Fifth TBM delivered for Mont Cenis base tunnel

Together with its twin TBM, which was delivered in December, the machine will excavate the longest and most complex section of the Mont Cenis base tunnel – the 18km 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 where the overburden is more than 2,200m.

Designed to meet the needs of the CO5 construction site, the TBM has several distinctive features, designed both to perform the initial lining of the tunnel and to tackle any geological difficulties.

The gripper TBM has a 10.4m diameter, is 334m long, has a capacity of 4,900kW and weighs 3200 tonnes. It has two lateral grippers which clamp to the tunnel walls and four pistons then push forward on the head, which rotates breaking the rock.

In the centre of the machine, a 50m-long beam allows curved beams of 25 tonnes each to be moved and laid at the base of the tunnel, where the network of underground utilities will pass.

While excavation is under way, shotcrete support will be applied to the walls, along with bolts and ribs. Behind each TBM, a 650m-long machine called the Würm (worm) will fix the final concrete lining.

A gripper TBM was chosen because of the geology of relatively compact, uniform and stable formations; the depth of the tunnels; and geotechnical considerations, such as detachments and convergence. There are also the high temperatures present at such depths, which were highlighted by the exploratory tunnel of La Maddalena in Chiomonte, which was used to establish the TBM’s specification.

Elyot, which comprises Eiffage Génie civil, Spie batignolles génie civil, Ghella and Cogéis, will launch the two TBMs from France, a few weeks apart.

At the same time as the 18km-long twin tunnels are excavated, 7.8km of tunnels will be dug using conventional methods. In addition to these works, all connected and logistical works will be carried out. In total, around 15km of tunnels will be built using the conventional method, including the underground safety area at Modane.