A recent development of the use of concrete and reinforced concrete is to construct piles of this material. A reinforced-concrete pile foundation does not materially differ in construction from a timber pile foundation. The piles are driven and capped with concrete ready for the superstructure in the usual manner. In comparing this type of piles with timber piles, they have the advantage of being equally durable in a wet or dry soil, and the disadvantage of being more expensive in first cost. Sometimes their use will effect a saving in the total cost of the foundation by obviating the necessity of cutting the piles off below the water line. The depth of the excavation and the volume of masonry may be greatly reduced, as shown in Fig. 54. In this figure is shown a comparison of the relative amount of excavation which would be necessary, and also of the concrete which would be required for the piles, thus indicating the economy which is possible in the items of excavation and concrete. There is also shown a possible economy in the number of piles required, since concrete piles can readily be made of any desired diameter, while there is a practical limitation of the diameter of wooden piles. Therefore a less number of concrete piles will furnish the same resistance as a larger number of wooden piles. In Fig. 54 it is assumed that the three concrete piles not only take the place of the four wooden piles in the width of the foundation, but there will also be a corresponding reduction in the number of piles in a direction perpendicular to the section shown. The extent of these advantages depends very greatly on the level of the ground-water line. When this level is considerably below the surface of the ground, the excavation and the amount of concrete required in order that the timber grillage and the tops of the piles shall always be below the water line will be correspondingly great, and the possible economy of concrete piles will also be correspondingly great.

Fig. 54. Comparison of Wooden and Concrete Piles.

Fig. 54. Comparison of Wooden and Concrete Piles.

The pile and cap being of the same material, they readily bond together and form a monolithic structure. Reinforced-concrete piles can be driven in almost any soil that a timber pile can penetrate, and they are driven in the same manner as the timber piles. A combination of the hammer and water-jet has been found to be the most successful maimer of driving them. The hammer should be heavy and drop a short distance with rapid blows, rather than using a light hammer dropping a greater distance. For protection while being driven, a hollow, cast-iron cap filled with sand is placed on the head of the pile.

Concrete and reinforced-concrete piles may be classified under two headings: (a) those where the piles are formed, hardened, and driven very much the same as any pile is driven; (b) those where a hole is made in the ground, into which concrete is rammed and left to harden.

Reinforced-concrete piles which have been formed on the ground are designed as columns with vertical reinforcement connected at intervals with horizontal bands. These piles are usually made square or triangular in section, and a steel or cast-iron point is used.

Fig. 55 shows the cross-section of a corrugated pile used in the foundations of the buildings for the Simmons Hardware Company, Sioux City, Iowa, and for John J. Latteman, Brooklyn, N. Y. The pile tapers from 16 inches at the large end to 11 inches at the small end. The reinforcement consists of Clinton electrically-welded fabric, the size being approximately 3/8-inch wires longitudinally, and 1/8-inch wires, 12 inches on centers, for the bands. The hole in the center is 3 1/2 inches at the top, and tapers to 2 inches at the bottom.

The piles were driven by means of a water-jet and hammer. The jet extended through the opening in the pile, and protruded three inches below the bottom of the pile. The pressure of the water was sufficient to dig a hole and carry the loosened soil up the corrugations, and the weight of the hammer drove the pile down. When the pile was nearly in place, the jet was removed, and the hammer was used to force the pile until it was solid. The cap was made as shown in Fig. 56; and in driving the pile, a hammer weighing 2,500 pounds was dropped 25 feet, 20 to 30 times, without injury to the head.