This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
When the heads of the piles are above water, a layer of concrete is usually placed over them, the concrete resting on the ground between the piles, as well as on the piles themselves. It is necessary to use a thick plate of concrete, so that a concentrated load will be distributed over a number of piles (see Fig. 52). Sometimes a platform of heavy timbers is constructed on top of the piles, to assist in distributing the load; and in this case the concrete is placed on the platform (see Fig. 53).
When the heads of the piles are under water, it is always necessary to construct a grillage of heavy timber and float it into place, unless a cofferdam is constructed and the water pumped out, in which case the foundation can be completed as already described. The timbers used to cap the piles in constructing a grillage are usually about 12 by 12 inches, and are fastened to the head of each pile by a drift-bolt (a plain bar of steel). A hole is bored in the cap and into the head of the pile, in which the drift-bolt is driven. The section of the drift-bolt is always larger than the hole into which it is to be driven; that is, if a 1-inch round drift-bolt is to be used, a 7/8-inch auger would be used to bore the hole. The transverse timbers of the grillage are drift-bolted to the caps.