EPB tunnelling in long, highly sensitive sections - The experience of Grand Paris Line 14 underpassing the Orly Airport

The correct definition and application of face confinement pressure is one of the main challenges for urban tunnels, where soil perturbation can affect the surrounding structures with undesirable or even severe consequences. The choice of this parameter needs to comply not only with minimal geotechnical performances, such as volume loss control, water pressure balance, and front stability, but also with optimization criteria (impact on production rate and Tunnel Boring Machine (TBM) components wear). The Grand Paris Line 14 South tunnel has emphasized this concept especially on the 1.5 km section below the Orly International Airport, with two constraints: the limitation of deformations on sensitive airport structures and the tight schedule due to the Olympic Games of 2024. The 8.83 m-diameter Earth Pressure Balance (EPB) TBM 'Koumba' successfully performed the excavation at 20 m depth. The design face pressure profile, varying between 0.8 and 2.6 bar, was studied. A complex monitoring system was also provided to validate design expectations and to update the TBM-soil interaction prediction model. This article focuses on the experience acquired on the EPB face confinement pressure management along this long, highly sensitive section. The first part presents the comparison between reference values and thresholds given by the designer and the real TBM drive oscillation. The second part is dedicated to the observed influence of the face pressure on the geotechnical aspect, machine mechanical parameters, and production rates.