bell-mouthed extensions thus forced the soil toward the center of the wells until the material was within reach of the dredging buckets.
This method of dredging through an opening is very readily applicable to the sinking of a comparatively small iron cylinder. As it sinks, new sections of the cylinder can be added; while the dredge, working through the cylinder, readily removes the earth until the subsoil becomes so firm that the dredge will not readily excavate it. Under such conditions the subsoil is firm enough for a foundation, and it is then only necessary to fill the cylinder with concrete to obtain a solid pier on a good and firm foundation.
One practical difficulty which applies to all of these methods of cribs and caissons, is the fact that the action of a heavy current in a river, or the meeting of some large obstruction such as a boulder or large sunken log, may deflect the pier somewhat out of its intended position. When such a deflection takes place, it is difficult if not impossible to force the pier back to its intended position. It therefore becomes necessary to make the pier somewhat larger than the strict requirements of the superstructure would demand, so that the superstructure may have its intended alignment, even though the pier is six inches or even a foot out of its intended position.
INTERIOR OF GARAGE OF THE GEORGE N. PIERCE COMPANY, BUFFALO, N. Y.
Courtesy of Trussed Concrete Steel Company, Detroit, Mich.