Foundation Engineering: Functions of Piles

When the soil bearing capacity is too low, then the land is too weak or too compressible to provide adequate support, hence the loads including dead and live loads are to be transferred to a more suitable material at a greater depth by drilling in piles of piers. 

Piles - structural members of small cross-sectional area compared to their length and are usually installed by a vibrator or hammer. Piles are grouped into clusters or rows each containing enough members to support a load delivered by a single column or wall

Piers - usually larger in cross section capable of transferring entire load from a single column to the supporting stratum

Columns with not much load can require just one single pile. However, field conditions should be considered too like the actual position of a pile that may be several inches away from planned location hence eccentricity of load may occur and can hardly be avoided. Consequently, the heads of piles are usually braced in two directions by grade beams. 

If only two piles are needed, heads may be connected by a concrete cap braced by grade beams in only one direction perpendicular to the line joining the two piles.

Three or more piles clustered together shall be provided with reinforced cap and are considered stable even without the support of grade beams.

Aside from its original purpose, vertical piles can also resist lateral loads such as winds and lateral earthquake at certain point. But when larger lateral loads are to be resisted, an inclined or batter piles are more applicable. A batter of four horizontal and twelve vertical represent about the greatest inclination that can be achieved with ordinary driving equipment. Economically, smaller inclinations are more favorable even if it requires more piles to be battered.