A pier, technically referred to as a drilled shaft pier, is constructed by drilling a hole in the ground, and placing concrete and reinforcing steel in the hole. The principal resistance to a shallow pier settling is end bearing, with additional support in skin friction along the length of the pier.
A pile is defined as a member that is driven (with a pile driver), pushed (pressed), or screwed into the ground. They are wood, concrete, steel, or helical (patented by A. B. Chance, Atlanta, Ga.). Typical piles are shown at the end of this article.
Driven piles are monolithic. They are usually either steel or concrete. They are driven into the ground using a pile driver. The pile driver can be suspended from a crane or part of a tower structure that can be angled to drive a pile into the ground at an angle. A typical pile driver is shown at the end of this article.
Pressed piles are segmental, that is they are pieces stacked on top of one another. The principal resistance to settling is skin friction along the length of the pile, with additional support in end bearing. When pressed piles are installed by foundation repair contractors, a hydraulic jack is used, with the building being the resistive weight. The distance that they can be pushed into the ground is limited by the capacity of the jack, the soil condition at the time of installation and by the weight of the building.
Piles and piers can be used to provide a deep foundation to reverse localized building settlement in single family dwellings and other "lightly loaded" structures. This settlement can be caused by soil movement typical of expansive soils (clays). Expansive soils are characterized by expanding when moisture is added, and shrinking when moisture is removed. Comparison of piles and piers are summarized in the chart below.