A day in the life of a station: Cité, the station that was not part of the plan

When metro line 4 opened in 1908, it was the first north-south line, and ran in two separate two branches, one on each bank. For the first time in its history, the metro had to cross the Seine River. The original plan was for metro line 4 to take the same path as rue du Louvre, go under the Seine River, then under Institut de France, to reach rue de Rennes on the other side. 

When the honourable institution rejected the proposal, the City of Paris proceeded to modify the line’s trajectory, shifting it further east, following a decision that was handed down in August 1903.

The line’s new path meant that, from Les Halles in the direction of rue de Rennes, there was no other option but to cross two branches of the Seine River, and to cut through Île de la Cité, once upstream, and once downstream. To accomplish this feat, two stations had to be built – one at Cité, and the other at Saint-Michel.

As the crossing under the Seine River was by far the most difficult stage in the construction of the metro line, a competition was launched to examine the possible solutions, and identify the most suitable project owner.

Following the call for projects, 13 candidates presented 33 projects to an ad-hoc committee. On 18 March 1905, the company Chagnaud was selected. It had previously built a complex, multilevel structure below Place de l’Opéra, to accommodate the intersection of metro lines 3, 7 and 8.

This time, Chagnaud’s project was selected because it presented two advantages: in addition to its idea of sinking caissons vertically into the riverbed, it also proposed to build a single tunnel containing twin tracks, instead of using two tunnels.

Construction works were split up into the following phases:  

• An underground tunnel between Châtelet station and the major branch of the Seine River,
• Three caissons to cross the major branch of the Seine River,
• Three caissons making up Cité station,
• An underground tunnel between Cité station and the minor branch of the Seine River,
• Two caissons to cross the minor branch of the Seine River,
• An underground tunnel leading to Saint-Michel station,
• Three caissons making up Saint-Michel station.

In the segments that crossed the major and minor branches of the Seine River, caissons were sunk into the riverbed. In theory, the idea seemed straightforward: pre-fabricated caissons simply needed to be placed end to end in a trench that was dug in the soil of the riverbed, before aligning them in a way that would create an actual tunnel as an extension to those that were dug in solid ground on both sides of the bank.

On the southern segment of the minor branch of the Seine River, and on a very short 14.5-m stretch, an unprecedented technique was used: freezing the ground.

As an existing railway line (today’s RER line C) prevented the caisson method from being executed, the poor-quality soil on that section was frozen to perform levelling works under the best conditions possible. The soil was frozen over a 40-day period by using freezing tubes filled with -24°C brine. The works on this section lasted for 10 months from December 1908 to September 1909.

A grand basilica, measuring at least 60m by 35m, the largest known basilica built in Gaul during that period, was partially unearthed below the Marché aux Fleurs during the construction of Cité metro station. The basilica was aligned with the decumanus (east-west oriented road in ancient Roman cities), leading out to the cardo maximus (north-south road) on rue de la Cité. The imposing structure was built out of stone blocks that originated from large, richly decorated mausoleums and funerary stelae.

Cité and Saint-Michel stations were first built at street level above their future underground locations, before being sunk into the ground once the stations were ready.   

Each of these stations was built from three caissons – the central caisson holding the station itself with its circular vaulted ceiling, and two caissons located on either side of the station platforms, containing shafts that were to become station entrances, stairwells, and lifts. The three caissons were then enclosed together in a metallic hull.

Following the completion of these colossal, spectacular works, the central segment of metro line 4 between Châtelet and Raspail stations was inaugurated in January 1910, thereby joining the northern segment that was commissioned in 1908, and the southern segment opened in 1909.

The long-awaited operation of a full-length metro line 4 lasted only a few days, as the Seine River was about to experience one of the heaviest floods in its history. The flood that year suspended traffic until 6 April 1910, when service resumed across the line.

Find out more with the history of metro line 4