Causes of pile foundation failure in permafrost regions: The case study of a dry bridge of the Qinghai-Tibet Railway

Pile foundations are widely used to support dry bridges crossing large extents of warm and ice-rich permafrost zones along the Qinghai-Tibet Railway (QTR). The performance of these pile foundations are related to permafrost conditions surrounding the piles. However, the impacts of permafrost degradation on the stability of pile foundations have been rarely investigated. In this study, permafrost degradation has been assessed around several pile foundations in the Tanggula Mountain area along the QTR 15 years after the first field investigation in 2001 for the construction of the QTR. This assessment is mainly based on drilling, geophysical surveys, and monitoring of the settlements affecting the pile foundations. The permafrost in contact with the piles has significantly thinned after the piles were casted-in-place and put into service about 8 years ago. Moreover, the thickness of residual permafrost is less than the embedment length for some piles and, therefore, the adfreeze bond between the piles and permafrost has significantly decreased. In addition, artesian sub-permafrost groundwater has been observed around the middle-lower and below the piles. Due to this sub-permafrost aquifer, the end bearing capacity of the piles and the friction between the piles and thawed soils have also probably decreased. The applied load on the piles is now supported by the residual permafrost resulting in large settlements of the piles. The thaw consolidation and settlement of degrading permafrost have also potentially contributed to the total settlement. According to the investigation presented herein, the occurrence of sub-permafrost aquifer which was induced by permafrost degradation is the primary cause of pile settlements. The failure mechanisms of the piles as revealed by this assessment are useful for the design and maintenance of piles in warm and ice-rich permafrost regions. Moreover, the use of geophysical methods for investigating pile foundation failure due to permafrost degradation has proved effective.