The engineering challenges of Grand Paris Express, Europe’s largest transport infrastructure project

From the article:

Part infrastructure upgrade, part social cohesion project, part carbon reduction strategy, the project itself is an impressive feat of engineering however you look at it. Tunnelling work started in 2018 and the first part of the project – an extension of the existing Métro line 14 – is due to open in 2024 in time for the Paris Olympics, while four completely new lines will come into service between 2025 and 2030.

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Although the world record for the most tunnel boring machines operating simultaneously on a single project goes to the Moscow Metro project which used 23 TBMs, the Paris Grand Express still boasts one of the largest fleets ever used, with 18 procured from German manufacturer Herrenknecht and two from Chinese manufacturer China Railway Engineering Equipment Group. At peak times, Doha Metro, constructed between 2014 and 2016, saw 20 TBMs working simultaneously while 19 machines worked together on the first phase of the Delhi Metro between 1998 and 2006. Construction of the Channel Tunnel in the 1990s was undertaken by a fleet of just eleven.

Despite the length of the completed tunnels, the Société du Grand Paris opted to split construction work on many of the lines into separate construction packages, while the TBM worked on a tunnel section, averaging just 3.5 kilometers each in order to speed up delivery and reduce the amount of time tunnel entrances and access shafts needed to stay open.

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For Cathelain, whose distinguished career includes managing large construction and engineering projects at both Paris Airports and French motorway company SANEF, one of the biggest challenges with creating the new metro is the fact that it is so deep below ground, effectively meaning that each of the 68 new Grand Paris Express stations is a highly complex construction project in its own right.

In order to avoid large amounts of existing utilities infrastructure, to compensate for the hilly topography of the Greater Paris metropolitan area and to enable tunnelling to take place in the city’s stable chalk beds, engineers have had to dig the new metro on average 30 meters below the city – at least twice as deep as the original network, completed at the turn of the twentieth century which averages just 10-15 meters under the surface (although just a fraction of the depths of the world’s deepest metro networks of Kiev, St Petersburg and Pyongyang).

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Cathelain says that one of the biggest problems with digging 200 kilometers of tunnels and 68 multi-storey buildings underneath one of the biggest cities in Western Europe is what to do with all of the earth and rock the project has displaced – 47 million tonnes to be precise.

To overcome this, Cathelain says, the Société du Grand Paris launched a competition for community groups and other organisations to come up with innovative uses for the material to meet its aim of re-using 70% of the spoils in local projects. So far, he says the plan has been extremely popular, providing ballast for community projects including new disabled accesses, community spaces and gardens.

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Low carbon concrete is one area where the Société du Grand Paris really hopes to capitalise on its investment into innovation. The Société has commissioned research from the Institute Mines-Télécom in Douai, northern France, to investigate methods of flash calcination, where excavated rubble from the project is rapidly cooked and used to replace the carbon-heavy clinker, the main constituent of traditional cement.