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  TBM excavation of the Frejus safety tunnel through highly deformable schistous rock mass under high rock cover

  The 13km-long safety tunnel of the existing Frejus Highway Tunnel at the French-Italian border, in the Western Alps, is currently under construction. The first 6.5-km lot is successfully excavated. The tunnel is being excavated by a 9.4m-diameter, single shield TBM and is lined with 0.4m-thick precast concrete rings. The tunnel is excavated through anisotropic schistose formation, under high cover (1800 m locally). The schistosity is almost parallel to the tunnel axis, forming a 45° angle with the horizon. The rock mass characterization and identification of risks was made based on specific investigations and on the construction experience of the adjacent highway tunnel excavated in conventional method. Relatively high convergence and potential buckling of the schistosity planes due to deconfinement were identified. The use of a hard rock, single shield TBM with a specific design suited for high deformable rocks was foreseen. The paper summarizes the main results obtained during the safety tunnel excavation, in terms of response of the excavated rock mass and TBM performances. The technical success of the first stretch construction was mainly due to the TBM design and to the technical follow-up during construction. The latter allowed to analyse construction data, to interpret the encountered behavior in the most critical sections, and to adapt both the countermeasures for reacting to anticipated problems and the criteria to activate such countermeasures during construction.

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