The world’s longest railway tunnel is currently being built in the Alps between Lyon, in France, and Turin, in Italy. The two parallel, 57.5km-long tunnels under Mont Cenis form the centrepiece of this new rail link. For the excavation of the base tunnel, the contracting joint ventures have ordered a number of Herrenknecht tunnel boring machines, including two 334m-long Gripper TBMs with two 650m-long lining and installation systems – the so-called ‘Wurm’. They are intended to support the tunnellers in completing the lining of the bored tunnels within the specified time frame.
1 – PROJECT OVERVIEW
The new 164km-long railway link between Lyon and Turin closes a gap in European infrastructure in terms of the rail network. It will form part of the Mediterranean Corridor within the EU’s Trans-European Transport Network (TEN-T). The centrepiece of the new line will be the 57.5km-long base tunnel under Mont Cenis, with the French-Italian TELT (Tunnel Euralpin Lyon Turin) responsible for the project.
Mont Cenis Base Tunnel is a complex system consisting of two parallel main tubes, four access tunnels and 204 safety bypasses. At the peak of tunnelling on the project, a total of seven TBMs will be in operation simultaneously. The joint ventures contracted by TELT have ordered a number of machines from Herrenknecht AG. One TBM has commenced tunnelling and four have been formally accepted at the plant. The first of the two TBMs for excavating the tunnel section on Italian soil has been ordered and is ready for acceptance in March 2026.
In Lot CO6/7 (Chantiers Opérationnels) ‘Saint Martin-LaPorte – Villarodin-Bourget/Modane’, three single shield TBMs with a diameter of 10.34m are each carrying out 8.3km of hard rock tunnelling with segmental lining. For the neighboring Lot CO5 ‘Villarodin-Bourget/ Modane – Chiomonte’, two Gripper TBMs with a diameter of 10.43m and two Wurm installations are being used to construct the 18km-long tunnels. A multi- mode TBM with a diameter of 10.02m is to excavate the tunnel on the Italian side using segmental lining.
The excavated material generated by the project will be reused across national borders. For the first time in Europe, the circular economy principle is being applied here on a binational jobsite.
Another six railway base tunnels are under construction or already completed to facilitate crossing the Alps with similarly flat routes: in Switzerland and Austria, these are the Gotthard (57km), Brenner (64km in total), Koralm (32km), Semmering (27km), Ceneri (15.4km), and Lötschberg (34.6km) tunnels.
1.1 Geological parameters
The Mont Cenis Base Tunnel will cross the outer Alps on the French side and the inner Alps on the Italian side and is located in one of the most complex geological systems in Europe. Here, very diverse rock types such as granular soils (alluvial deposits), gneiss, granites and rock formations (schist) are encountered, requiring different excavation methods depending on the geological sector being traversed. In order to get an accurate picture of the geological system of Mont Cenis, approximately 65km of drill cores were extracted and analysed, leading to the identification of seven geological areas and the decision as to which sections should be put out to tender for either mechanised or conventional tunnelling.
2 – GRIPPER TBMS FOR LOT CO5
The two identical Gripper TBMs with Wurm are being used by the joint venture of Eiffage, Spie batignolles, Ghella and Cogeis in section CO5 of the Mont Cenis Base Tunnel, which crosses the border between Italy and France underground. The construction lot begins at the Villarodin-Bourget/Modane access tunnel, crosses the Ambin massif and reaches the underground safety zone of Clarea.
In addition to the 18km of tunnel excavated by each of the two Gripper TBMs, two 7.8km-long tunnels are being built using conventional methods. Then there is all the associated logistical work and the underground safety zone in Modane. The overburden at the deepest point is over 2,200m.
The TBMs, specifically developed for the requirements of the CO5 jobsite, have numerous special features designed both for initial tunnel support and for overcoming any geological difficulties. For excavation, the TBM braces itself directly against the previously excavated tunnel wall with its two lateral gripper shoes. Four cylinders press the rotating cutterhead onto the tunnel face, which breaks up the rock ahead with its 62 disc cutters. The excavated material is taken from the tunnel face via the muck ring to a belt conveyor, which transports the material out of the tunnel.
Depending on the rock class, the excavated tunnel can be temporarily supported with shotcrete or steel arches. In addition, both exploratory drilling and rock anchoring can be carried out to secure the rock mass.
Even by Herrenknecht’s standards, at 334m in length, the Gripper TBM is very long. This is due to two bridges integrated into the back-up. The bridges will be used to help concrete 30m of invert every day, on which the Wurm will later also run.
The Gripper TBMs will drive two base tunnel tubes from France toward Italy. Five teams of 25 people will operate each machine in three shifts, 24/7.
Once each Gripper TBM, including the back-up, has completed the first 2km-3km two of tunnelling then the pre-installed Wurm can begin its work.
3 – THE WURM – A MOVING FACTORY
The 650m-long installation and lining system behind the Gripper TBM, the Wurm, was used for the first and so far only time over 20 years ago during construction of the Gotthard Base Tunnel. As at the Gotthard, the Wurm for the Mont Cenis Base Tunnel is also a completely separate system that operates independently of the TBM.
The use of the Wurm allows the interior lining to be installed while the TBM is still advancing. The Wurm is designed as a large, moving factory: in the front section, the supply lines, ventilation duct and tunnel belt are initially detached for the Gripper advance; the tunnel is reprofiled if necessary; the formwork for the inner shell is placed in block lengths; and, the inner shell is concreted – before the supply lines, ventilation duct and belt are reattached.
Development of the Wurm for the project is carried out at Herrenknecht in close consultation with the specialists from the contracting joint venture. One of the biggest challenges here was to route the belt conveyor through the installation in such a way as to minimise conflicts with all other work.
The design had to ensure that safety of the 20-25 workers on the Wurm was not compromised by the moving belt transporting the muck. At the same time, the performance of the belt conveyor must not be reduced over the entire length of the tunnel, i.e., around 18km.
The optimal solution was to find three different cross-sections with a defined position of the belt conveyor and the respective position changes in the Wurm: at the entrance to the Wurm; in the Wurm; and, at the end. The cross-sectional area of the Wurm where the inner shell is concreted, for example, is filled to the last square centimetre.
3.1 Work steps in the Wurm
The concreting of the inner shell determines the speed of the Wurm, which is why it must be able to work and advance completely independently up to 2km behind the TBM. This means that the speed of the Wurm depends solely on how quickly all work is carried out, including sealing, formwork, concrete pouring, formwork removal, etc. There are two large formwork sections in the Wurm, each 10m in length. It has its own completely independent speed, which is determined by the geology as well as the speed of concreting.
3.2 Stations in the Wurm
The space available in the Wurm is very limited, with an outer diameter of 10.43m and a later inner diameter of 9.40m. Fixed dimensions include the belt conveyor suspended from the tunnel ceiling, which is at least 1.50m-wide and up to approximately 2.50m high. Also on the ceiling is the ventilation duct, which has a diameter of approximately 2.40m.
In the bottom section of the tunnel is the more than 5m-wide concrete invert, on which the Gripper TBM and the Wurm advance. Here, a passageway approximately 2.50m-high and 4.30m-wide must be provided for rail-bound jobsite traffic travelling in both directions, above which the Wurm is located. This lower passageway and the outer shell of the tunnel define the space available for all work that must be carried out in the Wurm during the individual phases.
The approximately 126m-long infrastructure deck is located in the first part of the Wurm, closest to the Gripper TBM. It is here that all installations from the front area of the tunnel drive are diverted into the Wurm, including ventilation ducts, tunnel cables, the belt conveyor and other supply lines. Here there is a cooling and desanding plant, dewatering, generators, fans, toilets and crew cabin, a workshop, intermediate storage, diverse logistical openings, and areas as well as various cranes.
The following, approximately 60m-long, reprofiling section is separated from the first part by a dust separation wall. Here, the rock face is worked on using excavators, milling units, and a shotcrete plant, among other things. Logistical openings for materials and equipment to and from the reprofiling areas are located here, as is a crane.
Another dust separation wall protects the next area, the 17m-long shotcrete handling area with associated equipment.
This is followed by the 30m-long section for dewatering the bottom area with material, entrances and exits and intermediate storage.
The material storage area and the assembly crane for sealing are located in the approximately 58.4m-long upper sealing section.
Behind this section is a 3.6m-long interface: cylinders are mounted here, which can either be used to push the front Wurm section while the rear section is braked, or to pull the rear section while the front section is braked.
The conveyor interface is followed by another infrastructure deck, 27.6m-long, located in the middle section of the Wurm. This deck houses a workshop, a shelter, a staff room and toilets, drinking water tanks, generators and electrical installations, a small laboratory, concrete pumps, and the operator’s cabin.
In the subsequent 27.6m-long section, the reinforcement cages are temporarily stored and installed in the upper area by crane.
A further, 51.4m-long, deck area follows. This is where the formwork is stored and installed in two sections, as well as a 5m-long intermediate section for installing the ventilation pipes.
Then, a 42.6m-long infrastructure deck houses various facilities, such as the bentonite station for the formwork, logistical openings, compressors and pressure vessels, cooling plants, and service water tanks.
Connected to this are a 15m-long deck for lower lateral dewatering and a 7m-long deck for train unloading and intermediate storage.
A 42.6m-long section follows, with hose reels, a crane for pipe transport, a logistical opening and intermediate storage for pipes and grout, where the pipes are installed. The last section of the Wurm is 79m-long and contains extensive infrastructure facilities, such as cable drums, the belt conveyor, ventilation, fans, and logistical openings. In this deck, all connections are routed back into the tunnel with the final inner shell installed.
4 – CONCLUSION AND OUTLOOK
Long-distance mechanised tunnel drives, such as the completed or still-to-be-completed base tunnels in the Alps, pose particular challenges at all levels: from the reliable performance of the machine technology to the comprehensive planning and management of a multitude of large and small construction sections.
For hard rock drives with Gripper TBMs, the advantages of using an additional unit in the tunnel that is complex in design and execution, such as the Wurm, are clear: the lining of the tunnel interior can already be carried out in parallel 1km-2km behind the advance with the Gripper TBM; the requirements of the TBM drive on the one hand and the interior lining on the other can be specifically coordinated and the construction schedule optimised accordingly.
Furthermore, use of the Wurm can be compared to industrial production, as the same teams work continuously at the same location and only need to be trained once. Although the Wurm moves continuously, the work is more or less stationary.
This contributes to increased work safety, quality of the work performed, and cost-effectiveness. Another factor is that the individual work processes in the Wurm are independent of each other, both within the Wurm and throughout the entire drive.
