The two other forms of pile-drivers are the gunpowder driver and the water-jet driver. The gunpowder driver is seldom used and is worthy of mention chiefly for its novelty. It consists essentially of a heavy hollow cylinder with its bottom resting on the pile and its top open, which is called the gun. Into the top of the gun tits a sort of piston carrying on its top a mass of iron weighing about I.510 pounds, which is called the ram. In operation the ram is raised, a cartridge of from two to three minces of gun powder is placed in the gun, and the ram is let fall. In falling the piston enters the gun, coin the air and causing sufficient beat to explode the cartridge. when the expansive force of the powder forces the pile down and the rani up. A cartridge is thrown into the gun each time the ram ascends. With this machine thirty to forty blows per minute can he struck with a fall of from eight feet to ten feet. The water jet is not strictly a pile-driver, hut it is such a well known method of pile-driving that it merits mention here. The method is very simple: a jet of water is forced into the soil just below the point of the pile, thus loosening the soil and allowing the pile to sink either by its own weight or with very light blows. The water is conveyed to the point of the pile by a hose which is held to the pile by staples during sinking and pulled up to he used again after the pile is sunk.
Timber piles sunk by the meth ods just described support their loads partly by the friction of the soil on the sides of the pile and partly as a column by the point resting on an impenetrable mate rial. There are two methods of
determining the supporting power of a pile: 11) to note its re sistance to penetration under the last blow of the pile-driver ham mer, and (2) to load the pile and observe the weight that it will safely stand. The former method is the one generally used, the latter being used simply as confirmatory evidence in doubtful cases. See FOUNDATIONS.
As previously stated. the two most common forms of iron piles are screw piles and disk piles.
Sometimes the stem of such piles is of wood, while the screw and disk are of metal. Screw piles for engineering work usually have a shaft from three inches to eight inches in diameter, with screws from three feet to six feet in diameter. They are driven by rotating the shaft just as an ordinary wood screw is driven. capstan bars being usually employed to secure the screwing motion. although hydraulic screwing de vices have been occasionally employed. Screw piles will penetrate all ordinary soils. They are sel dom used in the United States, but are commonly employed abroad. Disk piles are sunk by the water-jet. At the ocean pier at Coney Island. New York, disk piles were used having wrought iron shafts S inches in diameter and disks two feet in diameter and nine inches thick. Some of these piles stand seventeen feet in the sand, and carry loads of over six tons per square foot of disk. Consult Baker, Treatise on Masonry Con struction (New York, 1000). Sue FOUNDATIONS; LIGHTHOUSE; BRIDGE.