ABSTRACT
TISSEO, the owner of the metro network in Toulouse-France, has launched the construction of a third line. To improve the whole network, they also decided to connect the second line, under operation since almost 20 years, to this new line. To do so, the line must cross the “Canal du Midi” which is classified as a UNESCO world heritage site.
The first design was a cut and cover with a temporary closure of the Canal, but its impact was not acceptable by the authorities and the communities. It was thus decided to cross the Canal underground. The final design by ARCADIS is not common: two shorts TBM tunnels only 250 meters long with an internal diameter of 4,66 m. The route of the 2 tunnels is also very complex with slopes between -8 and +8% and a long curve with a radius of 180 m. The minimum ground cover is only 6 m with the bottom of the Canal and the tunnels must also cross a live sewer with only 3 m cover.
The project was awarded to a JV made up of 3 companies from Vinci group (Bessac, GTM and Soletanche-Bachy). The JV reused an existing boom type TBM with confinement with compressed air. The machine was used more than 20 years ago for the construction of the second line of the metro. It was refurbished by Bessac in the factory close to the jobsite which limited the carbon impact of the project.
Moreover, the JV also decided to manufacture the segments in a precast plant only 7 kilometers from the jobsite and using an innovative concrete mix design, using metakaolin and classified as Very Low Carbon.
This paper addresses the multiple unusual characteristics for a subway project in soft ground such as short tunneling distances, steep slopes, tight curves, and proximity to sensitive historic sites and critical infrastructure and explains why the choice to use a pressurized face digger shield was made.
