Estimation on settlement of precast tower footings along the Qinghai-Tibet Power Transmission Line in warm permafrost regions

- Ground temperatures under precast footings were predicted in the coming 50 years.
- Thaw settlements of footings were calculated by using the layer summation method.
- Causes of long-term settlements of the footings in permafrost regions were analyzed.

In order to mitigate frost jacking of the foundations, most of the towers in permafrost regions along the Qinghai-Tibet Power Transmission Line from Golmud, Qinghai Province to Lhasa, Tibet Autonomous Region, China were installed on precast or cast-in-place footings. However, the monitoring data showed that these footings were suffering more from settlement rather than heave due to the settlement of the warm and ice-rich permafrost under the footings. In order to estimate the long-term settlement of these footings, a series of numerical simulations were conducted for predicting the changes in ground temperatures under the precast footings in the coming 50 years, taking three mean annual ground temperatures (MAGTs) of permafrost (− 0.5, − 1.0 and − 1.5 °C), two moisture contents of foundation soils (20, 35 and 55%), two embedding depths of footings (3.7 and 5.8 m), and the most intense scenario of climate warming (linear rate of + 0.052 °C/a) into account. Then, the settlement processes of the foundation soils were calculated by using the layer summation method in which the thaw consolidation of permafrost soils as well as the compression of plastic frozen soils under the footings were considered. The computation results show that due to climate warming and disturbances of the ground surfaces, major settlement of the foundation soils would occur after 20 or 30 years of operation. Additionally, the compression of plastic frozen soils under the footings could considerably contribute to the total settlement of foundation soils. Furthermore, embedding depth of the footings would have a significant effect on both thermal and mechanical stability of the footings. When the embedding depth is set at 3.7 m, thawing of the permafrost beneath the footings would always happen in warm (MAGT > − 1.0 °C) permafrost areas during the next 50 years, and the settlement of foundation soils could exceed 20 cm for all calculated conditions by the end of the 50th operating year. When it is 5.8 m, however, thawing of the permafrost would take place only in very warm (MAGT > − 0.5 °C) permafrost areas, and the settlement of foundation soils would be less than 20 cm by the end of the 50th operating year.