This paper deals with the cross-border section of the Lyon-Turin Line, i.e. the 57.5 km long Mont Cenis Base Tunnel between Saint Jean de Maurienne in France and the Susa valley in Italy. Works at Saint Martin La Porte started in 2015 including the 9 km TBM excavation along the south tube of the base tunnel between the access adits of Saint Martin La Porte and La Praz. In order to assemble the TBM, a large underground cavern has been excavated at the end of the Saint Martin La Porte access adit. The size of this cavern, with a length of approximately 45 m, a span of 23 m and a height of 22.2 m, and the geological and geomechanical conditions in the Carboniferous Formation at a depth of about 600 m made this work a challenge. The excavation and support methods adopted are described, together with the rock mass conditions and the observed ground behaviour. The monitoring data obtained during excavation are briefly presented, including the works schedule.