The impact of karst collapses on railway engineering spans the entire lifecycle of railway construction and operation, with train loads being a significant factor in inducing such collapses. To study the dynamic response characteristics of subgrades in karst areas and to select appropriate monitoring points and indicators for long-term effective monitoring, a numerical simulation method was employed to analyze the vibration response characteristics of the subgrade. A three-dimensional finite element model coupling the high-speed train, ballastless track, and subgrade foundation was established to study the vibration responses of subgrades when the train passes over a subgrade with an underlying soil hole and one without a soil hole. The results indicate that when there was a soil hole, both the dynamic displacement amplitude and vibration acceleration amplitude decreased, while the dominant frequency slightly increased, with the dominant frequency being higher at locations closer to the soil hole. The vibration response at the soil hole location showed significant attenuation, with the attenuation coefficient of dynamic displacement amplitude being higher than that of the vibration acceleration amplitude. Monitoring points were arranged at positions 0 m to 10 m from the toe of the slope, with vertical dynamic displacement, vertical vibration acceleration, the dominant frequency of vertical vibration acceleration, and corresponding amplitude selected as monitoring indicators. These indicators effectively reflect whether soil holes exist within the subgrade and help identify the locations of defects. This study summarizes the dynamic response characteristics of subgrades in karst areas under different conditions, providing a basis for the design and monitoring of railway subgrades in regions prone to karst collapse.

Long-term groundwater exploitation causes water-level fluctuations, which are a major driver of land subsidence and infrastructure deformation in sedimentary basins. To investigate the stress-history-dependent compression–rebound behavior of deep cohesive soils, soil samples along the Beijing–Tianjin High-Speed Railway were examined using high-pressure consolidation tests, cyclic loading–unloading tests, SEM, and XRD. Prescribed cyclic effective-stress paths were applied to simulate groundwater-level decline and recovery at different burial depths. The results showed that irreversible deformation was highly concentrated in the early loading stage. Under the groundwater-level decline paths, the first loading cycle alone accounted for approximately 77% and 94.8% of the total cumulative compression in the shallow and deep soils, respectively. During repeated loading–unloading between 0 and P₀, the plastic deformation decreased significantly with cycle number, whereas the elastic deformation remained relatively stable. Under the groundwater-level rise paths, the cumulative rebound ratios were only 9.8% for the shallow soils and 17% for the deep soils, indicating that unloading recovered only a small fraction of the preceding compression. In terms of microstructure, the shallow soils had relatively loose and pore-rich fabrics, while the deeper soils exhibited denser particle packing and stronger interparticle contacts. These findings demonstrate pronounced mechanical irreversibility. Together with the measured low permeability, they support a mechanistic interpretation in which slow pore-pressure and effective-stress adjustments may further contribute to delayed deformation. Overall, this study provides a quantitative experimental characterization of the evolution of irreversible compression and incomplete recovery of deep cohesive soils under cyclic effective-stress changes induced by groundwater-level variations.