In addition to being built in the city centre, in an area of old buildings with shallow foundations, this project was subjected to working in unpredictable and challenging geological conditions: very disturbed subsoil of rare geological complexity. The latter is characterised by a succession of extremely varied geological strata, like many piles of broken plates inserted between each other. The GTA – a suspended gantry rig equipped with drill arms and baskets for arch handling and placement – was used for the first time in France. In addition to saving space in this confined area, this type of machine also helps to ensure the safety of staff by keeping them at a distance from the tunnel rock face.