Hydrothermal Response of Railway Subgrade to Seasonal Rainfall in Permafrost Regions: Patterns and Mechanisms

The combined effects of global warming and increasing rainfall threaten the stability of railway embankments in permafrost regions of the Qinghai–Tibet Plateau. An indoor embankment model at a geometric scale of 1:17 was constructed based on a typical section of the Qinghai–Tibet Railway on the Chumar River High Plain. Experiments were conducted in a temperature-controlled environmental chamber using four rainfall classes with different durations, prescribed according to observed seasonal precipitation characteristics. Temperature and moisture variations were monitored at different depths beneath the embankment center, shoulder, and slope toe under rainfall and rain-free conditions. The study quantifies three regulatory effects of rainfall on the hydrothermal field: (1) The “seasonal differentiation effect” on the temperature field—rainfall generally reduces shallow embankment temperature (from April to October, average cooling of 0.8 °C, 0.6 °C, and 0.4 °C at 10 cm depth below the pavement, shoulder, and slope toe, respectively). Maximum cooling occurs in summer (1.8 °C beneath the embankment center in July); in autumn, cooling is observed beneath the embankment center and shoulder; in spring, shallow ground temperatures increased with seasonal warming, while rainfall generally reduced their values relative to the corresponding rain-free condition. (2) The “spatial heterogeneity effect” on the moisture field—rainfall significantly increases shallow moisture content (average increases of 3.1%, 4.3%, and 4.9% at the same locations), with the slope toe showing the largest increase due to surface runoff, while beneath the pavement the “pot-cover effect” intensifies vapor condensation. (3) Rainfall-associated temperature and moisture differences generally decreased with depth. Under the prescribed experimental conditions, moisture differences were relatively small at monitored model depths of 50 cm and greater. The results reveal the hydrothermal response mechanism of permafrost embankments under coupled rainfall and freeze–thaw cycles, providing refined experimental evidence for disease prevention and long-term stability evaluation.