3.2.2.1 Raft foundations

As a general rule, as with conventional power plants, nuclear power plant structures are laid on raft foundations, but the demands on the subsoil are extremely high, especially under the reactor building, with its high permanent loads combined with the exceptional actions of aircraft impact or earthquakes. Soil compression from perma­nent loads alone often reaches levels of approx. 500 kN/m2. If an earthquake occurs, or an aircraft hits a station, levels could exceed 1000 kN/m2.

This high level of soil compression often also calls for additional theoretical studies looking for weak points, such as cavities, in the soil; these have a major influence when designing the slab on ground. Such non-constant soil conditions must also be taken into account when considering the soil-structure interaction when designing to resist earthquakes.