This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
When piles are driven closely to confine puddle in a coffer dam, or for preventing the fall of an earth bank, they are called sheet piles, and consist of planking from 2 to 6 in. thick, the bottoms being cut at an angle which forms a driving toe, that tends to keep the pile close up to the adjacent one. The heads are kept in line by securing longitudinal stringers, or wales, to them by spiking or bolting.
Piles which sustain loads are called bearing piles. They may be square or round in section, but are usually round, and are from 9 to 18 in. in diameter at the top. They may be used either with or without the bark. White pine or spruce is suitable where the ground is soft; where it is firmer, Georgia pine may be used to advantage. Where the soil is very compact, hard woods, such as oak, hickory, ash, elm, beech, etc., are used. Piles are usually driven at intervals of from 2 1/2 to 4 ft. between centers, according to the nature of the soil and the weight to be sustained.
The following formula gives the safe load for a pile:
W=2wh / (k+1), in which W = safe load in pounds; w = weight of hammer in pounds; h = fall of hammer in feet; k = penetration of pile in inches at the last blow (head of pile in good condition, not split or broomed).
This formula gives a factor of safety of 6. Assuming that the fall of the hammer is 30 ft., its weight 2,000 lb., and its penetration at the last blow 1/2 in., or .5 in., the safe load is
(2X2,000X30) / (.5+1)=120.000 / 1.5 =80,000 lb., or 40 tons.
Where piles are spaced at 3 ft. between centers each way, the foundation area will safely sustain a load of from 3 to 5 tons per sq. ft., and when spaced at 2 1/2 ft. between centers each way, the load may be increased to from 5 to 7 tons per sq. ft.
Where the soil is very hard, it is necessary to shoe the piles with cast or wrought iron, to make them drive more easily. In order to preserve the heads from brooming and splitting, wrought-iron hoops, from 1/2 to 1 in. thick, and 2 or 3 in. wide, are used.
When driven, the piles are carefully sawed off to the same level, usually below the water-line, and capped by cross-rows of timbers or planking, forming a grillage upon which the masonry is laid. Sometimes large flat stones are laid directly on several of the piles. This method is good, provided the stones are set so as to bear evenly on the piles, without much pinning up by spalls, which are liable to be crushed. The heads of the piles may also be embedded in concrete for 2 or 3 ft., which makes a very satisfactory foundation.