Displacements of tower foundations in permafrost regions along the Qinghai-Tibet Power Transmission Line

In permafrost regions along the Qinghai-Tibet Power Transmission Line (QTPTL) from Golmud, Qinghai Province to Lhasa, Tibet Autonomous Region, China, 130 tower foundations were selected for foundation displacement observations. Based on observations, displacement characteristics and mechanisms of tower foundations in plateau permafrost soils were analyzed. The results showed that vertical displacement of the footings (VDF) was the dominant displacement type of tower foundations, while changes in footing span (horizontal distance between two tower footings) (CFS) and differences in vertical displacement (DVD) between two adjacent footings of each tower were relatively small. The VDF of most tower foundations (about a quarter of the 130 observed foundations) performed as settlement rather than frost jacking. Due to the effective cooling of thermosyphons, frost jacking of tower foundations mainly occurred in cold seasons (from October to the next May); and because of the hysteresis of heat transfer process in foundation soils, settlement happened in cold seasons, too. Combining with the analysis of the thermal regimes of foundation soils, it was found that the ground temperature under the foundation was the most important factor influencing the VDF of tower foundations. In addition, the creep of the underlying warm and high ice-content permafrost was inferred as the main cause for the foundation settlement. The relatively slight frost jacking of tower foundations mainly resulted from frost heaving in situ of foundation soils and heaving possibly due to considerable water migration within the foundation pit. On slope terrains, the downslope movement (creep) of active layer soils resulted in considerable CFS of tower foundations. DVD between two adjacent tower footings might be derived from jacking of tower foundations with thermosyphons and the artesian sub-permafrost water migration. The effect of the latter was stronger, especially when the artesian water resulted in the development of ground icing. These results and conclusions may provide valuable references for the design and maintain of similar structures in permafrost regions.