A deep foundation derives its support from competent strata at significant depths below the surface or, alternatively, has a depth to diameter ratio greater than 4. A deep foundation is used in lieu of a shallow foundation when adequate bearing capacity or tolerable settlements cannot be obtained with a shallow foundation. The term deep foundation includes piles, piers, or caissons, as well as footings or mats set into a deep excavation. This chapter discusses problems of placing footings and mats in deep excavations and design of drilled piers. Drilled piers (or caissons) are simply large-diameter piles, but the design process is somewhat different. An arbitrary distinction between a caisson and pier is that the caisson is 30 inches or more in diameter.