Description of Piles - pile, inches, head, driving, hood, hard and driven

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DESCRIPTION OF PILES.

Piles may be divided. into bearing piles and sheet piles. Bearing piles may be composed of wood, iron or steel, or concrete.

Wood Piles.

These are very important factors in founda tions on soft or swampy soil. All kinds of timber are employed. Spruce and hemlock answer very well, in soft or medium soils, for foundation piles or for piles always under water; the hard pines, elm, and beech, for firmer soils; and the hard oaks, for still more compact soils. Where the pile is alternately wet and dry, white or post oak and yellow or Southern pine are generally used. Piles should never be less than 6 inches, and preferably not less than 8 inches, in diameter at the small end, and never more than 18 inches, and preferably not more than 14 inches at the Large end. A pile should be straight, and should be trimmed close; and sometimes it is required that piles employed in foundations shall have the bark removed.

Pile Hood and Shoe. To prevent and splitting in driving, 2 or 3 inches of the head is usually chamfered off. As an additional means of preventing splitting, the head is often hooped with a strong iron band, 2 to 3 inches wide and to 1 inch thick. The expense of removing these bands and of replacing the broken ones, and the consequent delays, led to the introduction, recently, of a hood for the protection of the head of the pile. The hood consists of a cast-iron block with a tapered recess above and below, the chamfered head of the pile fitting into the lower recess and a cushion piece of hard wood, upon which the hammer falls, fitting into the upper one. The hood preserves the head of the pile, adds to the effectiveness of the blows (see Table 64, page 390), and keeps the pile head in place to receive the blows of the hammer. The device, above called a hood, is usually called a cap, which is unfor tunate, since the word cap is applied to a heavy horizontal timber placed on top of a row of piles.

A further advantage of the pile hood-is that it saves piles. In hard driving, without the hood the head is crushed or broomed to such an extent that the pile is adzed or sawed off several times before it is completely driven, and often after it is driven a portion of the head must be sawed off to secure sound wood upon which to rest the grillage or platform 0793-96). In ordering piles for any

special work where the driving is hard, allowance must be made for this loss.

Piles are generally sharpened before being driven; but many competent engineers claim that sharpening is of no advantage and is sometimes harmful. Sometimes, particularly in stony ground, the point is protected by an iron shoe; but some engineers claim that a shoe is no advantage. The shoe may be only two V-shaped loops of bar iron placed over the point, in planes at right angles to each other, and spiked to the piles; or it may be a wrought- or cast-iron socket, of which there are a number of forms on the market.

Splicing Piles. It frequently happens, in driving piles in swampy places, for false-works, etc., that a single pile is not long enough, in which case two are spliced together. A common method of doing this is as follows: After the first pile is driven its head is cut off square, a hole 2 inches in diameter and 12 inches deep is bored in its head, and an oak tree-nail, or dowel-pin, 23 inches long, is driven into the hole; another pile, similarly squared and bored, is placed upon the lower pile, and the driving continued. Spliced in this way the pile is deficient in lateral stiffness, and the upper section is liable to bounce off while driving. It is better to reinforce the splice by flatting the sides of the piles and nailing on, with say, 8-inch spikes, four or more pieces 2 or 3 inches thick, 4 or 5 inches wide, and 4 to 6 feet long. In the erection of the bridge over the Hudson at Poughkeepsie, N. Y., two piles were thus spliced together to form a single one 130 feet long.

Piles may be made of any required length or cross section by bolting and fishing together, sidewise and lengthwise, a number of squared timbers. Such piles are frequently used as guide piles in sinking pneumatic caissons (§ 885). Hollow-built piles, 40 inches in diameter and 80 feet long, were used for this purpose in constructing the St. Louis Bridge (I 889). They were sunk by pumping the sand and water from the inside of them with a sand pump (§ 877).