Numerical back-analysis of the southern Toulon tunnel measurements: A comparison of 3D and 2D approaches

Full-face excavation associated with ground reinforcement is a common technique to build large tunnels in soft rock or hard soil. The phenomenon of the interaction between the excavation process, the reinforcements and the ground reaction is a three-dimensional (3D) problem. Currently, software and hardware developments provide the option of a numerical analysis of a 3D tunnel excavation within a reasonable calculation time; however, twodimensional calculations based on the simplified convergence-confinement method are still the most common approach of engineers in current projects during the design phase. This study presents the numerical back-analysis of a monitoring section setup in the southern Toulon tunnel in France. The primary goal of this study was to investigate and compare the ability of the two numerical approaches (i.e., 2D and 3D) to reproduce the real behaviour of the tunnel measured in situ. The 3D calculation correctly simulates the in situ data, confirming that this tool can represent the complexity of a tunnel excavation. Fitting the 2D calculations onto the 3D results also enabled the determination of the stress release values corresponding to the real excavation process adopted in the Toulon project. This analysis produces two-dimensional numerical results that are globally satisfactory, considering the ground displacement. (C) 2015 Elsevier B.V. All rights reserved.