Risk Evaluation of Frost Jacking for Tower Foundations along Qinghai-Tibetan Transmission Line and Anti-Heave Measures

The Qinghai-Tibetan ±400 kV direct current grid interconnection project runs across 1038 km of permafrost and seasonally frozen-ground in the interior of the Qinghai-Tibetan Plateau. The mean annual air temperature of the Qinghai-Tibetan Plateau varies between -3 °C and -7 °C and the minimum air temperature is lower than -37°C in short durations. The active layer is subjected to annual freeze-thaw cycle and its thickness varies between 2 m and 3 m. significant heave force is expected due to the existence of extensive frost susceptible soils and cold weather. The tower foundations tend to be jacked out of the ground and result in expensive maintenance costs and foundation failure, which significantly threatens the safety and normal operation of the transmission line. Therefore, it becomes the first concern to prevent the frost jacking failure for design and construction of the Qinghai-Tibetan transmission line. To protect the transmission system from damage, it is necessary to evaluate the engineering risk and to employ some effective countermeasures to mitigate the frost-related damages. To evaluate the risk and provide reasonable suggestions for design and construction, a safety coefficient calculation which involves with the frost penetration, frost heave force, freezing strength as well as loads is conducted. The results show that the spread-type footing has more excellent performance to resist uplift loads than drilled shaft. To improve the safety of foundations, anti-heave measures including non-frost susceptive soil backfill, bevel foundation design, surface treatment and two-phase closed thermosyphon, etc were proposed and applied in the construction of the Qinghai-Tibetan grid project. The advantages and applicability of anti-heave technical solutions are described.