In the Gow pile a casing is sunk by use of a water-jet which removes the earth from inside the casing. A cutting tool is then used to widen the hole below the end of the pipe, the earth being removed by the water-jet. The hole is then pumped out and filled with concrete as the casing is removed.
Care is necessary, when using piles molded in place, that injury to the pile may not result from disturbance of the soil around the pile by driving other piles during the period of hardening—a danger which varies with the character of the soil. No pile should be driven near enough to be felt in the earth surrounding a green pile for a week after it is placed, unless the driving can be done before the initial set of the concrete takes place.
Pre-molded piles are reinforced like columns with lateral reinforce ment of wire hoops, spiral wrappings, or wire mesh, combined with longitudinal steel bars, the cross-section Most commonly employed being octagonal or square vvith chamfered corners. The diameters in general use are from 12 to 20 inches for lengths of 20 to 50 feet, although Larger and longer piles are sometimes employed and they are either of uniform section or given a slight taper, according to the service for which they are intended. When to be supported by friction upon their sides, tapering may be of value in increasing bear ing power, but at somewhat increased cost of construction. Pointed shoes are used at the bottom to facilitate driving.
Piles are molded in either horizontal or vertical position. The molding is easier to handle and readily subject to inspection when in horizontal position. When molded in vertical position, the surface of concrete as deposited is normal to the length of pile, but special care is necessary in placing the concrete to eliminate voids. The rein forcement is connected up and handled as a unit in placing in the forms, to assure its proper position in the pile. During the early period of hardening, special attention should be given to keeping the concrete moist, and it is customary to allow it to harden about thirty days before it is driven, though in some instances the hardening has been hastened by subjecting the piles to a steam bath.
The steel reinforcement in a pre-molded pile must be sufficient to carry the stresses which occur during handling and driving as well as those caused by the loads which come upon it afterward. In rais
ing the pile from a horizontal position or in moving it horizontally, the pile must be capable of carrying its own weight as a beam, sup ported near the ends or at the middle. Allowance for shocks and impact should be made. After driving, the pile may be in direct compression when supported laterally or it may act as a column when not so supported. The concrete used is ordinarily that known as 2000 pounds concrete of about 1 : 2 :4 mixture, although sometimes a richer mixture is employed.
On account of the weight of concrete piles, heavy drivers are necessary. Steam hammers are found most effective and occasion less damage to the piles than drop hammers. heavy drop hammers with low fall give better results than lighter ones with greater fall. Caps of various designs are employed to cushion the blow and pre vent shattering the head of the pile. A wooden block receives the blow of the hammer, and layers of old belting, rope ends, or bags of sawdust have sometimes been used upon the head of the pile to pre vent the shattering of the concrete. With proper precautions, it has been found practicable to drive pre-molded piles without injury where heavy driving was necessary.
When a jet is to be used in driving, a hole is cast through the center of the pile into which the jet pipe may be inserted—a tapering core, or a collapsible form, being used for this purpose, which is cheaper than casting the jet pipe in the pile. Solid piles are also some times driven by use of the outside jet as with wooden piles.
There are several forms of patented pre-molded piles in use.
The Chenowith pile is formed by spreading concrete over a wire mesh upon a platform, and rolling it over a mandrel, the longitudinal rein forcement being fastened to the wire mesh. Section for a Chenowith pile is shown in Fig. 113. The. Corrugated pile is reinforced with electrically welded wire fabric, and is generally octagonal in cross section, tapered, with grooves cut in each face. A section is shown in Fig. 11.1.