On February 29th, 2024 the fifth of the seven TBMs for the new Lyon-Turin railway line was delivered to the French-Italian group of companies ELYOT, consisting of Eiffage Génie civil (group head), Spie batignolles génie civil, Ghella and Cogéis.
Together with its twin TBM, delivered on 21 December 2023, this one will excavate the longest and most complex section of the Mont Cenis base tunnel, i.e. 18 km between the French and Italian border from the Villarodin-Bourget/Modane area to the Clarea underground safety site, in the deepest part of the tunnel.
The characteristics of the TBMs
The figures and characteristics of this ‘gripper’ TBM are the same as its twin TBM. It has a head with a diameter of 10.4 m, a weight of 3200 t, a length of 334 m and a capacity of 4,900kW. For the excavation process, the TBM clamps directly on the tunnel walls with its two lateral grippers. Four pistons then push forward on the head, which rotates breaking the rock thanks to its cutters. In the centre of the machine, a 50-metre-long beam allows curved beams of 25 t each to be moved and laid at the base of the tunnel, where the network of underground utilities will pass. The inverted arch of the tunnel, i.e. the upside-down arch at the base of the tunnel, which is intended to close the arch that has already been realised in the calotte, is completed with a concrete backfill.
Conceived specifically to meet the needs of the CO5 construction site, this TBM has several distinctive features, designed both to perform the initial lining of the tunnel and to deal with any geological difficulties.
The two TBMs will start up a few weeks apart, excavating each of the base tunnel tubes from France towards Italy. Five teams of 25 people each will be required to operate each machine in three shifts, 24 hours a day, 7 days a week. The excavated material is conveyed back on a conveyor belt that carries it to the outside world. Both TBM are also equipped with an ad hoc workshop to pre-classify excavated materials.
Behind each TBM another machine called the Würm (worm) advances. This is 650 metres long and carries out the final concrete lining of the tunnel.
Digging in the heart of the mountain
The choice of this type of TBM is dictated by several factors: the type of geology characterising the excavation areas, which here consist of relatively compact, uniform and stable formations; the great depth of the tunnels to be built, with more than 2,200 metres of overburden; and the associated geotechnical phenomena, such as detachments, or ‘popping conditions’. Not to mention the ‘natural’ high temperatures present at these depths, which were highlighted thanks to the exploratory tunnel of La Maddalena in Chiomonte, which made it possible to establish the specifications for this new TBM.
The CO5 construction site for the Mont Cenis base tunnel traverses the underground border between Italy and France, starting from the Villarodin-Bourget-Modane access adit, passing through the Ambin massif, and reaching the underground safety site of Clarea. In addition to the 18 km of tunnels excavated by twin TBMs (totalling 36 km), 7.8 km will also be dug using conventional methods. Besides these works, all interconnected and logistical tasks will be completed. Approximately 15 kilometres of tunnels will be constructed using conventional methods, including the underground safety area at Modane. The construction site, commencing after the completion of work on the four ventilation shafts of the tunnel under construction in Avrieux, will employ around 1,200 individuals.
