Comparative Study on Model Applicability for Longitudinal Seismic Response of Shield Tunnels Under Design Earthquake Loading

To investigate model applicability for the seismic analysis of shield tunnels in adverse geological sections, this study compares the beam–spring model (BSM) and mass–beam–spring model (MBSM). The Shantou Bay subsea shield tunnel, located in a Seismic Fortification Intensity Degree 8 region (PGA = 0.15 g), is used as the case study. Based on the Response Displacement Method, numerical simulations were conducted via ABAQUS and Python (Version 2.7) scripts to evaluate dynamic responses under unidirectional and tri-directional ground motions. Results indicate that while both models capture longitudinal response patterns, significant amplitude differences exist. Specifically, by accounting for soil inertial effects and shear transfer, the MBSM yields peak relative displacements, joint openings, and internal forces at soft–hard rock interfaces that are approximately 60–130% higher than those of the BSM. Furthermore, tri-directional input significantly amplifies structural responses, exhibiting distinct abrupt changes at geological transition zones. These findings provide a vital reference for the seismic design of shield tunnels traversing complex geological conditions.