Finite element modal analysis of a highway tunnel beneath a railway track system

Purpose The main aim of this study is to find out the natural frequencies and mode shapes of a highway tunnel that is placed beneath a railway track system. The purpose of the study is to improve the knowledge of key dynamic characteristics, including those determined by the presence of automobile live loads and seismic activity, to guarantee the structural integrity, safety and usefulness of such tunnel structures. Design/methodology/approach This paper uses a three-dimensional (3D) Finite Element Method (FEM) simulation in ANSYS software. The technique is constructing a detailed 3-D model encompassing the highway tunnel, the soil strata around it and the railway track system (including ballast, rails and sleepers). Several modal features, including mass participation factors, model length and model height, are assessed in the analysis. The FEM framework’s reliability was tested by comparison with the mathematical simulations of available sources. Findings The study indicates that natural frequencies of the tunnel model depend greatly on the structural arrangement of the model. Devoid of a track system, the models will have lower inherent frequencies because of lower rigidity. Span length: An increase in the model’s span length leads to a continuous decrease in the natural frequencies across all evaluated modes due to the dominant addition of mass relative to overall stiffness. Model height: A decrease in the model’s height increases the inherent frequency of the vertical modes. Originality/value This study thoroughly investigates the dynamic characteristics and dynamic interaction mechanisms of an integrated highway tunnel–soil–railway track system. This work explains the redistribution of stiffness-to-mass ratio that happens in the structure due to the superstructure of the top track, and does not assume simplified 2D plane-strain conditions that do not account for the skew angles and continuity of boundaries. Moreover, it reveals the physical mechanisms that govern spatial domain proportions and how they affect the vibration modes of vertical and transverse oscillations, providing essential technical data for vibration control and structural integrity evaluation of complex intersections of underground structures. Implication /limitation The investigation study at hand is limited to the exploration of the modal analysis of the framework. This query can also be evaluated by the tunnel and railway engineers and can be used to further dynamic testing of the highway tunnel that is located under a railway track exposed to different dynamic loads.