Impact of Excavation Phase Shift on Mechanized Twin Tunnels in Soft Ground: A 3D Numerical Study

This paper is interested in the control of ground surface settlements during shield driving of twin tunnels in urban areas. Considering the highly three-dimensional nature of the whole TBM process, a finite difference model using the Itasca Flac-3D code is proposed. The numerical results are compared, in terms of transversal ground movements, with field measurements carried out on a specific section (P1-S1) of the Lyon metro extension. The results show that the proposed 3D model can reasonably reproduce all the operations carried out by the slurry shield, in particular to reduce the deformations to acceptable levels (volume loss less than 1%). In particular, the study shows that an extramural distance between twin tunnels equal to or greater than three times the tunnel diameter minimizes the impact of the second tunnel excavation on the volume loss caused by the first tunnel. Conversely, this effect becomes more significant as the distance approaches 0.5 times the diameter. The study also examines the relative positions of the two cutting faces. In particular, the results suggest that starting the tunnel excavation below the retaining wall, together with alternate phasing of the excavation and a distance of approximately twice the tunnel diameter between the cutting faces, can improve the results in terms of volume loss. The theoretically best volume loss ratio achieved in this case is 0.45%, well below the generally accepted 1% threshold for urban tunneling, especially for twin tunnels.