Rock River Thresheree - Library - Art of Pile-Driving





The Art of Pile-Driving More than you ever wanted to know about pile-driving...
197. Bearing Power of Piles. Pile foundations act in a variable combination of two methods of support. In one case
the piles are driven into the soil to such a depth that the frictional resistance of the soil to further penetration of the pile is greater than any load which will be placed on the pile. As the soil becomes more and more soft, the frictional resistance furnished by the soil is less and less; and it then becomes necessary that the pile shall penetrate to a strata of much greater density, into which it will penetrate but little if any. Under such conditions, the structure rests on a series of columns (the piles) which are supported by the hard subsoil, and whose action as columns is very greatly assisted by the density of the very soft soil through which the piles have passed. It practically makes but little difference which of these methods of support exists in any particular case. The piles are driven until the resistance furnished by each pile is as high as is desired. The resistance against the sinking of a pile depends on such a very large variety of conditions, that attempts to develop a formula for that resistance based on a theoretical computation taking in all these various factors, are practically useless. There are so many elements of the total resistance which are so large, and also so very uncertain, that they entirely overshadow the few elements which can be precisely calculated. Most formulae for pile-driving are based on general proposition that the resistance of the pile, multiplied by its motion during the last blow, equals the weight of the hammer multiplied by the distance through which it falls. To express this algebraically:
then, according to the above principle, we have:
Practically, such a formula is considerably modified, owing to the fact that the resistance of a pile (or its penetration for
any blow) depends considerably on the time which has elapsed since the previous blow. This practically means that it is far easier to drive the pile, provided the blows are delivered very rapidly; and also that when a pause is made in the driving for a few minutes or for an hour, the penetration is very much less (and the apparent resistance very much greater), on account of the earth settling around the pile during the interval. The most commonly used formula for pile-driving is known as the Engineering News formula, which, when used for ordinary hammer-driving, is as follows:

This formula is fundamentally the same as the formula given above, except that,
In the above equation, s is considered a free-falling hammer (not retarded by hammer rope) striking a pile having a
sound head. If a friction-clutch driver is used, so that the hammer is retarded by the rope attached to it, the penetration of the pile is commonly assumed to be just one-half what it would have been had no rope been attached (that is, had it been free-falling). Also, the quantity s is arbitrarily increased by 1, to allow for the influence of the settling of the earth during ordinary
hammer pile-driving, and a factor of safety of 6 for safe load has been used. In spite of the extreme simplicity of this formula compared with that of others which have attempted to allow for all possible modifying causes, this formula has been found to give very good results. when computing the bearing power of a pile, the penetration of the pile during the last blow is determined by averaging the total penetration during the last five blows. The pile-driving specifications adopted by the American Railway Engineering & Maintenance of Way Association,
require that, "All piles shall be driven to a firm bearing satisfactory to the Engineer, or until five blows of a hammer weighing 3,000 pounds, falling 15
feet (or a hammer and fall producing the same mechanical effect), are required to drive a pile one-half (½) inch per blow, except in soft bottom, which special instructions shall be given." This is equivalent to saying (applying the Engineering News formula) that the piles must have a bearing power of
60,000 pounds. 198. Example 1. The total penetration during the last five blows was 14 inches for a pile driven with a 3,000 pound hammer. During
these blows the average drop of the hammer was 24 feet. How much is the safe load?
199. Example 2. It is required (if possible) to drive piles with a 3,000-pound hammer until the indicated resistance is 70,000 pounds.
What should be the average penetration during the last five blows when the fall is 25 feet?
200. The last problem suggests a possible impracticability, for it may readily happen that when the pile has been driven
to its full length its indicated resistance is still far less than is desired. In some cases, such piles would merely by left as they are, and additional piles would be driven beside them, in the endeavor to obtain as much total resistance over the whole foundation as is desired. The above formula applies only to the drop-hammer method of driving piles, in which a weight of 2,500 to 3,000
pounds is raised and dropped on the pile. When the steam pile-driver is used, the blows are very rapid, about 55 to 65 per minute. On account of this rapidity the
soil does not have time to settle between the successive blows, and the penetration of the pile is much more rapid, while of course the resistance after the driving is finished is just as great as is secured by any other method. On this account, the above formula is modified so that the arbitrary quantity added to s is changed from one to 0.1, and the formula becomes:
201. Methods of driving piles. There are three general methods of driving piles - namely, by using (1) a falling weight;
(2) the erosive action of a water-jet; or (3) the force of an explosive. The third method is not often employed, and will not be further discussed. In constructing foundations for small highway bridges, well-augers have been used to bore holes, in which piles are set and the earth rammed around them. 202. Drop-Hammer Pile-Driver. This method of driving piles consists of raising a hammer made of cast iron, and
weighing from 2,500 to 3,000 pounds, to a height of 10 to 30 feet, and then allowing it to fall freely on the head of the pile. The weight is hoisted by means of a hoisting engine, or sometimes by horses. When an engine is used for the hoisting, the winding drum is sometimes merely released, and the weight in falling drags the rope and turns the hoisting drum as it falls. This reduces the effectiveness of the blow, and lowers the value of s in the formula given, as already mentioned. To guide the hammer in falling, a frame, consisting of two uprights called leaders, about 2 feet apart, is erected. The uprights are usually wooden beams, and are from 10 to 60 feet long. Such a simple method of pile-driving, however, has the disadvantage; not only that the blows are infrequent (not more than 20 or even 10 per minute), but also that the effectiveness of the blows is reduced on account of the settling of the earth around the piles between the successive blows. On this account, a form of pile-driver known as the steam pile-driver is much more effective and economical, even though the initial cost is considerably greater.
DRIVING A "RAYMOND" CONCRETE PILE
The concrete core is held up between the leaders, ready to be driven. The steel shell on the right is drawn up high enough to be lowered into the shell just driven and then slipped up to the core.

203. Steam-Hammer Pile-Driver. The steam pile-driver is essentially a

hammer which is attached directly to a piston in a steam cylinder. The hammer weighs about 4,000 pounds, is raised by steam to the full height of the cylinder, which is about 40 inches, and is then allowed to fall freely. Although the height of fall is far less than that of the ordinary pile-driver, the weight of the hammer is about double, and the blows are very rapid (about 50 to 65 per minute). As before stated, on account of this rapidity, the soil does not have time to settle between blows, and the penetration of the pile is much more rapid, while, of course, the ultimate resistance after the driving is finished, is just as great as that secured by any other method.

204. Driving Piles with Water-Jet. When piles are driven in a situation

where a sufficient supply of water is available, their resistance during driving may be very materially reduced by attaching a pipe to the side of the pile and forcing water through the pipe by means of a pump. The water returns to the surface along the sides of the pile and thus reduces its frictional resistance. The water also softens and scours out the soil immediately underneath the pile, and enables the pile to penetrate the soil much more easily. In very soft soils, piles may be thus driven by merely loading a comparatively small weight on top of the pile while the force pump is being operated; and yet the resistance shortly after stopping the pump will be found to be very great. Of course the only method of testing such resistance is by actually loading a considerable weight on the pile. This method of using a water-jet is chiefly applicable in structures which are on the banks of streams or large bodies of water. The water-jet and the hammer are advantageously used together, especially in stiff clay.

205. Splicing Piles. On account of the comparatively slight resistance offered by piles in swampy places, it sometimes
becomes necessary to splice two piles together. The splice is often made by cutting the ends of the piles perfectly square so as to make a good butt joint. A hole 2 inches in diameter and 12 inches deep is bored in each of the butting ends, and a dowel-pin 23 inches long is driven in the hole bored in the first pile; the second pile is then fitted on the first one. The sides of the piles are then flattened, and four 2 by 4-inch planks, 4 to 6 feet long, are securely spiked on the flattened sides of the piles. Such a joint is weak at its best, and the power of lateral resistance of a joint pile is less than would be expected from a single stick of equal length. Nevertheless, such an arrangement is in some cases the only solution.

206. Pile-Caps. One practical trouble in driving piles, especially those made of soft wood, is that the end of the pile will
become crushed or broomed by the action of the heavy hammer. Unless this crushed material is trimmed off the head of the pile, the effect of the hammer is largely lost in striking this cushioned head. This crushed portion of the top of a pile should always be cut off just before the test blows are made to determine the resistance of the pile, since the resistance of a pile indicated by blows upon it, if its end is broomed, will apparently be far greater than the actual resistance of the pile. Another advantage of the steam pile-driver is that it does not produce such an amount of brooming as is caused by the
ordinary pile-driver. Whenever the hammer bounces off the head of the pile, it shows either that the fall is too great or that the pile has already been driven to its limit. Whenever the pile refuses to penetrate appreciably for each blow, it is useless to drive it any further, since added blows can only have the effect of crushing the pile and rendering it useless. It has frequently been discovered that piles which have been hammered after they have been driven to their limit, have become broken and crushed, perhaps several feet underground. In such cases, their supporting power is very much reduced. Usually about two inches of the head is chamfered off to prevent this bruising and splitting in driving the pile. A steel band
2 to 3 inches wide and ½ to 1 inch thick, is often hooped over the head of the pile to assist in keeping it from splitting. These devices have led to the use of a cast-iron cap for the protection of the head of the pile. The cap is made of two tapering recesses, one to fit on the chamfered head of the pile, and in the other is placed a piece of hardwood on which the hammer falls. The cap preserves the head of the pile.

BIBLIOGRAPHY:

Turneaure, Frederick E., ed. Cyclopedia of Civil Engineering - A General Reference Work, Vol. IV. Chicago: American School of

Corresponence/American Technical Society, 1913; pp. 123-129.