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 modifying the rigidity of these specifications, provided the most essential qualifications of strength and durability are 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 soil, it 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 cast-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 specification. The resistance to driving is considerably greater, while their ultimate bearing power is but little if any greater. If the driving of piles is considered from the standpoint of compacting the soil (as already discussed in section 177), then driving the piles with the small end down will compact the soil more effectively than driving them butt end down.

Fig. 49. Bevel Point for Sheet Pile.

Fig. 49. Bevel Point for Sheet Pile.

Fig. 50. Wrought Iron Pile Shoe.

Fig. 50. Wrought-Iron Pile-Shoe.

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 2V 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 teredo.

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."