The opening of the first part of EOLE, the extension of RER E to the west of the Île-de-France region, and the opening of three new stations served by an 8 km tunnel, where trains will travel at speeds of up to 120 km/h between Saint-Lazare station and Nanterre, mark the end of 8 years of work carried out at a depth of 30 metres beneath the capital. For customers, this means fewer passengers on lines A, B and D, and a 12% reduction in passenger numbers at Saint-Lazare station, eventually, but also easier travel thanks to this extended RER E, now the most interconnected line in the region.

Construction work in a densely populated area, in the heart of the capital region, extracting more than a million tonnes of rubble, using temporary foundations to support the CNIT and its more than 60,000 tonnes to build La Défense station, and "sliding" the equivalent of a Tour Montparnasse under Porte Maillot: Xavier Gruz, EOLE - NExTEO Project Director, looks back at the main technical challenges faced by SNCF Réseau, the project owner for this project, which was named "best underground project in the world" in 2023 by the International Tunnelling and Underground Space Association (AITES).

Xavier Gruz: We had to face two main challenges. The first was to succeed in "sliding" underground stations under La Défense and Porte Maillot, very large structures on particularly busy sites, both above and below ground. An underground structure, dug by our tunnel boring machine under Paris, links the two stations. It had to be supplied with construction materials, in particular segments, the large concrete rings that are laid to create the tunnel structure. The second challenge was to remove and evacuate the rubble safely, while limiting the impact on local residents.

X.G. : We built a base on the Seine, in Courbevoie, to be able to remove it by river, thus avoiding the equivalent of 250 lorry loads a day. The rubble was transported in mud injected under pressure to maintain the ground. The mixture arrived at our plant through huge pipes before being processed on site. The useful mud was returned to the site, while the excavated material from the tunnel was removed by barge.

X.G. : The first task was to find a way to build an underground station in this area, which is home to many high-rise buildings and where the subsoil is occupied by deep foundations. The decision was made to build La Grande Arche station under the CNIT. A set of tracks dedicated to maintenance enabled us to route the line there.

X.G. : The vault houses 11 floors of buildings representing a load of 60,000 tonnes. We put them on temporary foundations during the three years of construction work on the station, before resting them, so to speak, on the large columns that support the CNIT slab above the completed station. Building a station of this size while maintaining the operation of the buildings affected was an unprecedented operation for SNCF Réseau, which was 100% in charge of the project.

X.G. : For Neuilly-Porte Maillot station, the challenge was to build our station at a depth of more than 30 metres, in a subsoil already very much occupied by other infrastructures, such as metro line 1, without moving them. As a result, we succeeded in building 225-metre-long platforms. It's a bit like if we had brought the Tour Montparnasse under Porte Maillot. In addition, a 120-metre-long glass roof has been built on the surface to offer passengers even greater comfort. This brings natural light right down to the station platforms.

X.G. : We are going to gradually roll out a new train operating system. Called NExTEO, it will enable the RER to use the same supervision systems that are currently in operation on automatic metros. Thanks to assisted driving, the train will be able to accelerate and brake automatically on the 15 km central section of Line E between Nanterre and Pantin. Automatic control will make it possible to run trains in close succession, and thus optimise their circulation in the tunnel. A driver will of course remain on board to monitor the tracks and manage boarding at platforms.

X.G. : This second phase of works means that we will have to transform the Mantes-La-Jolie railway junction while maintaining a sufficient level of traffic to enable passengers on the Transilien and TER Normandie lines to continue taking their trains. These enormous works are of a level of complexity that we have rarely seen on a network in operation: They essentially involve moving the tracks and catenary systems safely enough to allow traffic to continue.

X.G. : Our role as project owner for the underground infrastructure and stations has meant that we have had to coordinate the company's various activities in order to bring together all our skills. We also went looking for talent from outside the company. At the height of the project, 5,000 people were mobilised, including 250 SNCF Réseau employees. So we know how to manage project management for very large-scale operations and keep delays to a minimum, even in extraordinary circumstances such as the health crisis we experienced.