Monitoring ground temperature and deformation of the cast-in-place footings in permafrost regions on the Qinghai‒Tibet Plateau

The long-term stability of the cast-in-place footings in permafrost regions has received much attention due to its climate sensitivity. The current research lacks long-term data validation, especially in the context of climate change. Based on the 13-year (2011–2023) temperature and deformation monitoring data from the Qinghai‒Tibet Power Transmission Line, this study investigates the characteristics of permafrost variation and its impact on the stability of tower footings under the cooling effect from thermosyphons. The results reveal that the thermosyphons effectively reduce the ground temperature around the footings. After the first freeze‒thaw cycle, the soil around the tower footings completed refreezing and maintained a frozen state. In the following 13 years, the ground temperature continued to decrease due to the cooling effect of thermosyphons. The duration notably exceeded the previously predicted 5 years. The temperature reduction at the base of the footings corresponded well with the frost jacking of the tower footings and could be divided into three distinct phases. In phase 1, the ground temperature around the footings rapidly reduced, approaching that of the natural field, while the footings experienced pronounced deformation. In phase 2, the ground temperature decreased at a faster rate, and the deformation rate of the footings slowed down. In phase 3, the frost jacking of the footings gradually retarded with the decrease in base temperature. Additionally, the ground temperature differences of over 1 °C were observed among different tower footings, which may lead to the differential deformation among the tower footings. The ground temperature differentiation is attributed to the difference in solar radiation intensity, which is shaded by the tower structure from different directions. This study provides theoretical support and empirical accumulation for the construction and maintenance of tower footings in permafrost regions.