This report is generally limited to piers of 30 in. (760 mm) or larger diameter, made by open or slurry stabilized construction methods. A 30 in. (760 mm) diameter boundary is an arbitrary size. Although smaller-diameter drilled piers have been designed and installed in accordance with this report, it is difficult to detect sidewall collapse. Refer to ACI 543R for concrete piles having diameters smaller than 30 in. (760 mm), piles installed by the use of hollow stem augers, or other pile types. Also beyond the scope of this report are rectangular columns on spread footings in deep excavations and foundations constructed without excavations by methods such as mortar intrusion or mixed-in-place.

Piers installed by tapping or ramming concrete or aggregate into an excavated shaft are beyond the scope of this report. Engineers and contractors have used the terms "caissons," "foundation piers," "bored piles," "drilled shafts," and "drilled piers" interchangeably. The term "drilled pier" is used in this report. A drilled pier with an enlarged base can be called a belled caisson, belled pier, or drilled and under-reamed footing. Drilled pier foundations excavated and concreted with water or slurry in the hole have been called slurry shafts, piers installed by wet-hole methods, or piers installed by slurry displacement methods.