The underground electrical substation with foundation tank is an effective solution for use in urban environments where above-ground utility structures such as an electrical substation cannot be left exposed.

Betoncablo S.p.a. produces prefabricated underground substations and structures in a variety of sizes, complete with an underfloor foundation tank, that meet all technical and plant needs and that fully comply with Enel DG 2092 requirements.

The basic configuration is an Enel switching station whose dimensions vary and are usually defined by the local Enel office. Another anteroom is prepared in front of the main entrance to the Enel station for lowering the electrical equipment into the substation. Personnel assigned to operate and run the system normally gain access through the trap door located above the substation anteroom, or by means of a lateral staircase constructed with a prefabricated stairway. Any additional room can be built according to the specific design requirements.
The structure is constructed using prefabricated sections designed for underground installation, using an interlocking block system and mechanical dry joints which are appropriately sized and shaped to support the loads and required vertical and horizontal movements. The walls and roof are made with a fire resistance rating no lower than R120. The roof of the substation anteroom has a galvanised steel trap door for lowering MV/LV equipment. It is appropriately sized and constructed using removable elements, complete with a manhole and ladder for authorised personnel. The trap door complies with the specifications of the Enel standardisation tables and has a live load capacity of 2,000 kg/m2.

To fulfil requirements under existing antiseismic legislation, a foundation system with seismic connectors suitable for class III use can be supplied. To ensure a reliable seismic connection system between the reverse foundation beams incorporated in the wall elements and the underlying foundation, a prefabricated foundation system can be used. The system is constructed with 50 cm high edge beams supporting the perimeter, to be completed with an additional casting on site. Special steel profile channels are incorporated in the top pulvino of the support curbs for connecting the steel earthquake restraint boxes. In accordance with Enel Unification Tables DG2092, a VCR segregation partition fitted with PVC flanges with predetermined fracture toughness, suitable to house cable gland systems with a 200 mm diameter is built between the prefabricated foundation tank and the subfloor of the substation anteroom.

Waterproofing the vertical walls in contact with the earth and the roof slab is usually carried out through the hot application of a 4 mm thick bituminous polyester sheath. In consideration of the particular need to waterproof the substation, also due to the possibility of rising groundwater, another layer of bituminous sheath is normally laid horizontally on the reinforced clean concrete substrate, which is then bound back on the vertical walls of the prefabricated structure. Prior to laying the sheaths, mortar cement collars and joins must be made so that they are laid in a workmanlike manner. To protect the waterproofing sheaths when backfilling, a sheath bonded with a polyester membrane which has truncated pyramid reliefs across all the vertical and horizontal surfaces is used. In addition, a further protective reinforced concrete screed up to 10 cm in thickness has proven to be effective on the horizontal surfaces.

Natural ventilation inside the station is achieved with insulating resin ventilation grilles (ENEL DS 4974 type) properly positioned near the substation anteroom, which is also ventilated via the boreholes at the top on the steel trap door. Otherwise, a system of prefabricated basement window wells complete with overhead galvanised steel grilles can be used as a viable alternative to the solutions described above.