Vert-de-Maisons, located between Alfortville and Maisons-Alfort, is one of the four underground stations of batch T2A of line 15 South of the Grand Paris Express transit system. As part of the Horizon consortium, Bouygues Travaux Publics is responsible for the civil engineering works.

A 40-meter-deep cavern

Comprising eleven levels, including nine underground, the station lacked the volume required to accommodate all of the platforms and passenger access corridors. To address this issue, the tunnel had to be widened: two lateral galleries, each 65 meters long, were therefore created either side of the tunnel, forming a cavern. 

Freezing the ground for safe tunnel excavation

The next step? The excavation of the mezzanine corridors intended to accommodate escalators and provide access to the platforms. This is where ground freezing comes in. This cutting-edge technique, monitored via sensors, consists in increasing the mechanical resistance of the surrounding ground and making it impermeable, by freezing the water naturally present within the soil to ensure safe tunnel excavation.

Teams launched these exceptional operations at the end of 2022 to enable the currently ongoing excavation of these tunnels, using traditional methods. Each manoeuvre is meticulously planned and particularly complex due to the gradient of the site. 

In the station cavern, the completion of lining concrete operations and dismantling of provisional installations enabled civil engineering teams to access level -9 and set about building the platforms, before moving on to the internal structures higher up (between levels -6 and -9). Completion of this crucial stage of the project will be a major milestone, giving teams from the Colas Rail-Alstom consortium access to the site to lay the rail tracks.

The art of coordination

In parallel with the operations carried out by the Horizon consortium, the contracting client wanted the architectural and technical finishing work batches (Equans) to be launched early on those levels where progress made it possible to do so. As site manager, the consortium is responsible for coordinating these various activities. 

With batch T2A in full swing, more than 100 people are currently mobilised. The Horizon consortium will have completed its work on the station by the end of 2023, and the facility is due to go live at the end of 2025.