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 Rail way 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 (1) inch per blow, except in soft bottom, when special instructions will be given." This is equivalent to saying (applying the Engineering Yews 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? 2 w h _ 2 X 3,000 X 24 _ 144,000 — 38,919 pounds.
s + 1 X 14) + 1 — 3.7 199. 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 ueadily happen that when the pile has been driven to its full length its indicated resistance is still far less than that desired. In some cases, such piles would merely be left as they are, and addi tional 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 05 per minute. On account of this rapidity the soil does not have time to settle between the successive blows, and the pene tration 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 dis cussed. 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 in 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 hoist ing 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, abort 2 feet apart, is erected. The uprights are usually wooden beams, and are from 10 to GO 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.
203. Stea nt-H Pile-Driver. The steam pile-driver is essentially a hammer which is attached directly to a piston in a steam qlinder. 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.