ARRANGEMENT OF THE FOUNDATION.
The length of the piles to be used is determined by the nature of the soil, or the conveniences for driving, or the lengths most easily obtained. The safe bearing power may be determined by equation 1 or 2, § 776, or from the data presented in § 780-85, or, better, by driving a pile and applying a test load. Then, knowing the weight to be supported, and having decided upon the length of piles to be used, and having ascertained their safe bearing power, it is an easy matter to determine how many piles are required. Of course, the number of piles under the different parts of a structure should be proportional to the weights of those parts.
It the attempt is made to drive piles too close together, they are liable to force each other up. To avoid this, the centers of the piles should be, at least, 2i or 3 feet apart. Of course, they may be farther apart, if a less number will give sufficient supporting power, or if a greater area of foundation is necessary to prevent overturning. When a grillage (§ 793) is to be placed on the head of the piles, great care must be taken to get the latter in line so that the lowest course of grillage timber, called capping, may rest squarely upon all the piles of a row. In driving under water, a convenient way of
marking the positions of the piles is to construct a light frame of narrow boards, called a spider, in which the position of the piles is indicated by a small square opening. This frame may be held in place by fastening it to the sides of the coffer-dam, or to the piles already driven, or to temporary supports. Under ordinary circumstances, it is reasonably good work if the center of the pile is somewhere under the cap. Piles frequently get considerably out of place in driving, in which case they may sometimes be forced back with a block and tackle or a jack-screw. When the heads of the piles are to be covered with concrete, the exact position of the piles is comparatively an unimportant matter.
In close driving, it is necessary to commence at the center of the area and work towards the sides; for if the central ones are left until the last, the soil may become so consolidated that they can scarcely be driven at all.