II-I-ought-iron Piles: Screw piles are also used to a considerable extent for the foundation of important structures (as, for example, light-houses exposed to the action of waves or of ice). These are shafts of rolled iron, from 5 to 8 inches in diameter, which, instead of being rammed into place, are furnished at the foot with a cast-iron screw 051. 5o, fig. II), by i.vhich they are screwed into place by men working at long levers attached to their tops, after the fashion of capstau-bars. This system was devised by Alexander Mitchell of Belfast.
The Saha' Pile for soft soils is an expedient that has grown out of the use of sand in filling foundations, trenches, etc. The mode of forming these piles is thus described by Trautwine: "A short, stout wooden pile is first driven 5 or io feet, or more, according to the case. It is then drawn out, and the hole is filled with wet sand well rammed. The pile is then again driven in another place, and the process repeated. The intervals may be from to 3 feet in the clear. Platforms may be used on these piles, as on wooden ones. If the sand is not put in wet, it will be in danger of after ward sinking, from rain- or spring-water. In this case, as i.vith fascines, it is well to test the foundation by means of trial loads. Some settlement must inevitably take place until all parts come to a full bearing, but it will be comparatively trifling." Afechanical Afifiliances: sitnplest, and also the least effective, machines designed for driving piles, are the hand-hammers of i.vood or iron, weighing from twenty-five to thirty-five pounds. In another form, a hammer weighing from six hundred and fifty to thirteen hundred and fifty pounds is allowed to drop from a height upon the head of the pile. The hammer is raised between the guides of the scaffold by a rope. This passes over a pulley at the top of the frame, where it divides into a nuinber of cords, which are drawn upon by twenty or more work men. The hammer in this case is not lifted more than 5 feet at most. The men deliver about twenty or thirty blows, and then rest for a few moments before repeating them. In another form the hammer is not .raised directly by hand, but by the intertnediation of some mechanical appliance (a winch, tread-wlicel, or the like). By a further improvement introduced with this form of the machine, the hammer, when raised to a certain height, is auto matically released, and drops upon the head of the pile; then the book that held the hammer follows, and seizes it again. In this form of the machine the weight of. the hammer varies from eleven hundred to seventeen hun dred pounds and the drop from zo to 3o feet. I3ecause of the greater dis tance of the drop, the last-described machine is much more effective than the preceding hand-machines. Aside from this, in the windlass-machines,
the strength of the workmen—of whom, usually, but from four to six are employed—is applied to better advantage. Pile-drivers operated by horse power have also been used.
modern improved forms of pile-drivers, actu ated by steam-power, are much more effective than those above described. In the power-machines in common use the steam is employed to actuate a winding engine by which the pile is lifted into position and the hammer raised to the proper height after each blow has been delivered. In another form of the machine, the steam-hammer pile-driyer of Nasmyth, a very heavy hammer (from three thousand to six thousand pounds in weight) is given a comparatively small drop (about 3 feet), and delivers a number of blows in quick succession. In this case the hammer is impelled by steam pressure in addition to its own weight.
Gunpowder work has been accomplished by the gunpowder pile-driver of Thomas Shaw, an American engineer (pl. 50, jig. 8), which is from four to eight times as effective as the ordinary steam pile-drivers. In this novel machine, the hammer is operated by small cartridges of gunpowder introduced one at a time in a species of mortar, which rests on the top of the pile. The falling of the hammer explodes the cartridge, driving the pile deeply into the soil, while the recoil at the same time throws the hammer back to its first position, ready for a second blow. From thirty to forty blows per minute can readily be delivered with this machine.
To protect the heads of the piles from being shattered by the blows of the hammer, it is customary to surround them with a stout hoop of iron. This precaution, however, does not always prevent the heads front split ting, and in hard drivinf.; it is frequently found necessary to saw off the head of a pile several tittles before it is driven home. This contingency must be considered in deciding upon the proper length of piles for work in soils difficult of penetration. It is worthy of note that the Shaw gunpow der pile-driyer does not injure the heads of the piles; in fact, the hammer never actually strikes the anvil, the cartridge being exploded, not by direct impact, but by the heat of air-compression in the recess in the top of the anvil, into which the piston of the hammer enters. The firm union of the piling may at times preferably be secured by filling in the space between them with concrete; or the heads of the piles may be permitted to project into the masonry-work of the foundation, in which case their connection by a grillage may be dispensed with.