Pile foundation is actually a slender column or long cylinder made of materials such as concrete or steel (most commonly, but also wood or plastic have been used) constructed by driving preformed units to the desired founding level, or by driving or drilling-in tubes to the required depth. Pile foundations are usually used to support structures with heavy, concentrated loads by transferring the load at the desired depth. Their load capacity derives from their end bearing and skin friction resistance. For that reason, most commonly, pile foundations are of three main types based on structural behavior: End Bearing Piles, Friction Piles and combinations of the two.
In this type of pile, the bottom end of the pile rests on a strong layer of soil or rock. The loads from the superstructure are directly transferred to the hard soil through bearing action of the pile bottom tip. As a result, the pile acts as a column. The ultimate load capacity of a pile foundation is equal to the load carried by the end bearing pile or pile group. End bearing piles are selected when the depth of hard soil strata or bedrock is within a reasonable range.
Friction pile transfers the load from the structure to the soil via the frictional force that develops between the surface of the pile and the soil surrounding the pile. This type of pile foundation is selected when a hard stratum is available at large depth and construction of end bearing pile becomes uneconomical. The ultimate load capacity of a pile foundation is equal to the load carried by the friction pile or pile group.
These Figures are from D.P. Zekkos personal colle...