Pile Foundations. Piles are made of wood, cast iron, con crete, or steel, but for ordinary building they are usually made front the trunks of trees, and should be straight and sound, and at least eight inches in diameter for heavy buildings. Spruce, hemlock, Georgia pine, and oak are the principal kinds of wood in use for piles. The usual method of driving piles is by repeated blows given by a block of iron called the hammer, which works up and clown between the uprights of a machine called a pile-driver. This hammer weighs from 1,200 to 2,500 pounds, ansl the fall varies front five to fifteen feet. Piles should be driven plumb, and any pile which has been driven for twenty feet or more, and refuses to sink more than half an inch under five blows of a 1,200 pound-hammer falling fifteen feet, may be con sidered as at its depth. Several f n.mule have been proposed for figuring the safe load upon piles, of which one of the latest, known as the Engineering News formula, is: 2 WH Safe load in pounds = 5 m 1 in which W = weight of hammer in pounds, H = its fall in feet, and S = the average set under the last blows in inches.
Piles should be spaced not less than two feet, nor more than three feet, on centers, and they must be cut off below low-water mark. The level at which piles are to be cut off will be given by the building laws of most large cities, and is established at a level which will insure of the pile being at all times under water. Under these ditions the piling will be subject to no decay, but alternate conditions of moisture and dryness will soon result in the rotting of the piles. The superintendence of piling will consist first in an examination of the piles as they are delivered, to see that they are of the requisite length and diameter, sound and straight. The lines of the building must be carefully established, and small stakes driven to fix the position of every pile. This should be verified by the superintendent according to the piling plan furnished. When the actual driving of the piles begins there should be kept a complete record giving the length of pile, the number of blows, and the distance which the pile has sunk at each of the last (ten) blows. From this data the bearing capacity of the pile may be computed by the foregoing rule. Another formula is known as Sounder's rule and is as follows: in which F fall of hammer in inches, S = sinking or pile at last blow, in inches, iI = weight of hammer in pounds, vA' r- safe lead for pile in pounds.
Besides this record, the pile should be carefully watched while being driven, to see that it does not get out of line, that the head, does not "broom" or split excessively. If there is danger of this, the head of the pile should be bound with a wrought-iron strap or a cast-iron cap. When the piles have been driven, trenches wide enough to accommodate the stone levelers must be excavated and kept free from water to a depth sufficient to allow of sawing off the piles at the required level. This is usually done by means of a cross
cut saw operated by two men, mid the tops must be cut off at a level with each other. Piles exposed to tide-water are usually driven with the bark on.
Footing Stones and Concrete. Whether the bearing he piles or the natural earth, the bottom of the foundation will usually consist of large stones or of concrete. If three rows of piles are used they may be covered as shown in Fig. 0G. Two rows of piles are generally capped by a series of levelers laid across the trench as in Fig. 97. A footing on earth will he laid in a like manner, and of a width made necessary by the load to be borne and the nature of the soil. The purposes of these wide footings are to spread the weight over a large area and also to add stability to the wall, and they may be of stone, brick, or concrete. For nearly all buildings on solid ground, concrete footings are probably the hest, and in many cases concrete enililiW1; for piles be used to advantage.
't'renc'hes for the footing way be dug below the regular excava tion and of the exact width required, and into this the concrete univ be tamped (Fig. 98). A good proportion is one part of cement, two parts of sand, and four parts of stone, for natural cement. The thickness of the footing should be one-quarter of its width (provided this does not figure less than twelve inches) put in by layers about six inches each. Tf this concrete is much wider than the wall over it, a stone leveler may be placed on the top, as shown in Fig. 9ti A.
If preferred, stone footings of a similar character may be used, as in Fig. 99, and for light buildings where stone is hard to obtain, brick footings may be used. If this is done, the offsets should never be more than one quarter of a brick, and the outside work should be all headers, with a double course at the bottom. (Fig.
100.) This course should be laid in a bed of mortar spread on the bottom of the trench, of hard burnt bricks, thoroughly wet if the weather is dry. Too mudi care can never be taken to insure a good foundation. If important footings are made of concrete, an inspector should be on the work during working hours, to see that the concrete is mixed in proper por tions, and put in to the full thick ness shown, and tamped and lev eled every six inches. The trenches must be kept free from water until the cement has set, o • it will become utterly worthless, by son of the water separating the cement from the sand. If the footing is of stone, the presence of water, if only a few inches deep, will do no harm, but the footing then must be bedded in firm sand or gravel instead of cement.