The error most frequently made in the application of the water jet is in using pumps with insufficient capacity. Both direct-acting and centrifugal pumps are frequently employed. The former affords the greater power; but the latter has the advantage of a less first cost, and of not being damaged as greatly by sand in the water used.
The pumping plant used in sinking the disk-piles for the Coney Island pier (see 4 735), "consisted of a Worthington pump with a 12-inch steam cylinder, 8i-inch stroke, and a water cylinder 7+} inches in diameter. The suction hose was 4 inches in diameter; and the discharge hose, which was of four-ply gum, was 3 inches. The bbiler was upright, 42 inches in diameter, 8 feet high, and con tained 62 tubes 2 inches in diameter. An abundance of steam was supplied by the boiler, after the exhaust had been turned into the smoke-stack and soft coal used as fuel. An average of about 160 pounds of coal was consumed in sinking each pile. With the power above described, it was found that piles could be driven in clear sand at the rate of 3 feet per minute to a depth of 12 feet; after which the rate of progress gradually diminished; until at 18 feet a limit was reached beyond which it was not practicable to go without con siderable loss of time. It frequently happened that the pile would `bring up' on some tenacious material which was assumed to be clay, and through which the water jet, unaided, could not be made to force a passage. In such cases it was found that by raising the pile about
6 inches and allowing it to drop suddenly, with the jet still in opera tion, and repeating as rapidly as possible, the obstruction was finally overcome; although in some instances five or six hours were con sumed in sinking as many feet." * Jet vs. Hammer. It is hardly possible to make a comparison between a water-jet and a hammer pile-driver, as the conditions most favorable for each are directly opposite. For example, sand yields easily to the jet, but offers great resistance to driving with the hammer; on the other hand, in stiff clay the hammer is much more expeditious. For inland work the hammer is better, owing to the difficulty of obtaining the large quantities of water required for the jet; but for river and harbor work the jet is the most tageous. Under equally favorable conditions there is little or no difference in cost or speed of the two methods.
The jet and the hammer are often advantageously used together, especially in stiff clay. The efficiency of the water jet can 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.