Utilization of renewable cold energy for sustainable stabilization of permafrost subgrades: field-calibrated 3D hydro–thermal–mechanical modeling of two-phase closed thermosyphons

Two-phase closed thermosyphons (TPCTs) are passive and energy-efficient heat transfer devices that utilize natural cold energy as a renewable resource to stabilize permafrost subgrades. In this study, a three-dimensional hydro–thermal–mechanical (HTM) coupled model of a TPCT–permafrost subgrade system was developed and validated using long-term field monitoring data from Hulunbuir, Inner Mongolia. Model verification was conducted by comparing simulated results with measured data through dual-variable (time–depth) three-dimensional temperature surfaces. Iterative adjustments of thermal boundary conditions were performed until close agreement was achieved, thereby confirming the model's reliability and yielding site-specific boundary parameters best suited to the study area. Results indicate that the model reproduces borehole ground temperatures with deviations consistently controlled within 0–2 °C, providing a solid basis for long-term prediction. Further analysis reveals that TPCTs efficiently store cold energy in winter and reduce heat accumulation in summer, leading to the upward migration of the 0 °C isotherm, improved thermal uniformity, and significant mitigation of differential settlement. Moreover, the mid-evaporator section demonstrates superior cooling efficiency compared to the bottom, underscoring its critical role in cold energy utilization and design optimization. Overall, this work establishes a reliable predictive framework for assessing the renewable cold energy potential of permafrost engineering and highlights the importance of TPCTs in advancing sustainable, low-carbon, and climate-resilient infrastructure development.