Changes of backfill soil of tower foundation in the permafrost regions with warm ice-rich frozen soil on the Qinghai–Tibet Plateau

In permafrost regions with warm ice-rich frozen soil, the changes in backfill soil of the cast-in-place bulb pile and the precast footing are studied over 2 years. The study results show that the freezing period is longer, but the thawing period is shorter for the backfill soil compared with those at the natural control sites. Depths of permafrost table at the backfill sites decrease apparently. The freeze–thaw actions accelerate the improvement of backfill soil, especially in the middle and upper layers. The increasing rates of backfill soil compactness at 3.0 m (middle layer) and at 5.0 m (bottom layer) are the fastest (39.2 and 13.1%) and the slowest (3.4 and 1.1%), respectively. The freeze–thaw actions also improve the backfill soil moisture, especially at the bottom layer. Soil temperature, compactness and moisture are found to be the most important factors affecting the freezing intensity of backfill soil. The improvement of soil compactness and moisture contributes to enhancing the freezing intensity of backfill soil, which benefits the stability of tower foundations. The cast-in-place bulb pile, with the backfill soil characteristics of lower temperature, shorter thawed period, longer frozen period, shallower permafrost table and greater frozen intensity, is more suitable for the warm ice-rich frozen soil.