[Objective] Pile foundations are one of the most commonly used foundation types in permafrost regions and are characterized by low thermal disturbance and high bearing capacity. Reducing engineering thermal disturbances and improving the long-term stability of pile foundations are key concerns in permafrost engineering. For friction pile foundations, bearing capacity mainly depends on the freezing strength at the interface between the pile and permafrost. Recently, global warming has increased, leading to an acceleration of the degradation of permafrost. It appears that the traditional design method relying solely on increasing pile diameter and length to improve pile bearing capacity is too conservative. Additionally, these methods do not have the bearing capacity reserves and may not be able to address the challenges of climate warming. Therefore, pile foundation settlement frequently occurs in permafrost regions. This study introduces solar cooling technology into permafrost engineering by proposing a solar cooling pile foundation. It comprises a solar power generation system, a vapor compression refrigeration system, and a concrete pile. This new structure actively cools the permafrost around the pile using a solar cooling system to protect the permafrost from climate warming effects. [Methods] In this study, onsite experiments were conducted using a model pile in the Qinghai-Tibet Plateau permafrost region. The model pile had a diameter and length of 0. 16 and 4. 5 m, respectively. We analyzed the actual cooling effects of the solar cooling pile, including the cooling temperature, cooling radius, and cooling power. Furthermore, we established a numerical model of the temperature field of solar cooling piles using finite element software (COMSOL Multiphysics). We conducted long-term cooling performance simulations under different cooling durations (6,9, and 12 h/d). [Results] The field test results demonstrated that the cooling temperature of the solar cooling pile sidewall could be reduced to a negative temperature, and the cooling radius reached 0. 65, 1. 24, and 1. 5 m after operating for 3, 10, and 30 days, respectively. The adequate cooling power of the solar cooling pile was estimated to be approximately 180 W through theoretical analysis and numerical simulation. The coefficient of performance was approximately 0.9. The simulation results revealed that the longer the cooling duration is, the greater the amplitude of the pile-side temperature and the lower the stable temperature is. The pile temperature corresponding to cooling durations of 6, 9, and 12 h/d were reduced to — 2. 39°C, — 3. 48°C, and — 4. 45°C, respectively; moreover, after ten years, the influence radius increased to 6. 68, 8. 34, and 9. 46 m, respectively. Even if the solar cooling pile stopped operating, the permafrost around the pile could remain in a stable low-temperature state for 2—4 years, providing ample time to maintain the solar cooling system. [Conclusions] The solar cooling pile can significantly reduce the permafrost temperature around the pile, effectively preventing permafrost degradation. In the future, it can be combined with remote control of the cooling temperature and duration, offering the prospect of achieving precise supplementary cooling for permafrost. This study provides a new method for designing and constructing piles in permafrost regions. © 2024 Tsinghua University. All rights reserved.
