When the bearing capacity of soil immediately below the structure is insufficient for a spread footing, then piles are used to transfer the load to deeper, firmer strata. Piles may also be used where the soil is particularly affected by seasonal changes, to transfer the load below the level of such influence.
The load carrying (bearing) capacity of a pile is the sum of the end bearing capacity and the skin friction capacity between the peripheral area of the pile and the surrounding soil. The contribution of each differs widely depending on the ground conditions. For example, the skin friction resistance in sandy soils is small compared to clayey soils.
Usually, the load to be supported exceeds the bearing capacity of a single pile, and so a group of similar piles is used.
The group is capped by a spread footing or a cap to distribute the load to all piles in the group. Where there are a large number of closely spaced piles, rather than provide individual caps, it may be more economical to provide just one large cap, thus forming a piled raft.