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Pile Foundations

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PILE FOUNDATIONS Classification of Piles.—A pile is a stick of timber or other material driven longitudinally into the soil for the purpose of increas ing its power to sustain loads, or to resist lateral pressures. Piles are usually of timber, concrete or metal, and are further classified according to the methods used in placing them or the uses for which they are intended.

Bearing piles are those which carry the weight of a structure rest ing upon them. They may act in either of two ways: (1) When they are driven through a bed of soft material to a firm stratum below, the pile acting as a column and receiving little or no support from the material through which it is driven. (2) When the piles reach no firm material, but are sustained by the frictional resistance of the material through which they are driven and the compacting of the soil about their upper ends.

Batter piles are driven at an inclination to resist lateral forces, and are commonly used where cross bracing of the vertical piles may not give sufficient lateral stiffness to the structure.

consists of piles driven in close contact for the pur pose of forming a tight wall to resist the pressure of water or soft material, being commonly used in cofferdams to prevent leakage into excavations.

Guide piles are frequently used to assist in holding a caisson in position during sinking, or to hold in position the horizontal timbers against which sheet-piling is to be sunk.

Screw piles consist of cast-iron or steel pipes with a broad screw upon the bottom for the purpose of giving large bearing area. They are driven by screwing them into the soil, and have sometimes been satisfactorily used in sand or gravelly soils.

Disk piles are pipes with horizontal circular plates stiffened by radial ribs, fastened to their bottom ends, as shown in Fig. 110. They are sunk by the use of a jet of water, which washes the soil from beneath the disk, allowing it to be forced down. The diameters of disks range from about 2 to 4 feet and of pipes from about 6 to 12 inches.

Sand piles are sometimes used for the purpose of increasing the bearing capacity of the soil by compacting it laterally for short depths below the surface. are placed by driving short Nvooden piles, then withdrawing them and filling the holes with sand, which should be damp when placed and should be tamped so as to compact it in the hole.

San.1 for this purpose possesses the advantage of transmitting a certain amount of pressure laterally, and adjusts itself to any slight settlement that may take place.

193. Pile-Drivers.—An ordinary pile-driving machine consists essentially of the leads, the sheaves and the hoisting drums, with their appurtenances. The leads are two parallel uprights between which the pile is held in position while being driven, and which form a guide for the hammer. At the upper end of the leads are the sheaves, over which the lines pass which are used to handle the pile in placing it in position and to raise and lower the hammer in driving. The leads are supported in position by a triangular framework braced with backstays. The platform or deck to which the framework is attached also carries a hoisting engine with friction drums for han dling the pile and hammer lines. The general arrangement is shown in Fig. 111. The details of arrangement and method of mounting vary widely according to the service for which the machine is intended.

Pile-drivers may be so mounted as to move forward, backward, and to the side by the use of rollers, or made to turn in any direction by mounting upon a turntable. For river work, they are usually rigidly connected to the deck of a barge which is moved to place the driver in position.

For railway work, drivers are commonly mounted upon cars, and many of them are very carefully designed to render efficient service under varying conditions. The cars are made self-propelling to make the machine independent of locomotive service, and leads which can be quickly raised and lowered are employed. The drivers are mounted upon turntables which permit driving upon either side, and the leads are arranged so that they may be turned to an inclined position for the purpose of driving batter piles. The stability of the machines when driving at the greatest reach from the cars is im portant and must be carefully considered in design. Combina tion machines, in which service as pile drivers is added to that as derricks or as excavators are also frequently employed.

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