cover method of the CMP's lines, then the standard in use on the Metropolitan system. As a consequence, the layout contains several difficult curves especially in the northern part where narrow streets have a winding route. The line had 23 stations, all with vaulted roof construction. An interchange with lines A and B was established at Saint-Lazare, but there was no connection with the CMP's lines. Work on the sub-fluvial tunnel underneath the Seine, between the Chambre des Députés station (now known as Assemblée nationale) and Concorde, took place between July 1907 and July 1909. This long section runs through a bed of limestone after a break in the sand on the Rive Gauche. The under-river passage was bored by two early tunnel boring machines, each with an external diameter of . The head of the machine cut into the rock, while in two intermediate chambers 24 hydraulic jacks exerted a pressure of on the rock walls so the machine could advance. At the back, a mobile arm turned on the axis of the tunnelling shield to install panels on walls of the tunnel. The two tubes, with an internal diameter of , were made from thick rings of cast iron. Each ring was constructed from ten panels, with one counter-key panel and a key panel to lock the segments in place. The casing extended downstream and upstream. The two tubes are not parallel: the distance between them varies from . Work was delayed by the 1910 Great Flood of Paris. During the works, the future Chambre des Députés station housed an air and water compressor. The compressed air prevented the collapse of the digging shield from the weight of the waters above. The tubes, initially unlined apart from the cast iron rings, were covered with an internal protective layer of masonry in 1920. The section from Porte de Versailles to Notre-Dame-de-Lorette was inaugurated on 4 November 1910 and opened to the public on the next day. Porte de Versailles was set up as a terminus station with a connection to a workshop. The latter was joined by to the Petite Ceinture rail line, which allowed for the transfer of trains between lines. At the other end, the provisional terminus at Notre-Dame-de-Lorette was set up with three tracks including a central depot. On the opening day, public representatives rode from Notre-Dame-de-Lorette to Porte de Versailles, and returned to Gare Saint-Lazare