Pile Foundations: the excavation is quite dry, the masonry for the foundation may be laid directly upon the ground, if the latter be firm enong-li for the purpose. When, however, solid bottom lies so deep that the excavation cannot be carried clown to it, recourse may be had to rows of piles to bear the load of the structure and transmit the same to the solid bottom. The piles (pi. 4r, fis,,-s. 15, 16) are driven at regular distances from one another, forming rows. When driven, their heads are sawed off at a uniform heig,ht, and they are then strongly bound top,-ether by two courses of stout timbers crossing at rig-lit angles(figs. 15b, 160), and firmly secured to the piles and to one another by bolts or tree-nails. In most cases, partly that the load to be sustained may be transmitted to the soil surrounding the "grillage" (as the pile structure is called; pt. 50, fig. 6), and partly to prevent undermining, the spaces between the piles are filled up to the level of their heads with concrete, broken stone, etc. (A/.
41, Ars. isa, 16a). To the top of the upper course of timber is bolted a close flooring, constituting a wooden platform, on which the masonry is laid; or the upper course of the grillage timbers for this purpose may be laid as a close platform.
Further to guard against undermining, the grillage is commonly sur rounded with a wall of sheet-piling (fig. I5b), or, with the same object in view, the outside row of piles is formed of close piles (fig. 16b), which are usually driven about tO 4 feet apart each way from centre to centre; their diameter should be at least 9 inches. The depth to which thew should be driven will depend on the nature of the soil and the weight of the structure they are intended to sustain. The entire woodwork of the grillage must be below the ascertained low-water mark, inasmuch as wood exposed to alternations of wet and dry speedily decays. Where the soil is stony or difficult to penetrate, it is usual to furnish the foot of the pile with a pointed or edged shoe of cast or wrought iron, to facilitate its descent.
cases where the access of water cannot be prevented, or can be prevented only by considerable outlay, one or the other of the following, modes of laying a subaqueous foundation is em ployed. W'here the upper strata of the bed of the river, lake, or sea are solid, a deposit of rough stone is simply thrown into the water until the surface is reached. The top is then levelled off, and the building is pro ceeded with above water. To insure against settlement, the larger stones
arc disposed around the outside of the heap. Foundations of this sort are known as " rip-rap," or " random-stone" (pl. 50, fig-. 4). In the construc tion of docks, breakwaters, sea-walls, and similar structures, in harbors, instead of using random-stone, enormous blocks of beton are used therefor, and these are placed by divers in proper position with reference to one another.
Piles protected lir the solid stratum is found to be at some depth below the surface of the river-, lake-, or sea-bottom, the overlying unreliable soil must be removed by dredging before laying the concrete or stone foundation. It may also be found advantageous to employ piling under such circumstances. Figure 5 exhibits a set of piles driven into the river-bed and protected from damage by a deposit of stone. The tops of the piles are sawed off to an even height a short dis tance below low water, and a strong- platform is erected upon them, from which the masonry is started; or the piling may be made to serve as the support of a crib of tiinber (floated to the proper position and sunk upon the piling by weighting with stones), and the masonry is started from a platform built upon the top of the crib. In this case the tops of the piles must be sawed off so far down that the top of the superincumbent crib shall also be below the level of low water.
Piles and of cast iron of various forms are somewhat used for sheet-piling; or, when intended to serve as bearing piles, cylinders open at both ends have been employed. These may be of con siderable size, made in sections, the successive lengths being- bolted to gether through internal flanges. (Such constructions have been described in the section on Bridges, p. 273.) Where tubes or cylinders of cast iron are employed (usually filled with concrete), they are generally carried not merely to high-water mark, but, in the case of bridge-piers, up to the superstructure proper. A modification of this method consists in surround ing a group of piles with an iron cylinder sunk to a solid bearing, after which the enclosed space is filled with concrete. In such an arrangement the piles may be carried above low water, since they are protected from decay by the enclosing cylinder and concrete filling. They are likewise protected from destruction by boring-worms. This plan was first proposed by S. B. Cushing, an American engineer, and is said to give very satisfac tory results.