The 57.5 km long Mont Cenis Base Tunnel will link up Saint-Jean-de-Maurienne in France to the Susa plain in Italy. In 2015 additional exploratory work was launched from the Saint-Martin-la- Porte decline access tunnel. This included TBM excavation of a 9 km stretch of the southern tube of the Base Tunnel linking the existing access tunnels of Saint-Martin-la-Porte and La Praz.

The purpose of this work was in part to check assumptions related to the geology and to acquire the experience required to excavate the Base Tunnel by means of a TBM through the Briançonnais Houiller zone. After excavating close to 300 metres, the TBM encountered a significant geological accident which disrupted progress and required adaptation work to be carried out.

The geological context is presented along with the characteristics of the TBM.

The major stages which enabled the geological accident to be successfully overcome will then be detailed along with the significant milestones involved in crossing the fault and the means used to strengthen supporting structures and terrains. The technical changes made to the cutting wheel which enabled boring to be resumed will also be detailed. The additional geological exploratory work by surveys and measurements of the deformations observed are presented.