Vibration Isolation of Concrete-Filled Trench for Pile–Net Composite Foundation under High-Speed Train Loading

To investigate the vibration isolation performance of concrete-filled trench in pile–net composite foundations under high-speed train loading, a validated three-dimensional finite-element model integrating track, embankment, pile, soil, and concrete-filled trench was established. The results indicate that in pileless foundations, stress waves concentrate in surface soils, whereas in pile–net foundations, they localize at interfaces between the concrete-filled trench and layered soils, distributing throughout the foundation depth and enabling full-depth isolation. Before the concrete-filled trench, pile–net composite foundations exhibit lower dominant frequencies but higher vibration amplitudes than pileless foundations. After the concrete-filled trench, the overall vibration isolation effect is improved, with a higher dominant frequency but a smaller vibration amplitude at the dominant frequency. Moreover, the trench effectively filters medium-to-high-frequency vibrations (above 20 Hz) behind the barrier, with residual energy concentrating below 20 Hz. Parametric analyses demonstrate that reducing the pile spacing can enhance both the vibration isolation effect and its stability. Increasing the pile length and pile diameter can improve the vibration isolation effect in the post-trench area, although the benefit of length diminishes beyond the combined thickness of the top two soil layers. Trench depth is the most influential geometric parameter. Although flexible barriers achieve superior isolation, rigid barriers such as concrete can provide more stable attenuation behind it and ensure structural stability in soft soils, making them a more practical choice in many applications. This study reveals the synergistic mechanism of pile–net foundations and concrete-filled trenches in redistributing stress waves and optimizing isolation, providing critical design insights for combined vibration mitigation in high-speed railway projects.