Comparative Study: Simplified vs. FEM for Seismic Forces in Circular Tunnels

Abstract

A comparative seismic study between simplified approaches and the finite element method (FEM) was conducted to estimate the internal forces of a circular tunnel. Plaxis2D software© was used for the FE analysis, employing the contraction method in phasing the 2D FE model. The study focused on the Algiers Metro as a practical case study, considering the impact of the horizontal and vertical components of the Boumerdes earthquake in 2003. The purpose of this study is also to use the maximum shear strain rate to determine the maximum vertical strain rate subsequently apply it in existing simplified approaches to calculate the internal forces under the impact of the vertical seismic component. The effect of the volume loss coefficient (VL) was considered. Results demonstrate that increasing VL values initially reduced axial thrust, followed by an increase. Shear force and bending moment proportionally increased with the VL ratio, remaining within the practical range of simplified solutions. Additionally, the total principal stresses around the tunnel increased with the VL ratio. The study underscores the crucial role of selecting the appropriate VL ratio in achieving accurate results in FE analysis.