The steam hammer is also employed for driving piles, and has certain advantages over the ordinary form, the chief of which lies in the great rapidity with which the blows follow one another, allow ing no time for the disturbed earth, sand, etc., to recompact itself around the sides and under the foot of the pile. It is less liable than other methods to split and broom the piles, so that these may be of softer and cheaper wood, and the piles are not so liable to "dodge" or get out of line.
When piles have to be driven below the end of the leaders of the pile driver a follower is used. This is made from a pile of suita ble length placed on top of the pile to be driven. To prevent its bouncing off caps of cast iron are used, one end being bolted to the follower and the other end fitting over the head of the pile.
Piles are also driven by the "water jet." This process consists of an iron pipe fastened by staples to the side of the pile, its lower end placed near the point of the pile and its upper end connected by a hose to a force pump. The pile can be sunk through almost any material, except hardpan and rock, by forcing water through the pipe. It seems to make very little difference, either in the rapidity of 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 efficiency of the jet depends upon the increased fluidity given the material into which the piles are sunk, the actual displace ment 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 moder ate force rather than smaller quantities with high initial velocity. In gravel, or in sand containing gravel, some benefit might result from a velocity sufficient to displace the pebbles and drive them from the vicinity of the pile. The error most frequently made in the applica tion of the water jet is in using pumps with insufficient capacity.
The approximate volume of water required per minute, per inch of average diameter of pile, for penetrations under 40 feet is 16 gallons; for greater depths the increase in the volume of water is approximately at the rate of 4 gallons per inch of diameter of pile per minute, for each additional 10 feet of penetration.
The number and size of pipes required for various depths are about as follows: When the descent of the pile becomes slow, or it sticks or "brings up" in some tenacious material, it can usually be started by striking a few blows with the pile-driving hammer, or by raising the pile about 6 inches and allowing it to drop suddenly, with the jet in operation. By repeating the operation as rapidly as possible the obstruction will generally be overcome.
It is an advantage to use an ordinary pile-driving machine for sinking piles with the water jet. The hammer being allowed to rest upon the head of the pile aids in accelerating the descent, and light blows can be struck as often as may appear necessary. The effi ciency of the jet can also be greatly increased by bringing the weight of the pontoon upon which the machinery is placed to bear upon the pile by means of a block and tackle.
Splicing Piles. It frequently happens in driving piles in swampy places, for false works, etc., that a single pile is not long enough, in which case two are spliced together. A common method of doing this is as follows. After the first pile is driven its head is cut off square, a hole 2 inches in diameter and 12 inches deep is bored in its head, and an oak treenail or dowel-pin 23 inches long is driven into the hole; another pile similarly squared and bored is placed upon the lower pile, and the driving continued. Spliced in this way the pile is deficient in lateral stiffness, and the upper section is liable to bounce off while driving. It is better to reinforce the splice by flattening the sides of the piles and nailing on with, say, f spikes four or more pieces 2 or 3 inches thick, 4 or 5 inches wide, and 4 to 6 feet long.