The buried substations were placed inside cofferdams, i.e. rectangular enclosures bounded by sheet piles. These were installed by vibrodriving, with continuous monitoring of vibration intensity, given the sensitivity of the project site. Once the enclosures had been excavated and pumped dry, a concrete slab was poured. The networks required to operate the substations were then installed in the cofferdams, followed by the transformer substations at the bottom of the excavations. Finally, steel and pinewood decking was installed to complete the works.
The presence of an electrical transformer in the enclosure meant that several safety features had to be installed in collaboration with ENEDIS, including an earthed network linking the sheet piles and the reinforcement of the invert.
Excavations revealed the presence of large concrete blocks on piles, which necessitated adapting the position of the substations. At first sight, these blocks may have belonged to the foundations of the old warehouses in the area.
The heritage plaque near the Jefferson substation has been preserved and enhanced. It commemorates the Frankton operation (more information: article from the Musée Mer Marine in Bordeaux).