204. Driving Piles with Water-Jet. When piles are driven in a situation where a sufficient supply of water is available, their resist ance during driving may be very materially rcrluced by attaching a pipe to the side of the pile and forcing water through the pipe by means of a pump. The water returns to the surface along the sides of the pile and thus reduces its frictional resistance. The water also softens and scours out the soil immediately underneath the pile, and enables the pile to penetrate the soil much more easily. In very soft soils, piles may be thus driven by merely loading a comparatively small weight on top of the pile while the force pump is being operated; and yet the resistance shortly after stopping the pump will be found to be very great. Of course the only method of testing such resist ance is by actually loading a considerable weight on the pile. This method of using a water-jet is chiefly applicable in structures which arc on the banks of streams or large bodies of water. The water-jet and the hammer are advantageously used together, especially in stiff clay.
205. Splicing Piles. On account of the comparatively slight resistance offered by piles in swampy places, it sometimes becomes necessary to splice two piles together. The splice is often made by cutting the ends of the piles perfectly square so as to make a good butt joint. A hole 2 inches in diameter and 12 inches deep is bored in each of the butting ends, and a dowel-pin 23 inches long is driven in the hole bored in the first pile; the second pile is then fitted on the first one. The sides of the piles are then flattened, and four 2 by fl inch planks, 4 to 6 feet long, are securely spiked on the flattened sides of the piles. Such a joint is weak at its best, and the power of lateral resistance of a joint pile is less than would be expected from a single stick of equal length. Nevertheless, such an arrangement is in some cases the only solution.
206. Pile Caps. One practical trouble in driving piles, espe cially those made of soft wood, is that the end of the pile will become crushed or broomed by the action of the heavy hammer. Unless this crushed material is trimmed off the head of the pile, the effect of the hammer is largely lost in striking this cushioned head. This crushed portion of the top of a pile should always be cut of just before the test blows are made to determine the resistance of the pile, since the resistance of a pile indicated by blows upon it, if its end is broomed, will apparently be far greater than the actual resistance of the pile.
Another advantage of the steam pile-driver is that it does not produce such an amount of brooming as is caused by the ordinary pile-driver. Whenever the hammer bounces off the head of the pile, it shows either that the fall is too great or that the pile has already been driven to its limit. Whenever the pile refuses to penetrate
appreciably for each blow, it is useless to drive it any further, since added blows can only have the effect of crushing the pile and render ing it useless. It has frequently been discovered that piles which have been hammered after they have been driven to their limit, have become broken and crushed, perhaps several feet underground. In suelpeases, their supporting power is very much reduced.
Usually about two inches of the head is chamfered off to prevent this bruising and splitting in driving the pile. A steel band 2 to 3 inches wide and to 1 inch thick, is often hooped over the head of the pile to assist in keeping it from splitting. These devices have led to the use of a cast-iron cap for the protection of the head of the pile. The cap is made with two tapering recesses, one to fit on the chamfered head of the pile, and in the other is placed a piece of hard wood on which the hammer falls. The eap preserves the head of the pile.
207. Sawing Off the Piles. When the piles have been driven, they are sawed off to bring the top of them to the same elevation so that they will have an even bearing surface. When the tops of the piles are above water, this sawing is usually done by hand; and when under water, by machinery. The usual method of cutting piles off under water is by means of a circular saw on a vertical shaft which is supported on a special frame, the saw being operated by the engine used in driving the piles.
20S. Finishing the Foundations. When the heads of the piles arc above water, a layer of concrete is usually placed over them, the concrete resting on the ground between the piles, as well as on the piles themselves. It is necessary to use a thick plate of concrete, so that a concentrated load will be distributed over a number of piles (see Fig. 52). Sometimes a platform of heavy timbers is constructed on top of the piles, to assist in distributing the load; and in this case the concrete is placed on the platform (see Fig. 53).
'When the heads of the piles are under water, it is always neces sary to construct a grillage of heavy timber and float it into place, unless a cofferdam is constructed and the water pumped out, in which case the foundation can be completed as already described. The timbers used to cap the piles in constructing a grillage are usually about 12 by 12 inches, and are fastened to the head of each pile by a drift-boll (a plain bar of steel). A hole is bored in the cap and into the head of the pile, in which the drift-bolt is driven. The section of the drift-bolt is always larger than the hole into which it is to be driven; that is, if a 1-inch round drift-bolt is to be used, a '-inch auger would be used to bore the hole. The transverse timbers of the grillage are drift-bolted to the caps.