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.
Piles are made of wood, cast iron, concrete, or steel, but for ordinary building they are usually made from the trunks of trees, and should be straight and sound, and at least eight inches in diameter for heavy buildings. Spruce, hemlock, Georgia pine, and oak are the principal kinds of wood in use for piles. The usual method of driving piles is by repeated blows given by a block of iron called the hammer, which works up and down between the uprights of a machine called a pile-driver. This hammer weighs from 1,200 to 2,500 pounds, and the fall varies from five to fifteen feet. Piles should be driven plumb, and any pile which has been driven for twenty feet or more, and refuses to sink more than half an inch under five blows of a 1,200 pound-hammer falling fifteen feet, may be considered as at its depth. Several formulae have been proposed for figuring the safe load upon piles, of which one of the latest, known as the Engineering News formula, is:
Safe load in pounds = 2 WH / S + 1 in which W = weight of hammer in pounds, H = its fall in feet, and S = the average set under the last blows in inches.
Piles should be spaced not less than two feet, nor more than three feet, on centers, and they must be cut off below low-water mark. The level at which piles are to be cut off will be given by the building-laws of most large cities, and is established at a level which will insure of the pile being at all times under water. Under these conditions the piling will be subject to no decay, but alternate conditions of moisture and dryness will soon result in the rotting of the piles.
The superintendence of piling will consist first in an examination of the piles as they are delivered, to see that they are of the requisite length and diameter, sound and straight. The lines of the building must be carefully established, and small stakes driven to fix the position of every pile. This should be verified by the superintendent according to the piling plan furnished. When the actual driving of the piles begins there should be kept a complete record giving the length of pile, the number of blows, and the distance which the pile has sunk at each of the last (ten) blows. From this data the bearing capacity of the pile may be computed by the foregoing rule. Another formula is known as Saunder's rule and is as follows:
FH / 8 S = W, in which F = fall of hammer in inches, S = sinking of pile at last blow, in inches, H = weight of hammer in pounds, W = safe load for pile in pounds.
Besides this record, the pile should be carefully watched while being driven, to see that it does not get out of line, that the head does not "broom" or split excessively. If there is danger of this, the head of the pile should be bound with a wrought-iron strap or a cast-iron cap. When the piles have been driven, trenches wide enough to accommodate the stone levelers must be excavated and kept free from water to a depth sufficient to allow of sawing off the piles at the required level. This is usually done by means of a crosscut saw operated by two men, and the tops must be cut off at a level with each other. Piles exposed to tide-water are usually driven with the bark on.