Freezeback behavior of a cast-in-place pile foundation surrounded by two-phase closed thermosyphons in permafrost regions

Abstract

The heat of hydration of cast-in-place pile foundations significantly warms, and even thaws, the surrounding permafrost, resulting in a substantial loss of bearing capacity. How to reduce the freezeback time is crucial to the construction schedule of superstructures; however, this has not been fully explored. To accelerate the freezeback process, we innovatively induced two-phase closed thermosyphons (TPCTs) to cool a cast-in-place pile foundation during construction. In this study, a laboratory model experiment and numerical simulations were conducted to highlight the refreezing behavior, cooling mechanism, and structural optimization of the pile-TPCTs system. The results indicate that the TPCTs can significantly accelerate the water–ice phase change, and increase the cooling rate of the subsequent cooling process by 1.75–2.8 times for the soil adjacent to the evaporator section. The pile-TPCTs system completed the freezeback approximately half the freeze–thaw cycle ahead of the traditional pile. Two TPCTs can satisfy the demand of freezeback under the conditions that 1) the condenser and evaporator section lengths must exceed 40% and 68% of the length of the underground pile, respectively, and 2) the distance from the TPCT to the pile side should be less than 5.26 times the TPCT diameter. The findings of this study will be valuable for controlling the construction schedule of cast-in-place pile foundations in permafrost regions.