The main work comprises an 8.3km-long, double-tube tunnel excavated by two Herrenknecht TBMs with a diameter of 9m. Complementary works include 45 connection tunnels and an intermediate exploratory tunnel. The Les Cluses adit tunnel was developed to study a sector characterized by difficult tunneling conditions, due to a regional-scale flat-lying tectonic shear zone.

Geological-structural study of the basic tunnel to enable the revision of the project geological reference model. Hydrogeological study of the interference between the works and the thermal sources of the Le Boulou spa. Water sampling from sources, wells and tunnels, chemical and isotopic analyses. Hydrogeological modeling of the groundwater flux systems and DHI (drawdown hazard index) assessment. Surface and tunnel CO2 flux measurements. Geological assistance to tunneling. Planning of geotechnical investigations and of test fields. Planning of reinforcements and supports in the exploration tunnel. Structural analysis of high-resolution satellite imaging. Geological field surveys, borehole logs analysis, structural underground surveys, high-resolution hybrid seismic surveys. Analysis of the squeezing-related behavior of the lead excavations; support efficacy assessment. Analysis of TBM driving methods and solutions for the crossing of the main tectonized zone (injection test fields, etc.). Revision of the geological-structural and geomechanical reference model of the Les Cluses exploration tunnel, as a consequence of unexpected difficult tunneling conditions. Structural and geometric 3D analysis for evaluating the geomechanical conditions along the main basis tunnels. Squeezing risk analysis and modeling within the tectonized zones, study of solutions aiming to prevent TBMs block.