Frost jacking behavior of photovoltaic support pile foundations under freeze-thaw cycles

During the operational period, frequent occurrences of frost heave and thaw settlement in pile foundations induced by frost heave forces have severely undermined the stability of the pile foundations. Current designs for PV pile foundations in cold regions often rely on experience from non-frost-susceptible areas or adopt excessively conservative solutions. Over the past three decades, due to discrepancies in experimental methods or insufficient fundamental test data, related research still faces numerous unresolved challenges, lacking mature, reliable, economical, and practical design solutions as well as a standard system for PV foundations in cold regions. To address this issue, this study developed a self-designed freeze-thaw cycling system based on laboratory model tests and numerical analysis, combined with practical engineering conditions. Frost heave model tests and numerical simulations were conducted on specially shaped pile foundations under multiple freeze-thaw cycles. By analyzing the evolution patterns of temperature, stress, and displacement fields in the frozen soil-pile foundation system, the influence of temperature, pile type, and number of freeze-thaw cycles on the frost heave-thaw settlement characteristics of pile foundations was investigated. Research findings indicate that: (1) Under the same freezing duration, temperature shows positive correlations with freezing depth and rate. Freezing temperature significantly affects pile uplift, with lower temperatures accelerating the uplift process. (2) Throughout the freeze-thaw cycles, tapered concrete piles demonstrate optimal frost resistance, reducing residual displacements by 40.98% and 55.98% compared to belled and uniform-diameter piles, respectively. After three freeze-thaw cycles, both cumulative frost jacking displacement and cumulative residual displacement tend to stabilize. (3) When the soil reaches the maximum ice front, the pile's side friction resistance also peaks. As the freezing temperature drops from -20℃ to -30℃, the absolute maximum side friction resistance increases by 21.3%. The depth range for maximum side friction resistance is 17-21 cm. Furthermore, side friction resistance shows an increasing trend with more freeze-thaw cycles. (4) Frost heave displacement under groundwater conditions is greater than without groundwater, with a 0.5-meter water level producing greater displacement than a 1-meter level. Therefore, in the design and construction of PV support pile foundations in seasonal frost regions, it is essential to consider cumulative frost heave displacement from multiple freeze-thaw cycles leading to uplift and tilting failures, and to minimize soil water content while cutting off water sources to achieve effective frost heave mitigation.