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.
A pile supports a load coming on an area of the foundation which is approximately proportional to the spacing between the piles. This area, of course, is several times the area of the top of the pile. It is therefore necessary to cap at least a group of the piles with a platform or grillage which not only will support any portion of the load located between the piles, but which also will tend to prevent a concentration of load on one pile and will distribute the load more or less uniformly over all the piles. Sometimes such a platform is made of heavy timbers, especially if timber is cheap; but this should never be done unless the grillage will be always under water; and even under such conditions the increasing cost of timber usually makes it preferable to construct the grillage of concrete. A concrete grillage is usually laid with its lower surface a foot or two below the tops of the piles. The piles are thus firmly anchored together at their tops. The thickness of the grillage is roughly proportional to the load per square foot to be carried. No close calculations are possible; a thickness of from 2 to 5 feet is usually made. When reinforced-concrete structures are supported on piles or other concentrated points of support, the heads of the piles are usually connected by reinforced-concrete beams, which will be described in Part III.