Degradation process of permafrost underneath embankments along Qinghai-Tibet Highway: An engineering view

Embankment settlement is closely related to the process of degrading of underling permafrost. Previous studies have focused on the process of permafrost degradation under natural ground surfaces, and the degradation process under roadway embankments has yet to be investigated. With the influence of climate warming, this study becomes more and more important in the analysis of embankment settlement. In this paper, based on the in-situ geothermal data of up to 15 years of 5 typical sections along the Qinghai-Tibet Highway (QTH), the characteristics of permafrost degradation are extensively investigated. According to the characteristics, the trends of thawing and temperature rise in the process of permafrost degradation are analyzed. Corresponding to the tendencies, four stages are defined according to the mean annual ground temperature (MAGT). Combining the features of permafrost degradation in each stage with the in-situ data of embankment settlements along QTH, the sources of the embankment settlement are identified. The main results are as follows. The permafrost degrades extensively in the five typical sections with different characteristics. With the increase in MAGT, the thawing rate firstly increases and then fluctuates as temperature rise rate increases and then decreases at MAGT of about − 0.5 °C. The four stages are divided: initial degradation stage, for MAGT ≤ − 2.5 °C; the intensive degradation stage, for − 2.5 °C < MAGT ≤ − 0.5 °C; the vertical talik stage, for − 0.5 °C < MAGT ≤ 0 °C; and the quasi-disappearance stage, for MAGT > 0 °C. When MAGT is less than about − 1.2 °C, the main contribution of total settlement is probably thawing settlement, with the negligible creep of permafrost; when MAGT higher than − 1.2 °C, creep of permafrost starts to have an increasing share in the total settlement.