196. Wooden Bearing Piles. Specifications for wooden piles generally require that they shall have a diameter of from 7 to 10 inches at the smaller end, and 12 to 15 inches at the larger end. Older specifications were quite rigid in insisting that the tree trunks should be straight, and that the piles should be free from various kinds of minor defects; but the growing scarcity of timber is modify ing the rigidity of these specifications, provided the most essential qualifications of strength and durability arc provided for. Timber piles should have the bark removed before being driven, unless the piles are to be always under water. They should be cut square at the driving end, and pointed at the lower end. When they are to be driven in hard, gravelly is often specified that they shall be shod with some form of iron shoe. This may be done by means of two straps of wrought iron, which are bent over the point so as to form four bands radiating from the point of the pile (see Fig. 50). By means of holes through them, these bands are spiked to the piles. Another method, although it is considered less effective on account of its liability to be displaced during driving, is to use a east-iron shoe. These shoes are illustrated in Fig. 51. It is sometimes specified that piles shall be driven with the butt end or larger end down, but there seems to be little if any justification for such a speci fication. The resistance to driving is con siderably greater, while their ultimate bear Mg power is but Uttle if any greater. If the driving of piles is considered from the standpoint of compacting the soil (as al ready discussed in section 177), then driv ing the piles with the small end down will compact the soil more effectively than driv ing them butt end down.
White pine, spruce, or even hemlock may be used in soft soils; yellow pine in firmer ones; and oak, elm, beech, etc., in the more compact soils. They are usually driven from 21- to 4 'feet apart each way, center to center, depending on the character of the soil and the load to be supported. Timber piles, when partly above and partly under water, will decay very rapidly at the water line. This is owing to the alternation of dryness and wetness. In tidal waters, they are destroyed by the marine worm known as the tercdo.
The American Railway Engineering & Maintenance of Way Association recommends the following specifications for piling: "Piles shall be cut from sound, live trees; shall be close-grained and solid; free from defects such as injurious ring shakes, large and unsound knots, decay, or other defects that will materially impair their strength. The taper from butt to top shall be uniform and free from short bends.
"All piles except foundation piles shall be peeled." 197. Bearing Power of Piles. Pile foundations act in a variable combination of two methods of support. In one case the piles are driven into the soil to such a depth that the frictional resistance of the soil to further penetration of the pile is greater than any load which will be placed on the pile. As the soil becomes more and more
soft, the frictional resistance furnished by the soil is less and less; and it then becomes necessary that the pile shall penetrate to a strata of much greater density, into which it will penetrate but little if any. Under such conditions, the structure rests on a series of columns (the piles) which are supported by the hard subsoil, and whose action as columns is very greatly assisted by the density of the very soft soil through which the piles have passed. It practically makes but little difference which of of support exists in any particular case. The piles arc driven until the resistance furnished by each pile is as high as is desired. The resistance against the sinking of a pile depends on such a very large variety of conditions, that attempts to develop a formula for that resistance based on a theoretical computa tion taking in all these various factors, are practically useless. There are so many elements of the total resistance which are so large, and also so very uncertain, that they entirely overshadow the few elements which can be precisely calculated. formula; for pile-driving are based on the general proposition that the resistance of the pile, multiplied by its motion during the last blow, equals the weight of the hammer multiplied by the distance through which it falls. To express this algebraically: If R = Resistance of pile; s = Penetration of pile during last blow; Weight of hammer; h = Height of fall of hammer; Practically, such a formula is considerably modified, owing to the fact that the resistance of a pile (orits penetration for any blow) depends considerably on the time which has elapsed since the previous blow. This practically means that it is far easier to drive the pile, provided the blows are delivered very rapidly; and also that when a pause is made in the driving for a few minutes or for an hour, the penetration is very much less (and the apparent resistance very much greater), on account of the earth settling around the pile during the interval. The most commonly used formula for pile-driving is known as the Engineering News formula, which, when used for ordinary hammer-driving, is as follows: This formula is fundamentally the same as the formula given above, except that, R = Safe load, in pounds; s = Penetration, in inches, w = Weight of hammer, in pounds; h = Height of fall of hammer, in feet.
In the above equation, s is considered a free-falling hammer (not retarded by hammer rope) striking a pile having a sound head. If a friction-clutch driver is used, so that the hammer is retarded by the rope attached to it, the penetration of the pile is commonly assumed to be just one-half would have been had no rope been attached (that is, had it been free-falling).