Dynamic response characteristics of tunnel-subgrade structure under high-speed rail and aircraft dynamic loads

To investigate the dynamic response characteristics under coupled dynamic loads of aircraft and trains during the underpassing of a high-speed rail tunnel through an airport airfield, with the Chongqing-Kunming High-Speed Railway underpassing Kunming Changshui Airport as the background, a "tunnel, pavement, surrounding rock, and subgrade" three-dimensional refined finite element model was established using ABAQUS. Vibration loads from CRH380 trains were implemented using Abaqus's tabular function, while aircraft taxi loads of the A380-800 were modelled with a Fortran-based DLOAD user subroutine. The dynamic equations were solved based on the Newmark-β method. Response characteristics of structure and subgrade were systematically compared under three working conditions: aircraft taxiing, train vibration, and their coupling. By arranging monitoring points at different depths of the subgrade and characteristic locations of the tunnel lining, response laws such as peak dynamic displacement, vertical dynamic stress, dynamic acceleration, and lining internal forces were obtained. Research results indicate that under coupled dynamic loads, the vertical displacement of the subgrade above the tunnel crown presents an "inverted trough-shaped" distribution. The transient peak reaches 48.33 mm, and it decays approximately linearly with depth. The coupling effect significantly expands the dynamic influence range. Based on the criterion of dynamic stress being 10% of the self-weight stress, the influence depth increases to 31.53 m, which is 2.93 times that under the aircraft taxiing load. Based on the acceleration threshold of 0.1 m·s^-2, the influence depth of the surrounding rock at the tunnel bottom is 22.2 m, which is 1.28 times that under the train vibration load. The acceleration response of the subgrade exhibits spatial heterogeneity; the acceleration in the upper subgrade shows an approximate linear attenuation with an influence depth of less than 24.2 m, whereas that in the surrounding rock at the tunnel bottom exhibits an exponential attenuation. The lining displacement is primarily controlled by the train vibration, and the additional influence of the aircraft taxiing load is negligible. Under coupled conditions, the dynamic tensile stress at the arch foot is 530 kPa, which is far below the material strength. The minimum safety factor of the full lining cross-section reaches 720.62, indicating a sufficient safety reserve. A dual threshold control criterion of a peak acceleration of 0.1 m·s^-2 and a dynamic stress of 10% self-weight stress is proposed, and the established coupled analysis method provides a theoretical basis for the vibration safety assessment and design optimization of underpass projects beneath air-rail intermodal hubs.