Analysis on the cooling effect of the thermosyphon buried in the subgrade of the permafrost regions of Inner Mongolia, China

Against the backdrop of global warming, the degradation of underlying permafrost following common highway construction in the patchy permafrost regions of Inner Mongolia has led to frequent road diseases, presenting a pressing issue that demands immediate resolution. To address this problem and enhance permafrost protection, thermosyphons of various shapes were installed along the Genhe-Labudalin first-class highway. However, as the thermosyphon-equipped subgrade has been in service for more than a decade, a crucial question has emerged: what is the current cooling effect of the thermosyphon buried in subgrade, and can such the thermosyphon subgrade effectively impede the degradation of permafrost? Based on this, temperatures on the thermosyphon side wall, the subgrade and the foundation have been monitored from 2014 to 2024. The temperature distribution characteristics in the vertical, horizontal and longitudinal directions as well as over time were analyzed in detail, further revealing the cooling effect of the thermosyphon on the subgrade. The research results show that after the common highway was built, the permafrost table has reached a depth of 8–10 m, and it has been degrading year by year. However, the thermosyphon has significantly reduced the temperature of the subgrade, and the surrounding permafrost table has dropped to 1.13–4.87 m. The temperature trough of the I-shaped thermosyphon is a symmetrical V-shape, and the effective working radius and the maximum working radius are 2.5 m and 4 m. While the temperature trough of the L-shaped thermosyphon is an asymmetrical single-sided stepped shape, the subgrade temperature in the relevant area with the L-shaped thermosyphon is significantly lower than that with the I-shaped one. The research results can provide technical support for highway subgrade construction and stability assurance in patchy permafrost regions.