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  Performance monitoring for back-analysis of hard rock TBM tunnelling: the case of the Frejus safety tunnel

  Phenomena such as convergence and anisotropic behaviour can be expected when tunnelling through schistose rocks under high overburden in the Alps. The 13 km-long safety tunnel of the existing Frejus Highway tunnel, at the French-Italian border, is currently under construction and the 6.5 km-long French Lot was successfully completed with a hard rock 9.4 Ø single-shield TBM, driven through such rocks. During construction a buckling phenomenon caused asymmetric convergence and load on the lining with consequent cracks. Several countermeasures were taken to control and reduce the problem. A specific monitoring was also created to qualify and quantify these phenomena. A comprehensive back-analysis based on this monitoring was done at the completion of the French Lot to validate the tools in view of improving the construction performances of the next section to be excavated (key TBM parameters, calibration of threshold values and countermeasure toolbox).

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