Effect of freeze–thaw cycles on the performance of cast‐in‐place piles in permafrost regions: Working state and action effect sharing

Owing to a minor thermal disturbance to the permafrost environment, cast‐in‐place piles are widely used for building and bridge foundations in permafrost regions. However, because of the dynamic and cyclic variation in frozen ground affected by the atmosphere, the load transfer mechanism is not yet clear, and the current design is economically insufficient. To illustrate the bearing pattern of cast‐in‐place piles subjected to freeze–thaw cycles, a systematic in situ investigation was carried out. Results show that the load from the superstructure has a marginal action effect, while freeze–thaw cycles have a more significant action effect. Freeze–thaw cycles have a decisive effect on the dynamic variations of the pile's working state and action effect sharing while the mechanisms are quite different, which vary with depths. Action effect sharing of the pile shaft and tip experiences a cyclic variation and is affected by the long‐term effect of freeze–thaw cycles. The shaft takes an increasing sharing proportion gradually and has a 19% rise after two freeze–thaw cycles, while the pile tip goes the opposite way. Two years after the building is completed, the bearing capacity is almost entirely provided by shaft resistance and mainly by the upper one‐third of the pile. This research clarifies several essential issues about the bearing pattern and provides solid scientific support and novel opinions for the pile design in permafrost regions.