Cost of Wood Piles. The cost of wood piles varies greatly with locality, and has been going up rapidly in recent years. The chief value of the following prices is to show the difference for dif ferent lengths and qualities. In 1908, in the North Central States, prices were about as follows: White or burr oak, 6-inch top and 12-inch butt, 20 to 30 feet long, 16 to 18 cents per foot; same, 40 to 60 ft. long, 21 to 25 cents according to length. Long-leaf yellow pine, 40 to 60 ft. long, 18 to 23 cents; and short-leaf pine, 14 to 15 cents; other soft woods 1 to 2 cents per foot less.
Iron Piles. Cast-iron piles were formerly used to a limited extent in the prairie States for supports for highway bridges; but have been abandoned—largely because of the introduction of concrete for piles and as a substitute for stone masonry. The horizontal cross section was cruciform, lugs or flanges were cast on the sides of the piles to which to attach sway braces; and after being driven, a cap with a socket in its lower side was placed upon the pile to receive the load. The advantages claimed for cast-iron piles are: (1) they are not subject to decay; (2) they are more readily driven than wooden ones, especially in stony ground or stiff clay; and (3) they possess greater crushing strength, which, however, is an advan tage only when the pile acts as a column (see § 771). The principal disadvantage is that they are deficient in transverse resistance to a suddenly applied force, which objection applies only to the handling of the piles before being driven and to such as are liable after being driven to sudden lateral blows, as from floating ice, logs, etc.
and steel sections were used to a limited degree for piles for supporting highway bridges, but their use was dis continued in favor of massive concrete substructures.
Steel tubes have been used as bearing piles to a limited extent, chiefly in New York City. The diameter varies from 6 to 16 inches, the thickness from } to inch, the length from 60 to 80 feet. The tubes are made in sections, the ends faced in a lathe, and the sections screwed together. They are sunk with a pneumatic hammer, and the material is removed from the inside of the tube with a water jet. If bowlders or hard pan are encountered, a chop ping drill is operated through the tube. After the pipe reaches the bed-rock, all the soil is removed from the inside of the tube, and rein forcing rods held apart by separators are placed inside, and then the tube is filled with cement mortar or rich concrete made of fine stone.
for light bridges.* The screw pile usually consists of a rolled-iron shaft, 3 to 10 inches in diameter, having at its foot one or two turns of a cast-iron screw, the blades of which may vary from 11 to 5 feet in diameter. The piles ordinarily employed for lighthouses exposed to moderate seas or to heavy fields of ice have a shaft 3 to 5 inches in diameter and blades 3 to 4 feet in diameter, the screw weighing from 600 to 700 pounds. For bridge piers, the shafts are from 6 to 10 inches and the blades from 4 to 5 feet in diameter, the screw weighing from 1,500 to 4,000 pounds.f For founding beacons, etc., the screw pile has the special advan tage of not being drawn out by the upward force of the waves against the superstructure. Even when all cohesion of the ground is de stroyed in screwing down a pile, a conical mass, with its apex at the bottom of the pile and its base at the surface, would have to be lifted to draw the pile out. The supporting power also is consider able, owing to the increased bearing surface of the screw blade. Screw piles have, therefore, an advantage in soft soil. They could also be used advantageously in situations where the jar of driving ordinary piles might disturb the equilibrium of adjacent structures.
These piles are usually screwed into the soil by men working with capstan bars. Sometimes a rope is wound around the shaft and the two ends pulled in opposite directions by two capstans, and sometimes the screw is turned by attaching a large cog-wheel to the shaft by a friction-clutch, which is rotated by a worm-screw operated by a hand crank. Horse-power, steam, and hydraulic power have been used for this purpose.$ The screw will penetrate most soils. It will pass through loose pebbles and stones without much difficulty, and push aside bowlders of moderate size. Ordinary clay does not present much obstruction; but clean, dry sand gives the most difficulty. The danger of twisting off the shaft limits the depth to which they may be sunk. Screw piles with blades 4 feet in diameter have been screwed 40 feet into a mixture of clay and sand. The resistance to sinking increases very rapidly with the diameter of the screw; but under favorable circumstances an ordinary screw pile can be sunk very quickly. Screws 4 feet in diameter have, in less than two hours, been sunk by hand labor 20 feet in sand and clay, the surface of which was 20 feet below the water. For depths of 15 to 20 feet, an average of 4 to 8 feet per day is good work for wholly hand labor.