A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints (like property lines). There are different terms used to describe different types of deep foundations including the pile (which is nalogous to a pole), the pier (which is analogous to a column), drilled shafts, and caissons.
Piles are generally driven into the ground in situ; other deep foundations are typically put in place using excavation and drilling. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and prestressed concrete. Prefabricated piles are driven into the ground using a pile driver. Driven piles are ither wood, reinforced concrete, or steel. Wooden piles are made from the trunks of tall trees.
Concrete piles are available in square, octagonal, and round cross-sections (like Franki Piles). They are reinforced with rebar and are often prestressed. Steel piles are either pipe piles or some sort of beam section (like an H-pile). Historically, wood piles used splices to Join multiple segments end-to-end when the driven depth required was too long for a single pile; today, splicing is common with steel piles, hough concrete piles can be spliced with mechanical and other means.
Driving piles, as opposed to drilling shafts, is advantageous because the soil displaced by driving the piles compresses the surrounding soil, causing greater friction against the sides of the piles, thus increasing their load-bearing capacity. Driven piles are also considered to be “tested” for weight-bearing ability because of their method of installation; thus the motto of the Pile Driving Contractors’ Association is “A Driven Pile… ls a Tested Pile! “.