Thermal performance of a combined cooling method of thermosyphons and insulation boards for tower foundation soils along the Qinghai–Tibet Power Transmission Line

Preliminary field observations along the Qinghai–Tibet Power Transmission Line (QTPTL) show that foundation soils of some shallow footings (with an embedding depth of 3.7 m) with thermosyphons in permafrost regions still suffer substantial and rapid thaw settlement in warm seasons. In this paper, a series of numerical simulations on the long-term thermal performance of foundation soils of these shallow footings are carried out. The simulated results show that thermosyphons could do effectively cool foundation soils at depths 2.5 to 8.0 m under the footing. However, in warm season from mid-May to mid-October, the rapid warming of shallow foundation soils near the footing caused by heat transfer through the concrete footing could not be prevented. The maximum thaw around the footing could be as much as 1.0 m deeper than that in the natural ground. Under a warming climate, the maximum thaw around the footing would go deeper than the embedding depth of the footing with four thermosyphons in very warm (≥− 0.5 °C) permafrost regions and with two thermosyphons in warm (≥− 1.0 °C) permafrost regions during a 50-year operational period. To retard thaw penetration around the footing, a combined cooling method of thermosyphons and insulation boards is proposed for foundation soils. Numerically simulated results show that the additional placement of the boards on ground surface could prevent the rapid warming of shallow foundation soils and effectively reduce the maximum thaw around the footing. The method could also effectively delay permafrost warming under the footing. Thus, it is recommended to be used at shallow footings in very warm, ice-rich permafrost regions along the QTPTL.