Through the Alps at 1.4 metres a day
Project CO08 TELT –Saint-Julien-Mont-Denis
5,6 km | of tunnel to excavate |
1'018 m | of umbrella vaults |
1'078 | centring frames to install |
11 | safety plants |
600'000 m3 | groundwork |
2'700 m2 | acoustic hangars |
over 127,000 m2 | construction site facilities |
5 years | construction time |
EUR 228 million | size of contract |
More than 300 people | on site at peak times |
In order to build the tunnel entrance at Saint Julien Mont Denis, the Implenia-led joint venture set up camp in a sustainable wooden construction office. The project involves building two 2,839 m long tubes in the direction of Italy, along with 11 safety crossings and 140 m of cut-and-cover tunnel. Preparatory work, which began in November 2021 and lasted about a year, is already complete. A concrete mixing plant and water treatment plant have been installed, as well as 794 jet grouting columns to improve the mechanical properties of the ground in the alluvial zone. A “Parisian wall” was built to enable excavation through the soft ground. 90,000 tonnes of material were dug out for the pre-cut and the portal zone – a total of 600,000 m3 for the entire project.
Conventional tunnelling instead of a tunnel boring machine
Things really got going after the ground-breaking ceremony in December 2022. A loose ground tunnelling technique with a pipe screen is being used for the first section of about 500 m. This involves drilling a “screen” of metal pipes and driving them into the ground to support the ground before excavation begins, plus the installation of temporary support in the form of steel arches made of HEB profiles and fibre shotcrete. Anchors and drainage boreholes are used to stabilise the face, i.e. the foremost part of the tunnel where the rock is removed and the cavity is made. This procedure temporarily secures the freshly made tunnel before the in-situ concrete inner shell is installed.
Machine engineer Philippe Liotard explains what equipment is needed for this initial section: “Conventional tunnelling uses various types of equipment. Excavation is typically done by tunnel diggers or conventional diggers fitted with demolition hammers or hydraulic cutters, wheel loaders with side tipping buckets, and dump trucks to transport the excavated material. Drilling rigs, anchors, shotcrete pumps and teleloaders are used underground.” By 30 April 2023, the joint venture had excavated more than 300 m on both routes, equivalent to an average of 1.40 m per day. Once the 509 m loose ground section has been excavated, the tunnel will made by blasting, which will make for faster progress.
Take a tour on TikTok
TELT in focus: If you go on TikTok, colleagues will show you the concrete laboratory, explain the noise protection measures, take you to the sustainably built construction offices and guide you into the new tunnel. The six parts of this mini-series are labelled “TELTC008”, which is the name of our construction site. You can also find the videos on the Intranet.
Watch the videos: Implenia (@implenia) Official | TikTok
Digitalisation is also helping the project keep to the planned rate of progress. The joint venture uses an innovative, participatory approach, allowing it to work more fluently, react more quickly, increase productivity and improve quality. This is done by centralising, analysing and sharing information, and making it available to all the different stakeholders anywhere, any time.
