Pile Foundation

a substructure in which piles are used to transfer a load from a structure to the soil. A pile foundation consists of piles and a connecting foundation mat (Figure 1). The choice between a pile foundation and a conventional foundation on a natural base is governed by economic and technical considerations. These considerations include the engineering and geological conditions at the building site and the features of the planned building or structure. Pile foundations are particularly appropriate when the building or structure is to be erected on soil that is not stable or is saturated with water. In many cases, such foundations substantially lessen the required earthwork and the expenditure of concrete.

The spacing of piles depends on the kind and magnitude of loads that act upon the foundation. Single piles are used under independent footings; rows of piles are placed under continuous footings; clusters of piles are used under columns; and

Figure 1. Pile foundation: (1) foundation mat and (2) pile

fields of piles are used under structures occupying a small area and exerting substantial vertical loads. Batter piles are used where substantial lateral forces act upon the foundation. The length of piles is chosen with regard to soil properties at the construction site. The lower ends of the piles must be sunk in compact soil.

Piles are classified into two categories according to the properties of the soil at the lower end of the pile. End-bearing piles are supported by practically incompressible soil; floating piles are sunk in soil of uniform consistency and transmit their load to the soil both through their lower and lateral surfaces.

A correct determination of the load-bearing capacity of a pile foundation is basic to the design of a reliable and economical foundation. The load-bearing capacity of the piles can be established from engineering and geological surveys, from data obtained by static probing of soils, and from the results of testing the piles by static and dynamic loading. The testing of piles by static loads is the most reliable method. However, since this method requires a large expenditure of labor, particularly for drilled piles, its application is limited to buildings and structures with heavy loads in cases where existing geological conditions are unfavorable.