31 May 2022
With the inauguration of two new stations on 31 May 2022, line 12 completes an extension that began with SYSTRA in 2007 to improve transport services in northern Paris.

After the arrival of line 4 at Bagneux last February, the Paris metro is continuing its extension to better connect to Greater Paris: line 12 now links Mairie d'Issy to Mairie d'Aubervilliers and serves two new stations, Aimé Césaire and Mairie d'Aubervilliers - Plaine des Vertus, which were inaugurated on 31 May.

These access points enable more than 330,000 local residents to reach Saint-Lazare in 20 minutes, with a metro every 130 seconds during rush hour. RATP expects to welcome more than 40,000 additional passengers per day.  Eventually, the new terminus will connect with line 15 of the Grand Paris Express.

This is the end of a 15-year story for SYSTRA's teams. It began in 2007 with the first extension of line 12 to Front Populaire station, which was opened in 2012. The line, which is the backbone of the network running from the north to the south of Paris, now totals over 15 km and 31 stations.

SYSTRA's expertise at the heart of the project

As project manager for the extension of line 12, SYSTRA drew on its expertise in urban transport and underground structures. Our teams first carried out all the design studies before supervising the construction of the two new stations and the entire site.

The extension of line 12 is a complex project because of the technical challenges involved in its construction. It has prompted us to innovate and rethink our working methods, in an extremely dense urban environment, while trying to limit the nuisance to local residents as much as possible. The result is a structuring extension in the heart of Greater Paris, two stations, including the largest in the capital's metro system, and the use of unprecedented techniques.

Guillaume Chapponnais, Line 12 SYSTRA Project Director

An ice shield to contain groundwater

The project was the opportunity to carry out a world first: the freezing of soil with liquid nitrogen. The nature of the soil, sandy and damp due to a shallow water table 4 metres below the surface, led us to use this technique. This technique, which is based on the principle invented by Fulgence Bienvenüe in 1909 to allow line 4 to cross the Seine, reinforces the mechanical strength and impermeability of the wet ground during the construction work.

The spandrels of the stations, which form the junction between the tunnel and the platforms and constitute the sensitive points of infiltration, were therefore frozen at -35°C and then at -196°C in a closed cycle, in order to obtain a shield of frozen earth 1.80 m thick.

Other feats were accomplished, such as the construction of the two stations after the demolition of the previously built tunnel.

Plaine des Vertus, one of the largest metro stations in France

The new terminus at Mairie d'Aubervilliers is one of the largest metro stations in France. In addition to its platforms, it houses the line's new maintenance workshop and two additional evacuation shafts, bringing the total length of the station to 280 metres. By comparison, the stations on line 14, which are among the largest in France, have 120 metre platforms...

This station required the construction of more than 25,000 m² of moulded walls, a slab, and then the excavation of its interior volume using a method called "mole excavation", for a volume of 125,000 m3 excavated. The choice of this method made it possible to guarantee pedestrian and road traffic on the surface, but also to maintain the soil while minimising the impact on the existing buildings, while ensuring the watertightness of the station with an anchoring depth of up to 40 metres, below the water table.

Lastly, this new terminus is low-energy since it relies on a geothermal collection system integrated into the moulded wall panels.

Minimised construction impact

To reduce the impact of the site as much as possible, our teams delivered the materials and evacuated the spoil from the earthworks by river, via the Saint-Denis canal, thus reducing the number of lorry movements on the roads. The road network was preserved by the construction of a temporary overpass while the Aimé Césaire station was being excavated.

We carried out work in the centre of Aubervilliers. It was a challenge, especially on market days, but it was also a driving force to transform our methods. For example, transporting the goods by river meant that 50,000 lorries were not on the roads. I would like to thank all our teams who worked on this project, including those who communicated with local residents, whose patience has now been rewarded.

Franck Coulaud, SYSTRA project director from 2016 to 2019.

In the space of sixteen months, this is the third major metro extension to be opened by SYSTRA in the Paris region, following the extension to line 4 at Bagneux and line 14 at Saint-Ouen. The next extensions to the existing network are expected on line 14 to the south and line 11 to the east of Paris.

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