In geotechnical engineering, a caisson (/ˈkeɪsən/ or /ˈkeɪsɒn/; borrowed from French caisson, from Italian cassone, meaning large box, an augmentative of cassa) is a watertight retaining structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships. Caissons are constructed in such a way that the water can be pumped out, keeping the work environment dry. When piers are being built using an open caisson, and it is not practical to reach suitable soil, friction pilings may be driven to form a suitable sub-foundation. These piles are connected by a foundation pad upon which the column pier is erected.