Engineering and Construction

Probably the most common type of deep foundation is the pile foundation. Piles are defined as relatively long, slender, column-like members. Piles are usually driven into specific arrangements and are used to support reinforced concrete pile caps or a mat foundation. For example, the building load from a steel column may be supported by a concrete pile cap that is in turn supported by four piles located near the corners of the concrete pile cap. Typical pile configurations are shown in Fig. 5.3.

Piles can consist of wood (timber), steel H-sections, precast concrete, cast-in-place concrete, pressure injected concrete, concrete filled steel pipe piles, and composite type piles. Examples of cast-in-place piles are shown in Fig. 5.4. Various types of prestressed piles are depicted in Fig. 5.5.

A discussion of the typical pile characteristics and uses are presented in Table 5.1.

Piles are either driven into place or installed in predrilled holes. Piles that are driven into place are generally considered to be low displacement or high displacement depending on the amount of soil that must be pushed out of the way as the pile is driven. Examples of low displacement piles are steel H-sections and open-ended steel pipe piles that do not form a soil plug at the end. Examples of high-displacement piles are solid section piles, such as round timber piles or square precast concrete piles, and steel pipe piles with a closed end.

Various types of piles, in terms of their support capacity, are as follows:

• End-bearing pile. A pile the support capacity of which is derived principally from the resistance of the foundation material on which the pile tip rests. End-bearing piles are often used when dense or hard strata underlie a soft upper layer. If the upper soft layer should settle, the pile could be subjected to downdrag forces, and the pile must be designed to resist these soil-induced forces.
• Friction pile. A pile the support capacity of which is derived principally from the resistance of the soil friction and/or adhesion mobilized along the side of the pile. Friction piles are often used in soft clays where the end-bearing resistance is small because of punching shear at the pile tip. A pile that resists upward loads (i.e., tension forces) would also be considered to be a friction pile.
• Combined end-bearing and friction pile. A pile that derives its support capacity from combined
end-bearing resistance developed at the pile tip and frictional and/or adhesion resistance on the pile perimeter.
• Batter pile. A pile driven in at an angle inclined to the vertical to provide high resistance to lateral loads.

An important consideration is to design and construct the foundation so that it can span unsupported between the piles if the underlying soil is expected to settle. Figure 5.6 shows an example of floor slab settlement due to consolidation and secondary compression of peat at the “Meadowlands,” which is a marshy area in New Jersey. Piles have been used to support bearing walls, but as shown in Fig. 5.6, there is often cracking and deformation of the floor slabs around the piles (Whitlock and Moosa, 1996). The reason for the settlement and damage to the floor slab shown in Fig. 5.6 is because the floor slab was not designed to span unsupported between the piles. The construction of a structural floor slab that can transfer loads to the piles is an important design feature for sites having settling compressible soil, such as the peat layer shown in Fig. 5.6.