In Boston it is obligatory to cap the piles with blocks of granite, which rest directly on the tops of the piles. If the stone does not fit the surface of the pile, or a pile is a little low, it is wedged up with oak or stone wedges. In capping with stone a section of the foundation should be laid out on the drawings showing the arrangement of the capping stones.
A single stone may rest on one, two or three piles, but not on four, as it is practically impossible to make the stone bear evenly on four piles. Fig. 10 shows the best arrangement of the capping for three rows of piles. Under dwellings and light buildings the piles are often spaced as in Fig. 11, in which case each stone should rest on three piles. After the piles are capped large footing stones, extending in one piece across the wall, should be laid in cement mortar, as shown in Fig. 12.
In New York a very common method of capping the piles is to excavate to a depth of 1 foot below the top of the piles and 1 foot outside of them, and fill the space thus excavated solid with rich Portland cement concrete, deposited in layers, and well rammed. After the concrete is brought level with the top of the piles additional layers are laid over the whole foundation until it reaches a depth of 18 inches above the piles. On this foundation bed, the brick or stone footings are laid as on solid earth. Many engineers consider this the best method of capping. There is certainly no question of its durability, and it is believed that the concrete will preserve the heads of the piles from rotting, provided the water is at all times up to the bottom of the concrete. A concrete beam 18 inches thick would also serve to distribute the pressure over the piles better than the stone capping, although not to such an extent as heavy grillage. If the soil is at all firm under the concrete, it will also assist the piles in carrying the load when concrete capping is used. Under very heavy buildings the space between the piles to the depth of 1 foot should be filled with concrete, whatever kind of capping is employed.