## 6 Conclusions  
This study advances the understanding of frost heave and thaw settlement mechanisms in buried water pipelines through a novel thermal-hydraulic-mechanical (THM) coupling framework, integrating ice-water phase transitions, layered soil heterogeneity, and dynamic pipeline-soil interactions. By leveraging field-validated numerical simulations, the work provides groundbreaking insights into the design and management of pipelines in seasonally frozen regions, with three key innovations:  
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First, shallow burial can effectively alleviate frost heave effects in the surrounding soil and reduce the accumulation of residual surface displacement. However, shallow burial may also lead to pronounced non-uniform surface settlement, which poses potential risks to surface structures and vegetation. Therefore, pipeline installation in cold regions requires a careful balance between minimizing frost heave effects and mitigating uneven surface settlement. Second, increasing the pipeline radius has been shown to significantly reduce surface frost heave deformation and optimize stress distribution in the surrounding soil. This reduces localized stress concentrations and enhances the long-term operational stability of the pipeline. Furthermore, the thermal and physical properties of the surrounding soil play a critical role in regulating the freeze-thaw cycle. High-specific-heat soil layers can absorb and release heat gradually, thereby reducing the freezing depth, delaying the onset of frost heave, and improving the recovery capacity of the soil during thawing. Similarly, pipeline materials with high thermal conductivity can accelerate heat transfer, effectively reducing the freezing range and promoting soil rebound during thawing, which optimizes the mechanical response of the surrounding soil. Finally, the heat conduction of water within the pipeline significantly mitigates frost heave effects. The stable water temperature in the pipeline creates a thermal buffering zone, reducing frost heave displacement in the overlying soil. This mechanism is crucial in alleviating vertical pressures on the pipeline and limiting deformation in the surrounding soil. Moreover, as freeze-thaw cycles progress, the interactions among the temperature field, moisture field, and stress field intensify, resulting in horizontal compression and vertical stretching of the pipeline. These forces may lead to the formation and propagation of micro-cracks in the soil. Such micro-cracks alter the soil's stress state, potentially reducing its bearing capacity and threatening the long-term stability of the pipeline. In summary, this study systematically unveils the intricate interplay among moisture migration, temperature variations, and mechanical behavior in frozen soil. These findings provide a theoretical foundation for the design and maintenance of pipelines in cold regions, particularly in optimizing burial depth, selecting suitable soil materials, and designing pipeline geometric parameters. The developed COMSOL-based THM platform, validated against field data (<20% error), offers a robust tool for simulating freeze-thaw dynamics in layered soils. The proposed THM coupling framework, though developed for water pipelines, holds significant potential for broader cold-region infrastructure applications. For road embankments, substituting the pipeline geometry with trapezoidal cross-sections allows direct adoption of the temperature-dependent permeability model to predict frost heave in clay-rich subgrades. In building foundations, recalibrating the stress-strain equations with concrete thermal parameters enables tilt prediction under cyclic freezing. For buried oil/gas pipelines, extending the phase change model to multi-phase flow conditions can quantify thaw-induced axial strains, as validated by the China-Russia pipeline case.