Cooling effects of thermosyphons in tower foundation soils in permafrost regions along the Qinghai–Tibet Power Transmission Line from Golmud, Qinghai Province to Lhasa, Tibet Autonomous Region, China

Abstract

With global warming and increasing engineering activities, one effective approach to mitigating frost hazards in warm permafrost regions is proactive cooling of foundations soils. Thermosyphons are important proactive cooling engineering measures. Evaluating the effectiveness of thermosyphons is necessary for engineering design and stability analysis of tower foundations. Based on field monitoring data, the current paper presents the analysis on the evaporator temperatures, the effectiveness of thermosyphons, and the characteristics in changing hydrothermal regimes of foundation soils along the Qinghai–Tibet Power Transmission Line (QTPTL) from Golmud, Qinghai Province to Lhasa, Tibet Autonomous Region, China. The results showed that thermosyphons effectively cooled the surrounding soils from early October to early May of the next year. In cold seasons (from October to the next May when thermosyphons were working), the evaporator temperatures changed synchronously with air temperature, and the temperature differences along the evaporator were relatively small (less than 3°C). At the monitoring site, with the help of thermosyphons, the backfilled soils around the tower foundation re-froze completely and the thermal disturbance caused by the construction activities disappeared after the first cold period from mid-November 2010, when the foundation was constructed, to early May 2011. Due to the functioning thermosyphons, the cooling effect of the thermosyphons on foundation soils in cold seasons not only offset the warming effect of the building and operating of tower footings, but also progressively cooled the foundation soils during the first four operating years from 2011 to 2014. The more remarkable heat release was mainly because the cooling process in every year began much earlier. This resulted in the extension of the cooling period and the shortening of the warming period, resulting in an ongoing cooling of the foundation soils. As a result, the bearing capacity of the foundation soils was improved and sustained. Numerical simulations may be conducted in future to evaluate the necessary quantity and design layout of the thermosyphons around a tower foundation.