Finite Element Modeling of TBM Tunneling in Mixed-Face Ground Conditions

Abstract

ABSTRACT This article aims at demonstrating how the use of advanced numerical methods such as the Finite Element Method can provide a reliable prediction of the different ground formation behavior and development of structural forces during tunnel construction. It will particularly illustrate how the challenges associated with TBM excavation in mixed faced conditions could be efficiently dealt with using the finite element analysis. More specially detailed numerical analysis techniques for dealing with the construction of closed twin tunnels in mixed face conditions will be provided using the FEA software PLAXIS 2D. INTRODUCTION With the ongoing advancement of Tunnel Boring Machine (TBM) capabilities, the ability to tunnel through mixed face conditions has been improved dramatically and the risks have been reduced significantly. Still, mixed-face ground encountered in TBM tunneling presents great design and construction challenges for the tunnel engineers. From a geological viewpoint, the mixed-face condition is defined as the simultaneous occurrence of two or more geological formations with remarkably different properties in rock/soil mechanics, engineering geology as well as hydrogeology, or the same geological formation with different weathering grades. This paper aims at demonstrating how the use of advanced numerical methods such as the Finite Element Method can provide a reliable prediction of the different ground formation behavior along with development of structural forces and remarkably help geotechnical engineers to reach cost-effective and reliable tunnel design and construction. One of the first challenges is the consideration of the specific in-situ stress conditions with a large variation of initial orthotropic stresses from one geological formation to the neighboring one with possible significant initial locked stress in certain rocky layers that must be accounted for. The soft soil at the top of the face and the hard rock at the bottom makes it difficult to maintain a proper face-support pressure and face stability, and increases the risk of excessive cutter wear, face collapse, sinkholes, or damage to surrounding structures. This is another challenge that should also be properly addressed. To avoid undesirable ground surface settlement as well as provide appropriate safety measures and limit impact on existing assets, tunnel engineers must also be able to reliably predict the amount of settlement under all given ground conditions. The ground movement around the excavated ground mass will lead to stress changes (arching) the extend of which will define the effective load on the tunnel lining and will greatly condition its design. The mixed-face ground conditions will also induce uneven loading on the segmental lining that must be accurately assessed with the proper consideration of segment-to-segment non-linear contact interaction.