La Presse in France | Grand Paris Express: rare controversial aerial sections

(Paris) Can metro networks installed on overhead structures fit into the built environment of a city? Press traveled to Copenhagen, The Hague and Paris to visit three of the “inspirations” mentioned by CDPQ Infra for its Montreal project of the Eastern REM. The answer is: yes… in certain specific contexts.

Maxime Bergeron
Press

“There may be one per century of projects like that,” says Marie Vannieuwenhuyse, before activating the construction lift which will take us about fifteen meters underground.

The engineer and works manager of Colas Rail is accompanying us in one of the many tunnels under construction these days in the Parisian basement, in the suburb of Champigny-sur-Marne. His group was commissioned to install rails, catenaries and other systems in several portions of the Grand Paris Express, the largest infrastructure project in Europe. “It’s very nice to see this become concrete,” she emphasizes.

Titanic, pharaonic, ruinous: all the qualifiers have been added to this megaproject. After years of heated debate, the Société du Grand Paris (SGP) was created in 2010 to literally double the size of the capital’s metro. Some 200 km of rails and 68 new stations are planned; more than 90% of the route will pass underground.

Work has progressed at a good pace since the launch of the first digging sites in 2018. The subcontractors appointed by SGP have already drilled 50 km of very deep tunnels under the capital. It is as if three quarters of the Montreal metro had been dug in three years.

The tunnels are drilled by tunnel boring machines at depths of 25 to 30 m on average, in a wide variety of soils, and pass under an abundance of existing infrastructure (water and electricity pipes, metro lines). Some stations are dug up to 52 m underground. The only sections in height – approximately 20 km in total – will be built mainly in agricultural or industrial zones and are still the subject of strong contests.

Air mode displeases

CDPQ Infra cited certain underground stations of the Grand Paris Express as “inspiring examples of integration” for its REM de l’Est project. The thing may seem paradoxical, since the promoters of the Parisian project are doing just about everything that the Quebec group refuses to do: dig very deep tunnels and stations in urbanized environments where underground obstacles abound.

Here as in Montreal, the issue of overhead sections has given rise to often heated debates, recognizes Bernard Cathelain, one of the three members of the SGP management board, in an interview with Press.

As soon as the project was announced in 2010, some architects proposed to build a larger portion of the Grand Paris Express in height, he recalls. After all, several segments of the metro and RER (the Île-de-France regional express network) are already laid on such elevated structures in the French capital.

The most recent aerial sections of the Paris metro were designed “in the 1960s”, adds Mr. Cathelain, met in the brand new offices of the SGP, a stone’s throw from the Stade de France. “It would no longer work today in urbanized areas. “

Why ? “Sensitivity to environmental issues has become much stronger, and sensitivity to nuisances, which are real today, is no longer the same at all,” emphasizes Bernard Cathelain. Having a metro that passes under your windows is not something that today we are ready to accept. “

Protest … in rural areas

The ultimate goal of the Grand Paris Express is to relieve congestion in the heart of the capital by connecting several remote suburbs and the three airports. New neighborhoods will be built around the 68 future stations, a welcome development in this metropolis of 12 million inhabitants where the lack of housing is chronic. Several connections are planned with the existing public transport network.

The bill for the project now stands at 36 billion euros (52 billion CAN); almost double the 20 billion originally planned.

Even if it is often criticized for its exorbitant costs, the project is generally well received in the 130 municipalities of the metropolitan region that it will cross. But not everywhere.

In the Plateau de Sarclay, an agricultural area where line 18 is to switch to aerial mode, 250 academics, as well as local elected officials and farmers, asked the SGP last summer to give up a portion of the route, or at the very least to bury it. The Company proposed a compromise, that is to lower a few kilometers of rails to the ground level, which does not allay the fears of the opponents. A public inquiry is underway on this section.

Without speculating on the outcome of the conflict, Bernard Cathelain underlines that the choice of aerial sections is “very clearly” based on economic considerations for the SGP. And that, only in sectors where this mode is “compatible” with the urban environment.

“On a given section, we can triple the cost of civil engineering by being underground,” he explains. On line 18, we had looked at the results of the undergrounding of the western part of 8 km; we had 200 to 300 million euros in additional costs, ”he explains.

Very varied soils

Digging is expensive, and even more so in mixed soils like those of the French capital. We find here “almost all possible geological profiles”, says Bernard Cathelain. “We have marls, we have sands, we have gypsum, we have old quarries in places. ”

Despite all the preliminary soil analyzes, “surprises” were found during the work. The head of the SGP gives as an example the recent discovery of “sandstone blocks” which have greatly slowed down the work of a tunnel boring machine, in addition to damaging its blades. In Montreal, the leaders of CDPQ Infra argued that the mixture of loose soil, rock and large “erratic” stone blocks as well as the presence of the water table would make the digging of a tunnel in the center too complex and expensive. -city.

Despite several construction hazards, the SGP intends to inaugurate its first line in time for the 2024 Olympic Games. The rest of the network, which shows fairly significant delays, will be completed gradually by 2030, for the most part, according to current forecasts.

Read our report: “The ups and downs of aerial structures in Denmark” Read our report: “A” fishnet stockings “… and a tunnel in the Netherlands”