It is frequently claimed that the steam-hammer can drive a pile against a greater resistance than the drop-hammer. As compared with the old style drop-hammer, i.e., without the friction-drum and the pile-hood, this is probably true. The striking of the weight upon the head of the pile splits and brooms it very much, which materially diminishes the effectiveness of the blow (for an example, see Table 64, page 390). In hard driving with the drop-hammer, without the pile-hood, the heads of the piles, even when hooped, will crush, bulge out, and frequently split for many feet below the hoop. For this reason, it is sometimes specified that piles shall not be driven with a drop-hammer.
The rapidity of the blows is an important item as affecting the efficiency of a pile-driver. If the blows are delivered rapidly, the soil does not have sufficient time to recompact itself about the pile. With the steam-driver the_blows are delivered in such quick succession that it is probable that a second blow is delivered before the pile has recovered from the distortion produced by the first, which materially increases the effectiveness of the second blow. In this respect the steam-hammer is superior to the drop-hammer, and the friction clutch driver is superior to the nipper driver. • In soft soils, the steam-hammer drives piles faster than either form of the drop-hammer, since after being placed in position on the head of the pile it pounds away without the loss of any time.
- In a rough way the first cost of the two drivers—exclusive of scow or car, hoisting engine, and boiler, which are the same in each—is about $80 for the drop-hammer driver, and about $800 for the steam-driver. Of course these prices will vary greatly. The per cent for wear and tear is greater for the drop-hammer than for the steam-hammer. For work at a distance from a machine-shop the steam-driver is more liable to cause delays, owing to breakage of some part which can not readily be repaired.
_ _ _ _ Driving Piles with Dynamite. Occasionally piles are driven by exploding dynamite placed directly upon the top of the pile. It is a slow method, but might prove valuable where only a few piles were to be driven, by saving the transportation of a machine; or it might be employed in locations where a machine could not be oper ated. The higher grades of dynamite are most suitable for this purpose.
The method is very simple. A jet of water is forced into the soil just . below the point of the pile, thus loosening the soil and allowing the pile to sink, either by its own weight or with very light blows. The water may be conveyed to the point of the pile through a flexible hose held in place by staples driven into the pile; and after the pile is sunk, the hose may be withdrawn for use again. An iron pipe may be substituted for the hose. It seems to make very little difference, either in the rapidity of the sinking or in the accuracy with which the pile preserves its position, whether the nozzle is exactly under the middle of the pile or not.
The water jet seems to have been first used in engineering in 1852, at the suggestion of General Geo. B. McClellan. It has been extensively employed on the sandy shores of the Gulf and South Atlantic States, where the compactness of the sand makes it difficult to drive piles for foundations for lighthouses, wharves, etc. An other reason for its use in that section is that the palmetto piles— the only ones that will resist the ravages of the teredo—are too soft to withstand the blows of the drop-hammer pile-driver. By em ploying the water jet the necessity for the use of the pile-hammer is removed, and consequently palmetto piles become available. The jet has also been employed in a great variety dlrways to facilitate the passage of common piles, screw and disk piles, cylinders, caissons, etc., through earthly material; and also to loosen the soil around piles preparatory to pulling them out.
The efficiency of the jet depends upon the increased fluidity given to the material into which the piles are sunk, the actual dis placement of material being small. Hence the efficiency of the jet is greatest in clear sand, mud, or soft clay; in gravel, or in sand con taining a large percentage of gravel, or in hard clay, the jet is almost useless. For these reasons the engine, pump, hose, and nozzle should be arranged to deliver large quantities of water with a moderate force, rather than smaller quantities with high initial velocity. In gravel, or in sand containing considerable gravel, some benefit might result from a velocity sufficient to displace the pebbles and drive them from the vicinity of the pile; but it is evident that any prac ticable velocity would be powerless in gravel, except for a very limited depth, or where circumstances favored the prompt removal of the pebbles.