Two-phase closed thermosyphon-induced frost jacking of piles and foundation instability in a thawed permafrost area

Two-phase closed thermosyphon (TPCT) is widely used to cool foundation soil and improve the stability of structures in cold regions. However, along with the cooling of soil surrounding a TPCT, frost heaving of soil can occur, which might pull the structures out, but this phenomenon has rarely been reported. In this paper, based on field observations along the Qinghai–Tibet power transmission line, we present a case study on the frost jacking of piles, with the largest jacking amount being up to 65 mm from December 2011 to November 2018, because of the application of TPCTs. Owing to a greatly enlarged frozen section around the piles due to the cooling of TPCTs, moisture migration toward and frost heaving around the pile were greatly intensified. This resulted in an obvious increase in the tangential frost heaving force and a decrease in the anti-frost jacking force. Thus, the pile might be jacked out from the ground. In areas with shallow developed artesian ground water, the frost jacking of the pile with TPCTs was determined to be more probable because the tangential frost heaving force will be greatly increased. The influence of the decreasing efficiency, or even the failure of TPCTs applied in engineering structures, might give rise to the development of differential deformation, threatening the stability of the tower on the piles. The results indicate that the application of TPCTs for engineering in talik or seasonally frozen areas should be more careful and increasing concern should be given for the engineering influence of TPCT failure.