Finishing the Foundation

In some experiments made at the Poughkeepsie bridge (§ 847), it was found that a 1-inch rod driven into a 44-inch hole in hem lock required on the average a force of 2i tons per linear foot of rod to withdraw it; and a 1-inch rod driven into a finch hole in white or Norway pine required 5 tons per linear foot of rod to with draw it.

Concrete Cap.

A thick layer of concrete, resting partly on the heads of the piles and partly on the soil between them, is fre quently employed instead of the timber grillage as above. One advantage of the concrete cap over the timber grillage is that there is less danger of the concrete's sliding off. A second advantage is that the concrete adds materially to the supporting power of the foundation, since it utilizes the bearing power of the soil between the piles as well as the supporting power of the piles themselves. An other advantage of this form of construction is that the concrete can be laid without exhausting the water or sawing off the piles. A fourth advantage is that with concrete there is less excavation, and con sequently less trouble and expense during construction in keeping the foundation pit dry. In recent years in most localities concrete is cheaper than timber.

The substitution in recent years of concrete for block masonry has practically eliminated this method of finishing a pile foundation. However, the concrete of the masonry superstructure is frequently deposited around and over the heads of the piles; for example, see Fig. 116 (page 527), Fig. 118 (page 528), Fig. 142 (page 557), Fig. 148 (page 561), and Fig. 150 (page 563).

Cushing Pile Foundation.

The desire to utilize the cheapness and efficiency of ordinary piles as a foundation for bridge piers, and at the same time secure greater durability than is possible with piles alone, led to the introduction of what is known as Cushing pile foundation, first used in 1868, at India Point, Rhode Island. It consists of square timber piles driven close together or in intimate contact with each other. Surrounding the pile cluster is an envelope of cast or wrought iron, sunk in the mud or silt only deep enough to protect the piles, all voids between piles and cylinders being filled with concrete.

Several such foundations have been used, and have proved satisfactory in every respect. The only objection that has ever been urged against them is that the piles may rot above the water line; but experience seems to show that this is not likely.

In making a foundation according to the Cushing system, the piles may be driven first and the cylinder sunk over them, or the piles can be driven inside the cylinder after a few sections are in place. In the latter case, however, the cylinders may be subjected

to undue strains and to subsequent damage from shock and vibration; and besides, the sawing off of the piles would be very difficult and inconvenient, and they would have to be left at irregular heights and with battered tops. On the other hand, if the piles are driven first, there is danger of their spreading and thereby interfering with the sinking of the cylinder.

The special advantages of the Cushing piers are: (1) cheapness, (2) ability to resist scour, (3) small contraction of the waterway, and (4) rapidity of construction.

The railroad bridge over the Tenas River, near Mobile, rests on Cushing piers. There are thirteen, one being a pivot pier. Each, excepting the pivot pier, is made of two cast-iron cylinders, 6 feet exterior diameter, located 16 feet between centers. The cylinders were cast in sections 10 feet long, of metal 11 inches thick, and united by interior flanges 2 inches thick and 3 inches wide. The sections are held together by 40 bolts, each 11 inches in diameter. The lower section in each pier was provided with a cutting edge, and the top section was cast of a length sufficient to bring the pier to its proper elevation.

The pivot pier is composed of one central cylinder 6 feet in diam eter, and six cylinders 4 feet in diameter arranged hexagonally. The radius of the pivot circle, measuring from the centers of cylinders, is 121 feet. Each cylinder is capped with a cast-iron plate 21 inches thick, secured to the cylinder with twenty 1-inch bolts.

The piles are sawed pine, not less than 10 inches square at the small end. They were driven first, and the cylinder sunk over them. In each of the large cylinders, 12 piles, and in each of the smaller cylinders, 5 piles were driven to a depth not less than 20 feet below the bed of the river. The piles had to be in almost perfect contact for their whole length, which was secured by driving their points in contact as near as possible, and then pulling their tops together and holding them by 8 bolts 11 inches in diameter. In this particular bridge the iron cylinders were sunk to a depth not less than 10 feet below the river bed; but usually they are not sunk more than 3 to 7 feet. The piles were cut off at low water, the water pumped out of the cylinder, and the latter then filled to the top with concrete.