Usually, the deter mination of the point at which a pile is considered as having been driven to a firm bearing depends greatly upon the judgment of the en gineer in charge, based upon his ex perience in the par ticular locality in which the work is being performed. The hest practice, how ever, affords the fol lowing safe specifica tions of allowable penetration. For piles meeting a hard re sistance a penetration of one inch under each blow of a 2,000 pound hammer fall ing 10 feet, and n penetration of two inches under the blows of a 2,000-pound hammer falling 15 feet. The minimum distance between centres depends upon the hardness of the sub-strata, the size of the butts and the weight to he carried. Spruce piles may be often driven 24 inches be tween centres, while large and long piles need not usually be driven closer than 30 inches between centres. Another factor that must be carefully considered is the supporting power of the soil as a whole. For example, tf that power is equal to two tons per square foot and each pile is capable of sustaining 18 tons, it is useless to place the piles closer than three feet between centres.
In connection with the use of timber piles in places where the durability of the timber is very liable to be seriously impaired by the at tacks of the Teredo worm, the latest practice appears to favor the system of armoring them with concrete. In such cases the timber piles are usually driven in sets of three to a firm bearing. They are then cut off, one two feet, one four feet and one eight feet, below the level of the wharf platform. A wooden stave yiinder of three-inch planking is then placed around the piles and driven into the mud to a depth of .perhaps 12 feet, and the bottom of the cylinder scaled. The contained water is then pumped out and a cylinder of expanded metal Is set .within the wooden cylinder, and the re maining space filled with concrete.
Another method still more recently intro duced consists in the use of terra-cotta pipes as an outer covering for the piles. In this case the armoring is limited to individual piles.
"Skyscraper" Foundations.— In the con struction of the tall steel and concrete build ings commonly called "skyscrapers," which are so common in Manhattan, N. Y., the usual plan is to excavate the foundations 30 to 50 feet, and then to sink steel caissons as may be necessary to permit concrete piers to go to bedrock. The size and number of these piers is determined by the weight of the superstruc ture. If it is not possible to spread the foundations beyond the line of the walls, steel trusses are placed at the footings of the walls to transfer the weight to the centre of the piers, distributing it uniformly. It is common to cap the piers with large blocks of granite or stout iron castings. The foundations of one of these tall buildings, when nearing com pletion, presents the appearance of a group of concrete piers rising from the soil to the level of the cellar. A great deal of ingenuity is re
quired to sink deep foundations in a large city without disturbing the support of adjacent buildings.
Platform Foundations.— These structures are designed to distribute a concentrated weight over a large area in soft substrata when piles are not employed. They may consist of beds of concrete, or of masonry or brick inverted arches sprung between supporting piers or columns, as shown by Fig. 4. They may also consist ofplatforms of timber grillage— tim bers placed across each other in alternate layers, or of platforms of concrete or masonry reinforced with a grillage of steel bars, as illustrated by Fig. 5, which shows the platform foundations of a World's Fair building in Chicago, consisting of a concrete bed rein forced with a steel rail and I-beam grillage, to support the base castings of the main columns.
Subaqueous Foundations arc those con structed in the substrata of river beds or other bodies of water, or where the existing condi tions necessitate the building of structures in water and below the level of its surface. This class of work is accomplished by the use of cofferdams, cribs and open caissons, pneumatic caissons, by dredging through wells, by forcing cement into the substrata under pneumatic pressure, and by by freezing the substrata.
Cofferdam use of coffer dams is limited to the construction of founda tions in shallow waters where the depth of the necessary excavation to reach a firm bearing is small, or in water-bearing substrata on land. They are usually constructed by driving a double row of sheet piles — heavy timber or steelplanking—around the area in which the foundation is to be built. The space between the piles is then tamped in solidly with clay, and the water pumped out of the enclosure, so that the work of excavating and thC subsequent masonry construction may be carried on in the open air. Cofferdams are sometimes con structed of walls composed of bags of clay piled around the foundation area, and rein forced with barrels of sand banked on the outer side of the walls.
Crib and Open Caisson Under this method, the foundation bed is first pre pared by dredging until a solid material is reached, or by driving piles to a sufficient depth to reach a firm bearing. When piles are em ployed they are cut off at a uniform depth be low the level of the water surface and con stitute the supporting bed of the caisson. The caisson consists of a water-tight box-like structure open at the top. It is floated over the position of the bed, previously prepared by dredging, and the masonry work built up in its interior until it gradually sinks and rests upon the bed. The side walls are then re moved, leaving the bottom of the structure—. the crib— in the foundation between the ma sonry and the supporting bed. Fig. 6 illus trates the foundation for a pier constructed by this method, the rib being supported by piles.