Apart from any special designs associated with spring foundations, the sub-foundation must be a massive and rigid concrete base designed to transmit and distribute the static and dynamic loads of the support structure and plant above it to the subsoil.
Rigidity is necessary to limit differential settlement of the bases of the support columns to a value acceptable to the turbine-generator, and the sub-foundation mass is established from design analysis to limit plant vibration. For the sub-foundation of a modern turbine-generator installed on a CEGB station, the mass is typically 1.3 times the combined mass of the plant and structure above it.
The thickness of the sub-foundation should not be less than one tenth of its length. The need for piling is determined by the nature of the subsoil at a particular site. Discontinuity between the sub-foundation, the basement floor of the station and the foundations of other plant is attempted to prevent transmission of vibrations to other plant and the building structure.