Deformation causes of the Embankment-Bridge transition section in warm permafrost: The case study of a dry bridge along the Qinghai-Tibet railway

Differential embankment settlement and expansion joint failure in the embankment bridge transition section (EBTS) are two main issues endangering the safety of the Qinghai-Tibet Railway (QTR) in permafrost. A better understanding of the causes of EBTS deformation can offer valuable guidance for engineering maintenance. In this study, a comprehensive field investigation and a series of simulations were carried out to reveal the formation causes. The results revealed significant permafrost degradation beneath the EBTS after 15 years of operation, with temperatures nearing the thawing point. Despite contact between the abutment and beam end, the expansion joint exhibited continuous narrowing, particularly during the cold season. Permafrost degradation was primarily attributed to the heating effect of adjacent bridge structures. Foundation ground settlement emerged as the primary cause of embankment settlement in the transition section, contributing up to 80 % of the total settlement. Differential embankment settlement of the EBTS primarily stemmed from the uneven warming or thawing of permafrost, induced by the thermal impact of the adjacent bridge structures. The weakening of the transition section had a concentrated impact on embankment settlement within a 12-meter span behind the abutment, inducing approximately 30 % to 50 % of the embankment settlement. The backfill soils near the abutment experienced severe lateral frost heave in cold seasons. Horizontal displacement of the abutment caused by the lateral frost heave was identified as the main reason for expansion joint failure. Additional cooling measures are necessary to ensure the long-term stability of the EBTSs along the QTR, considering the faster degradation of ambient permafrost than expected. The findings offer valuable insights for the maintenance and design of railways in permafrost regions.