pile, post of timber, steel, or concrete used to support a structure. Vertical piles, or bearing piles, the most common form, are generally needed for the foundations of bridges, docks, piers, and buildings. Slender tree trunks, roughly trimmed and about 10 in. (25.4 cm) thick at the butt, are used in foundations for houses. Wooden piles last a very long time underwater but are subject to decay when buried underground. They are shaped for driving and sometimes have a pointed iron shoe set on the sharp end, with the butt end encircled by an iron band to prevent brooming under the blows of the pile driver. Their length is usually 20 to 60 ft (6.1-18.3 m), and they are generally spaced 3 or 4 ft (.9 or 1.2 m) apart from center to center. Concrete piles are generally of two types, the precast and the cast-in-place. They are very strong and durable, do not deteriorate when wholly in the ground, and are immune to the attacks of boring insects. Precast piles are made of concrete reinforced with steel bars looped one to the other and are tipped and topped with protective steel when driven into the ground. The steel is not needed when the piles are set by the force of jets of water; in this method an iron pipe is set in the center of the pile, and water under pressure is sent down to wash away the sand, silt, or soft earth that it is to displace. Only in such subsurfaces can the water-jet system be employed. Cast-in-place piles are variously made. One method consists of driving a steel shell into the ground and filling it with concrete, after which the shell is withdrawn and the molded concrete is in place. Sheet piling consists of wooden boards or interlocking steel plates and is used largely as a cofferdam to keep water from structural work, piers, and buildings. Concrete sheet piling is also used. Pilings are driven into the ground by pile drivers using drop hammers, diesel hammers, steam hammers, or compressed-air hammers. More recently, high-powered ultrasonic vibrators have come into use for driving piles.