Numerical modelling of pipe umbrella system for shallow tunnels

Context Shallow tunnels represent a critical condition during excavation. To improve ground stability and reduce deformations, pre-support techniques are often adopted. Among them, the pipe umbrella system consists of installing steel pipes ahead of the excavation face to enhance the mechanical response of the surrounding ground. Objectives This thesis investigates simplified two-dimensional approaches for modelling the pipe umbrella system under axisymmetric conditions, assessing their consistency, limitations and reliability. Explicit three-dimensional modelling, although potentially more accurate, is beyond the scope of this work because of its significantly higher computational cost. Methodology An analytical model based on the Convergence-Confinement Method was first developed to define the tunnel response. A reference problem was then implemented in the FEM software Lagamine and Plaxis 2D, reproducing different excavation stages. Three equivalent strategies were introduced to simulate the reinforced ground: linear elastic equivalent volume, anisotropic material and enhanced soil. The obtained results were compared in terms of stresses, deformations and deconfinement. Results The adopted strategies produced different deformation responses. The linear elastic equivalent volume showed a very stiff behaviour and may not adequately represent the real system. The anisotropic material better reproduced the increased axial stiffness introduced by the pipes, while the enhanced soil approach provided deformation magnitudes comparable to the anisotropic model, although with a less beam-like response. Conclusions The results indicate that some 2D approaches can reproduce part of the global effect of the pipe umbrella system despite not representing its actual three-dimensional nature. Future work could include the development of an equivalent 3D model and the introduction of more realistic assumptions, such as gravity and anisotropic in-situ stresses.