Piles should be prepared for driving by cutting off all limbs close to the trunk and removing the bark.
* " A Practical Treatise on Foundations." W. M. Patton, C. E.
The small end should be sharpened to a point 2 inches square, the bevel being from 18 to 24 inches long. The large end should be cut square to receive the blows from the hammer.
Experience has shown that in soft and silty soils the piles can be driven in better line without pointing. A pointed pile, on striking a root or similar obstruction, will inevitably glance off, and no available power can prevent it; while a blunt pile will cut or break the obstruction without being diverted from its position.
Piles that are to be driven in, or exposed to, salt water should be thoroughly impregnated with creosote, dead oil of coal tar, or some mineral poison to protect them from the "teredo" or ship worm, which will completely honeycomb an ordinary pile in three or four years.
Ringing. - When the penetration at each blow is less than 6 inches, the top of the pile should be protected from " brooming" by putting on an iron ring about 1 inch less in diameter than the head of the pile, and from 2½ to 3 inches wide by 5/8 inches thick. It is better to chamfer the head so the ring will just fit on than to drive the ring into the wood by the hammer, as the latter method is liable to split long pieces from the pile. When driving into compact soil, such as sand, gravel or stiff clay, the point of the pile is often shod with iron, either in the form of straps bolted to the end of the pile, as at a, Fig. 9, or by a conical cast steel shoe about 5 inches in diameter and having a 1¼ -inch dowel 12 inches long fitting into a hole in the end of the pile and a ring put around the pile, as shown at b, to prevent it from splitting. The latter method should be used in very hard soils. If straps are used, as at a, they should be 2½ inches wide, ½ inch thick and 4 feet long.
The usual method of driving piles is by a succession of blows given with a block of cast iron called the hammer, which works up and down between the uprights of a frame or machine called a pile-driver. The machine is placed over the pile, so that the hammer descends fairly on its head, the piles always being driven with the small end down. The hammer is generally raised by steam power, and is dropped either automatically or by hand. The usual weight of the hammers used for driving piles for building foundations is from 1,200 to 1,500 pounds, and the fall varies from 5 to 20 feet, the last blows being given with a short fall.
In driving piles care should be taken to keep them plumb, and when the penetration becomes small the fall should be reduced to about 5 feet, the blows being given in rapid succession.
Whenever a pile refuses to sink under several blows, before reaching the average depth, it should be cut off and another pile driven beside it.
When several piles have been driven to a depth of 20 feet or more and refuse to sink more than ½ inch under five blows of a 1,200 pound hammer falling 15 feet, it is useless to try them further, as the additional blows only result in brooming and crushing the head and point of the pile, and splitting and crushing the intermediate portions to an unknown extent.
"Sometimes piles drive easily and regularly to a certain depth, and then refuse to penetrate farther; this may be caused by a thin stratum of some hard material, such as cemented gravel and sand or a compact marl. It may require many hard and heavy blows to drive through this, thereby injuring the piles, and perhaps getting into a quicksand or other soft material, when the pile will drive easily again. If the depth of the overlying soil penetrated is sufficient to give lateral stability, or if this can be secured by artificial means, such as throwing in broken stone or gravel, it would seem unwise to endeavor to penetrate the hard stratum, and the driving should be stopped after a practical refusal to go with two or three blows. The thickness of this stratum and nature of the underlying material should be either determined by boring or by driving a test pile, to destruction if necessary. In the latter case the driving of the remaining piles should cease as soon as the hard stratum is reached." *
If the hard stratum, however, is only 2 or 3 feet thick, with hard pan not more than 40 or 50 feet from the surface, the piles should be driven to hard pan for heavy buildings; but if the soft material continues for an indefinite depth below the hard stratum, the piles should be stopped when the stratum is reached. In such cases, however, the actual bearing power of the piles should be tested by loading one or more of the piles, as described in Section 40.
*"A Practical Treatise on Foundations." W. M. Patton, C. E.