Steel Grillages
Depending on the size and weight of the structure being supported, a traditional foundation of this type (sometimes called a floating foundation) is a large box-like structure, not unlike the mesh cages used to reinforce concrete structures. Unlike the latter, where the reinforcement is ultimately surrounded by concrete, a steel grillage is a rigid construction that's placed at sufficient depth below ground that the weight of soil removed to place it equals the weight of the supported structure. Increasingly, for structures such as road signs or railway gantries, steel grillages consist of one or more steel beams (an interface) mounted on underlying helical piles.