FEM 3D Analysis of Settlements Induced by Shallow Tunnels in Coarse-Grained Soils

The paper is focused on some aspects of 3D numerical modelling of shield-driven tunnels in coarse-grained soils. Different approaches for modelling soil-shield interaction are considered and their impact on the predicted settlement trough is analyzed. These approaches have been applied for modelling two well documented case histories of one tunnel bored in green field conditions. The tunneling process has been simulated step-by-step in order to represent relevant features, such as, application of face pressure, installation of a segmental lining, grout injection and time-dependent variation of grout stiffness. The soil mechanical behavior has been modelled by the Hardening soil model with small strain stiffness implemented in the Plaxis 3D software. Soil deformation around the shield has been modelled by three different approaches: Shield contraction, E-modulus reduction and Strain applied method. Settlement distributions obtained by E-modulus reduction and imposed strain methods are generally in better agreement with field monitoring data but require a specific calibration. Numerical results concerning the structural forces in the lining and the stress paths of representative soil elements are also discussed.