Timber Piles are generally round, the diameter at the butt varying from 9 to 18 inches. They should be straight-grained and as free from knots as possible. The variety of timber is usually selected according to the character of the soil. Where the piles will be always under water and where the soil is soft, spruce and hem locks are used. For firm soils the hard pines, fir, elm and beech are preferable. Where the piles will be alternately wet and dry, white or black oak and yellow pine are used. Piles exposed to tide water are generally driven with the bark on. It is customary to fix an iron hoop to the heads of piles to prevent their splitting, and also to have them shod with either cast- or wrought-iron shoes.
Timber piles when partly above and partly under water, decay rapidly at the water line owing to the alternations of dryness and moisture. In tidal waters they are destroyed by the marine worm called the "teredo navalis." To preserve timber in such situations several processes are in use. The one most extensively employed is known as "creosoting," which consists of injecting creosote or dead oil of coal tar into the pores of the timber.
The frame of timbers placed on the top of the piles is called the grillage. The piles are sawed off square below low water, a timber called a cap is placed on the ends of the piles and fastened with drift bolts, and transverse timbers called strips are placed on the caps and drift-bolted to them. As many courses as necessary may be added, each at right angles to the one below it, the top courses being either laid close together to form a floor or else covered with heavy plank to receive the masonry.
In some cases the grillage is omitted, a layer of concrete being used instead, with the heads of the piles embedded therein, as shown in Fig. 7. The name gril lage is also applied to a combination of steel beams and concrete.
Iron and Steel Piles.
Cast iron, wrought iron, and steel are employed for ordinary bearing piles, sheet piles, and for cylinders. Iron cylinders are usually sunk either by dredging the soil from the inside or by the pneumatic process.
Cast-iron piles are used as substitutes for wooden ones. Lugs or flanges are usually cast on the sides of the piles, to which bracing may be attached for securing them in position. A wooden block is laid on top of the pile to receive the blows of the hammer, and after being driven a cap with a socket in its lower side is placed upon the pile to receive the load. Solid rolled-steel piles are driven in the same manner as timber piles, either with a hammer, machine or water-jct.
Screw Piles are piles which are screwed into the stratum in which they are to stand. They are ordinary piles of timber or iron (the latter usually hollow), to the bottom of which a screw disk, consisting of a single turn of the spiral, similar to the bottom turn of an auger, is fastened by bolts or pins. Instead of driving these piles into the ground they are forced in by turning with levers or machinery suitable for the purpose. The screw disks vary in diameter from 1 to 6 feet. The water jet is sometimes employed by applying it to the under, upper, or both sides of the disk for the purpose of reducing the resistance.
Concrete Piles. Two methods of forming these piles are in use. (1) The piles are made in moulds and carried to the place of use and driven in the same manner as timber piles. (2) Holes are made in the ground and filled with concrete.
Moulded Concrets Piles. Fig. 8 shows the moulded pile. This pile is made in moulds and contains four vertical rods a at the corners, the rods are stayed by loops or hooks b of large wire sprung into place across the sides of the pile and held transversely by horizontal strips of thin metal. The feet of the piles are either wedge shaped or pyramidal and are protected by cast iron points with side plates which turn in at c to lock with the concrete. The upper ends of the piles are shouldered in to give clearance for the driving cap d. This is a cast steel hood which fits loosely around the neck of the pile, and is filled with dry sand or a bag of sawdust d' retained by a clay ring and hemp jacket e at the bottom of the cap.