This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Calculations For Piles. The efficient bearing power of piles in different soils is very indefinite. A pile may go down through a stiff clay or gravel for some distance, sinking very little at each blow of the hammer, and may then penetrate a soft stratum, so that the pile will sometimes sink nearly as much at the last blow as at the first. In this case, the friction on the sides of the pile keeps it in place. But this friction is so great that even in marshy ground the ultimate bearing capacity of a pile 30 feet long is given as from 13,000 to 20,000 pounds, or from 6 1/2 to 10 tons. In alluvial soil, or moderately soft clay, the bearing capacity is from 20,000 to 40,000 pounds, or 10 to 20 tons; and in stiff clay, or compact sand, from 40,000 to 100,000 pounds, or 20 to 50 tons.
Multiply the weight of the ram in pounds, by the height it falls in inches, and divide the product by eight times the set or penetration at the last blow of the hammer.
The formula being:
8a in which h = fall in inches;
W = weight of hammer in pounds; a = penetration at last blow in inches;
P = safe load in pounds.
For example, suppose the weight of hammer to be 1,500 pounds; height of fall, 15 feet, or 180 inches; penetration of pile at last blow, 1 inch.
Then P = 1801500 =33,750 8x1 pounds, or nearly 17 tons, the load the pile will carry.
78. In all calculations of the bearing power of piles, a large safety factor is necessary, owing to the many uncertainties connected with the subject. The New York building laws require that a pile shall not be less than 5 inches in diameter at the smallest end, and that when walls, piers, or posts rest on them, they shall be spaced not more than 30 inches from centers, and shall not carry more than 20 tons to each pile.
Shoeing Piles. "When piles are driven through soft material to rock or hard gravel, the force of the blows of the hammer has a tendency to split the piles, after rock or hard gravel has been reached, thus greatly impairing their bearing capacity; to prevent this, piles are often protected at the end with wrought or cast iron shoes. Fig. 23 illustrates three different methods of shoeing the ends of piles. At (a) is shown a 2"x2 1/2" wronght-iron strap a bolted through the pile b, forming a shoe, which is the same on both sides of the pile. At (b) is shown a cast-iron conical shoe fitted over the end of the pile b; the head of the shoe c protects the end of the pile, and the straps a, one on each side, hold the shoe in place. One of the best forms of cast-iron shoe is shown at (c), Fig. 23. In this case, the pile has a blunt end from 4 to 6 inches in diameter shown at b. The shoe has a solid conical point c, the top of its base being about the same diameter as the end of the pile; the straps then extend up on the sides of the pile, and are bolted or spiked to it, as shown. The straps and bolts hold the shoe in place, while the plate end of the pile receives the effect of the blow. A shoe of this kind, will, to a great extent, prevent the end of the pile from brooming.