Thermal stability of permafrost under U-shaped crushed rock embankment of the Qinghai-Tibet Railway

The U-shaped crushed rock embankment (UCRE), of which widely utilized in the permafrost regions along the Qinghai-Tibet Railway, has the capability to rapidly reduce the ground temperature of the underlying permafrost. However, there remains uncertainty regarding the adaptation of UCRE to climate change and its long-term cooling trend. This study focuses on nine UCRE monitoring sites along the QinghaiTibet Railway to analyze the dynamic variations of the ground temperature underlying permafrost from 2006 to 2020. The efficiency of UCRE in stabilizing permafrost temperature in different permafrost zones is evaluated by considering the permafrost table, ground temperature, and MAGT, as well as the temperature difference between the top and bottom of the crushed rock layer and the ground temperature variation index (GTVI). The results show that UCRE is suitable for application in extremely unstable warm permafrost regions where the MAGT is higher than -0.5 degrees C. Moreover, UCRE effectively diminishes the disparity in permafrost thermal stability between the sunny and shaded shoulders of the embankment. The short-term and long-term effect of cooling permafrost is experiencing a change related with permafrost stability. Notably, in stable cold permafrost regions with MAGT lower than -1.5 degrees C, the long-term cooling effect of UCRE on permafrost seems to gradually diminishes, but UCRE continues to fulfill the role of stabilizing the underlying permafrost thermal state over the long-term. These results show that UCRE can quickly restore and stabilize the thermal state of permafrost in the early stages of construction, and adapt to the influence of future climate change. The findings provide important guidance for understanding the variations of permafrost thermal stability beneath the embankment in permafrost regions, as well as for improving the embankment stability and operational safety of the Qinghai-Tibet Railway.