Future of Tunnelling  |  Mont Cenis Base Tunnel

Modal shifter

Construction of the world’s longest railway tunnel is progressing with all excavation contracts now awarded. When complete, it will boost rail freight capacity between France and Italy. Sotiris Kanaris reports.

Trade between France and Italy is expected to increase after the new 270km freight and passenger railway between Lyon in France and Turin in Italy is complete.

The new line is also expected to reduce the Alpine region’s carbon emissions by 3M.t per year by taking 1M lorries off the roads.

Building it involves the construction of the 57.5km long twin tube Mont Cenis base tunnel, between the railway stations at Saint-Jean-deMaurienne in France and Susa in Italy. The tunnel is the railway project’s key infrastructure asset.

Currently trains travelling between the two countries in this region have to climb the Mont Cenis mountain on gradients of up to 3% to a height of 1,300m. This requires up to three locomotives to haul the trains. Rail traffic must then go through the 13.7km Fréjus Rail Tunnel, which was inaugurated in 1871. This tunnel does not meet current safety standards as it is a single tube.

The Mont Cenis base tunnel will create a railway with flatter gradients, allowing for increased capacity and trains capable of carrying maximum loads of 1,500t compared to today’s 700t maximum.

Trains will also be able to reach higher speeds – up to 120km/h for freight and 220km/h for passenger services. This means a non-stop journey from Lyon to Turin will take 1 hour 47 minutes compared to 3 hours 47 minutes today. In addition, it is estimated that trains on the lower gradient railway could use up to 40% less energy per journey.

A total of €8.6bn (£7.4bn) of funding has been allocated for the 65km cross border section of the Lyon-Turin line, which includes the base tunnel and 7.5km of above ground infrastructure. The European Union provided 40% of the funding, with 35% coming from the Italian Government and 25% from the French Government.

State-owned Italian-French project promoter Tunnel Euralpin Lyon Turin (Telt) is responsible for constructing and managing the base tunnel. It was established in 2015, with the final route decided in the same year.

SPLITTING THE WORK

The Mont Cenis base tunnel will cover a distance of 45km in France and 12.5km in Italy. The twin tube tunnel will have an internal diameter of 8.4m. It will have a total of 205 cross passages between 8.4m and 75m long and with an internal diameter of 3.5m. Base tunnel excavation started in 2016 as part of the exploratory work programme.

A consortium of six contractors – led by Spie Batignolles TPCI and including Eiffage Génie Civil, Ghella, CMC di Ravenna, Cogeis and Sotrabas – won the contract to excavate a section of the southern tube of the Base Tunnel linking the existing Saint-Martin-la-Porte and La Praz access tunnels.

The exploratory work was carried out to evaluate assumptions about the geology in that area. It was also carried out to acquire experience in driving tunnel boring machines (TBMs) through the challenging ground conditions of the Briançonnais Houiller zone.

“At the moment, we have activity on all the lots. But what is more active is the French side of the project

A TBM excavated 9km between July 2016 and September 2019. The contractors then excavated an additional 1.5km using drill and blast in an area with a long coal fault, completing this work in May 2022.

Although this was exploratory work, the tunnel was built on the same axis and to the same dimensions as the base tunnel, says Telt deputy general manager for France Lionel Gros.

While the exploratory work was underway, Telt awarded the contracts to bore the base tunnel. It split the work into four construction lots (see diagram), three in France and one in Italy. The £2.7bn contracts for the work in France were awarded in 2021, while the £860M contract for the lot in Italy was awarded in August 2023.

“At the moment, we have activity on all the lots. But what is more active is the French side of the project because around three quarters [78%] of this tunnel is on the French side. So that’s why the activity started earlier,” says Gros.

GROUND CONDITIONS

Telt says there are 14 different types of ground condition along the tunnel route. According to “The Mont Cenis Base Tunnel” paper published in Geomechanics & Tunnelling in 2017 the ground conditions include granular soils, complex rock mass formations and high strength or abrasive rock masses.

Seven TBMs will be used to excavate 85km out of the 115km length of the twin tunnels. Drill and blast will be used for sections shorter than 7km where TBM use is not economically viable and where the ground conditions are not suitable for mechanised excavation - like the lot near the French tunnel entrance.

Swiss contractor Implenia won the contract to excavate the 2.8km section between the French portal at Saint-Julien-Mont-Denis and towards Saint-Martin-la-Porte. It is leading a consortium which includes NGE, Itinera and Rizzani de Eccher.

“With these very big projects, it’s very difficult not to have dramatic accidentsDescribing the ground conditions in this lot, Implenia’s project director Alexander Heim explains that there is 500m of loose ground at the tunnel portal. Apart from loose ground, the ground conditions in this lot consist of flysch and schist, with the latter expected to be found closer to the Saint-Martin-la-Porte end. “But the medium resistance of the rocks should not be higher than 120MPa,” Heim adds.

He says that ground conditions will be more challenging on other lots, pointing to the higher overburden and the hard rock conditions on the section between Modane and the Italian border. “It is the granite type. We had this on the Gotthard Base Tunnel construction and resistances could reach up to 180MPa,” he explains.

This does not mean that the work in the base tunnel’s westernmost lot will be easy, particularly when it comes to excavating through the area of loose ground near the entrance.

The base tunnel will be predominantly excavated by seven TBMs

Heim says the New Austrian Tunnelling Method (NATM) – also known as the Sequential Excavation Method – is being used for this section and more than 400m has already been excavated.

The Austrian Society of Engineers and Architects defines NATM as “a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ringlike support structure. Thus the formations will themselves be part of this supporting structure.”

Reinforcement techniques will be used to limit tunnel excavation-induced settlement. As the tunnels are only 4m apart at the beginning of this section, jet grouting for the first 50m was used to stabilise the ground before excavation began.

Implenia’s team is using the umbrella arch method for ground reinforcement for the rest of the area of loose ground. This involves the installation of perforated steel pipes in the ground along the tunnel crown and injecting grout through them, ahead of excavation works.

Heim says the team will use drill and blast to excavate the remaining 2.3km, with this work expected to start in February 2024. Due to the tight schedule, the drill and blast and lining work will be carried out in parallel. Heim believes this will be the biggest challenge for the team as the logistics must be planned in a way that excavation performance will not be impacted by the lining operations.

CHALLENGES

Gros cites ground conditions as one of the project’s main challenges. However, he believes that sharing knowledge acquired from major tunnelling projects in the region – such as Gotthard Base Tunnel and Brenner Base Tunnel – will be vital.

Safety is another consideration. “The main challenge is to keep these construction sites safe, as you know with these very big projects, it’s very difficult not to have dramatic accidents,” Gros stresses. Telt anticipates that 4,000 people will be working on the various sites at the peak of construction in 2025.

To tackle the challenge of keeping the project safe, Telt created the Mission S strategy – S stands for safety – aimed at eliminating accidents and achieving zero fatalities.

The starting point is that safety is a responsibility shared by all the stakeholders and that it is crucial to learn from even the smallest incident if future accidents are to be prevented.

The parties involved in the project commit to:

• Constant training and information for improvement of health, safety and comfort conditions at work
• Shared assessment of worksite performance
• Control of work sites to monitor events such as near misses, accidents and injuries with in-depth analysis of the causes of the events to prevent their recurrence
• Using innovative technologies and new work practices to improve health and safety at work
• Collaborative organisation of interlot emergency management plans.

“The other challenge, common to big projects, is to be able to complete the work on time and on budget,” says Gros.

Telt anticipates that the project will be completed in 2032 and it has established a strategy which could lead to faster base tunnel excavation.

Gros says that adjacent lots are working towards each other so that if contractors finish their lot and there are delays in the neighbouring one, they could be asked to excavate further than the original plan to earn more money. He says this creates competition, which could boost efficiency. For example Heim says that in Implenia’s contract there is the option for it to excavate an additional kilometre of the base tunnel.

“It [Mont Cenis base tunnel] is scheduled to be completed in 2032. This is what we work every day for. We will be adapting the planning with everything that can happen on such a big project, but as most of the big contracts are already signed [tenders for equipment and maintenance have been launched but not awarded yet] I would say we are more confident about this completion date,” Gros concludes.