Factors Controlling Heave Deformation of a High-Speed Railway Subgrade–Culvert Transition Section on an Expansive Soil Foundation: A Case Study of the Xi’an–Ankang High-Speed Railway

To reveal the heave deformation characteristics and influencing factors of high-speed railway subgrade–culvert transition section (SCTS) on expansive soil foundations under water immersion, this study takes the Xi’an–Ankang High-Speed Railway as the engineering background. An in situ vertical swelling pressure test was conducted, and the equivalent thermal expansion coefficient was back-calculated based on the temperature–moisture equivalence theory. A three-dimensional numerical model was then established. The effects of immersion zone width, swelling potential, and immersion location were analyzed. The results show that, as the immersion center gradually deviates from the culvert center, the maximum subgrade heave deformation exhibits a non-monotonic trend of first decreasing and then increasing. When the immersion zone is locally distributed beneath the culvert, differential heave deformation of the SCTS is likely to occur. Gray relational analysis indicates that the maximum heave deformation is highly associated with both immersion zone width and swelling potential, while the width of heave area is more closely associated with immersion zone width. In engineering practice, the immersion range of the foundation near the SCTS should be carefully controlled, and drainage, anti-seepage, waterproofing, and soil improvement measures should be adopted to reduce the risk of differential heave deformation.