Nanterre (AFP) – Among the cranes, on the construction site of line 15 of the future circular underground metro around Paris, a conveyor belt carries black sand with a metallic shine.

Published on: 03/09/2026 - 07:57

Right next door, tons of concrete are being poured to build the Nanterre-la Folie station which will serve the line west of Paris. Objective: complete the Grand Paris Express by 2031, the “largest infrastructure project in Europe” according to its promoter the Société des Grands Projets (SGP).

For the European Investment Bank (EIB), which on Wednesday provided additional financial support of 3 billion euros to this gigantic infrastructure project totaling 36.1 billion euros, the objective is above all "to help succeed in the decarbonization of transport" and "to improve the daily life" of residents.

But in this titanic underground construction site, it is very difficult for an untrained eye to perceive what the positive climatic effect of such an avalanche of concrete could be.

“We must above all look at the modal shift that the new metro lines from suburb to suburb will allow, that is to say the number of cars which will be removed from the roads”, responds to AFP René Laurent Ballaguy, head of the EIB office in Paris during a visit to the site organized by the SGP on Wednesday.

“environmental profitability”

The European financial institution based in Luxembourg, which presents itself as a "climate bank", insists on financing projects "enabling the achievement of the objectives set by the Paris climate agreement in 2015".

“During our examination of the files, we of course integrate economic profitability, but (...) also an internal carbon price to verify its environmental profitability”, specifies Ambroise Fayolle, vice-president of the EIB.

In addition to line 15, the future Grand Paris Express network is made up of three other automatic driverless metro lines (16, 17 and 18), and extensions of line 14 to Orly in the south and Saint-Denis in the north, put into service just before the Paris Olympic Games in 2024.

In total, 68 new stations will be spread over 200 kilometers of new tracks. By connecting the suburbs together, this network should make it possible to double the size of the historic Paris metro.

Since 2018, the vast Ile-de-France project, which has already suffered several delays, has mobilized 36 tunnel boring machines, four of which are still drilling into the depths of the basement.

On the decarbonization side, the SGP argues that 90% of the Grand Paris Express uses “low carbon” concrete.

Its manufacturing emits up to 40% less CO2 than traditional concrete.

This material uses steelmaking waste to replace clinker, the most polluting component of cement, which emits greenhouse gases that warm the climate.

Some segments, the arc-shaped prefabricated concrete blocks that make up the tunnel walls, are made of "ultra-low carbon" concrete, says an engineer, with even lower CO2 emissions.

Next to the conveyor belt which spits out the black sand waste (pyrite) excavated by the tunnel boring machine, a crane suddenly raises a segment into the sky.

The drilling machine will automatically add it to the spine of the tunnel, embedding it in liquid concrete. Since December, the tunnel boring machine has progressed an average of 15 meters per day towards the La Défense district.

"Eocene, Lutetian and Ypresian"

The future metro will be deeper than the historic metro, and than the RER built in the 1970s, underlines Elodie Leygnac Chese, engineer responsible for the sector.

In Nanterre-la-Folie, the construction site is located 25 meters underground. In Saint-Cloud, west of Paris, “we will be 50 meters underground,” she says.

While digging, the four tunnel boring machines still in operation named Marine, Tiphaine, Rita and Yandé, cross "several levels of underground water tables" in Ile-de-France, adds the engineer, showing a "diaphragm wall", a concrete technique injected 80 centimeters thick to keep the land in a wet zone.

In the Paris basin, "we are digging in the Eocene, in the Lutetian and in the Ypresian" she adds, three geological layers of the subsoil whose formation dates back several tens of millions of years.

© 2026 AFP

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