By Luca Matteucci
The future Saint Maur - Créteil station is set to open 2026. October 18, 2023. Sciences Po/Luca Matteucci
SAINT-MAUR-DES-FOSSÉS, France, Oct 23 (Sciences Po) – It’s hard to miss when arriving in town. A big dark mouth yawns in the middle of a bustling construction site, beneath an ageing regional express network station.
In that borehole, 52 metres (171 feet) lower, France’s deepest metro stop is taking shape, in a Paris suburb just outside the péripherique ring road. The future Saint-Maur - Créteil station will connect with Line 15 of the Grand Paris Express, a €36 billion ($38 billion USD) expansion project of the Paris Metro aimed at making direct travel between suburbs faster. The rail tracks will be much deeper than in the current Paris and France record holder, the Abbesses metro station, sunk 36 metres (118 feet) into the Montmartre hill. Société du Grand Paris, the state-owned project developer, expects line 15 to enter service in 2025, forming a large loop around Paris. However, trains won’t stop in Saint Maur - Créteil until 2026, when the station becomes fully operational.
The new station will be an outlier in Paris, where metro stops are more shallow than in most other large European cities. Parisian stations are generally just between five and seven metres below ground, while the London Underground’s mean depth is 24 metres, according to Transport for London.
“It took some digging!” said Seydou Wele, on-site representative for Société du Grand Paris.
“It is the most complex station of the whole Grand Paris Express project,” he added.
Three workers are taking an underground break. October 18, 2023. Sciences Po/Luca Matteucci
The four new metro lines included in the infrastructure program will have a mean depth of 29 metres. In comparison, the world’s deepest subway network based on average depth, the Peterburgskiy Metropoliten, cuts on average 60 metres below the Russian city. Historical Saint Petersburg was built on a number of islands, and the builders had to dig deep to go below the water-filled and unstable ground. When it comes to individual records, the 106-metre-deep Hongyancun station in Chongqing, central China, became the deepest station on the planet in 2022, toppling the Arsenalna station in Kyiv. Constructed at the height of the cold war, the Arsenalna tunnels have doubled as a bomb shelter since the Russian invasion of Ukraine began in February 2022. Another deep network is Pyongyang's metro, in North Korea, which can double as a nuclear bunker too.
It wasn’t bombing risks that prompted engineers to dig at great depths in Saint-Maur, but geology.
A complex station “This depth of 52 metres was necessary because of the clay and marshy soils near the surface,” said Seydou Wele.
The construction started in 2018 with the demolition of a 375-lot car park. Then, the reinforced concrete walls of a waterproof “station box” were cast and buried in pre-dug trenches, creating an underground parallelepiped as large as the station structure.
The 375-lot car park destroyed in 2018 to build the metro station. October 18, 2023. Sciences Po/Luca Matteucci
The walls are out of the ordinary, reaching a depth of 70 m and a thickness of 1.80 m, so as to withstand the pressure exerted by the clay.
Deep digging was essential to reach the hard ground, but also to avoid weakening the foundations of nearby facilities, including a bus hub and the regional express railway (RER A) station.
Given these constraints, construction got off to a late start. And in 2020, a gargantuan tunnel boring machine passed through the station box before the 100,007 cubic metres of earth – the equivalent of 40 olympic swimming pools – were excavated.
The Regional Express Railway RER A in Saint-Maur-des-Fossés, a wealthy Paris suburb. October 18, 2023. Sciences Po/Luca Matteucci
Builders therefore employed the top-down technique, excavating seven stories one after the other in a downward fashion.
This method is very different from how the first subways were dug, more than a century ago, when tracks were laid mostly in a shallow cutting excavated along the street, which was then roofed over. The first subway line, the London Metropolitan line, opened in 1863, needed to be close to the surface for locomotives to let off steam periodically. Electric traction and the invention of the Greathead shield, the ancestor of modern computerised tunnel boring machines, then allowed deeper lines to be built below the layers of unstable gravel near the surface in much of London.
An underground train arriving in metro station Abbesses, on line 12. October 9, 2023. Sciences Po/Luca Matteucci
Narrow, circular tunnels were dug, a structure that distributes the stress more evenly and therefore resists pressure better, but has the downside of being somewhat cramped.
“The Tube feels claustrophobic,” said Andrew Martin, a writer of several books on the London underground and a book published in June, Metropolitain: An Ode to the Paris Metro. “Whereas in Paris, because they were near the surface, they could build an elegant vault, which is not such a strong shape as a tube, but didn't need to be so strong because there isn't much above it near the surface. That is the reason why it is not rare to see two lines next to each other in Paris,” he said. A long-standing record falls However, some Parisian stations were built deeper due to the presence of old quarries. Notably in the north-west, where for centuries miners have dug up gypsum, a white crystal-like mineral used in false ceilings, wall surfaces, or as fireproof agent.
Metro users waiting for their train to stop at metro station Abbesses. October 9, 2023. Sciences Po/Luca Matteucci
Abbesses, on line 12, is one of these stops tangling with quarries. Many people stopping there are tourists, ready to wander Montmartre’s cobbled streets or take the nearby cable car to go all the way up the hill, to the Sacré-Cœur basilica.
Tourists are also typically the only ones to use Abbesses' 181-step spiral staircase. Locals tend to use the lift.
“That’s because they can read French!” said Bonnie, an American tourist from Georgia, referring to a discrete panel near the platform exit that warns users about the large number of steps. The panel is translated into English, though in small characters.
Bonnie and Kim, two tourists from the US, climbing the stairs in metro station Abbesses. October 9, 2023. Sciences Po/Luca Matteucci
Abbesses’s long-standing record as France’s deepest metro stop – the station was built in 1913 at 36 metres – will shortly fall to the new Saint Maur - Créteil station. To access the platforms of the future station, the 45,000 users expected daily will either take one of the 11 elevators set up around the shaft, or a dizzying helicoidal staircase. Computer-generated imagery gives a sense of the sleek, white and gray design chosen for the stairs. A nebula made of thin cables will draw a ribbon-like shape in the central opening.
Carmen Atias, cultural mediator at Société du Grand Paris, shows the Grand Paris Express masterplan. October 18, 2023. Sciences Po/Luca Matteucci
For now, the Saint Maur - Créteil station is just a massive building site, but a privileged few have plunged into its bowels.
“It made me feel like an ant,” said Carmen Atias, cultural mediator at Société du Grand Paris.
“I cannot wait for the public to be impressed too. This station really is a showcase of the best France has to offer when it comes to engineering and architecture.”
Comments