This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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.
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 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 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.
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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.
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.