As part of the Grand Paris Express project, the biggest urban project currently under way in Europe, the Horizon consortium, led by Bouygues Travaux Publics, is working on batch T2A of Line 15 South, the section between Villejuif Louis-Aragon (not including the station) and Créteil l’Echat stations. This batch includes the construction of 6.6 kilometres of main tunnel, five ancillary structures and a disconnection structure between Line 15 South and the connection to the infrastructure maintenance site (SMI) at Vitry-sur-Seine, via a 1,100-metre single-track tunnel. The project also includes civil engineering work for four underground stations: Créteil l'Echat, Le Vert de Maisons, Les Ardoines and Vitry-Centre. The technical challenges of this batch include the crossing of multiple and complex geological formations - in particular Beauchamp sands - and the presence along the route of several singular points, such as the construction of structures under very heavy water loads and under operational train lines. To adapt to these difficult and constantly changing conditions, the Horizon consortium used three variable density TBMs, capable of adapting their digging mode to the geology encountered, for the first time ever on a project in France. Finally, it should be noted that excavated material from the tunnels is removed by barges on the Seine.