Qinghai-Tibet Railway (QTR) is the highest and longest railway in the high altitude regions of the world, which cut through approximately 550 km continuous permafrost. As the subgrade soil of QTR, the permafrost is very sensitive to the external environment change. 

Under the background of global climate warming, the permafrost of Qinghai-Tibet Plateau is regionally degenerating. The change of the permafrost, which is under the railway embankments, will directly impact the thermal stability of QTR embankments and seriously threaten the railway operational safety. 

To ensure the thermal stability of railway embankments, an active-cooling principle was used in the construction of the QTR. This paper analyzes the long-term thermal regimes of four representative embankments, based on ten years of ground temperature measurements (2003-2013). Research data indicate that the long-term thermal regimes within embankments of QTR vary significantly with different embankment structures. 

Obvious asymmetries exist in the ground temperature fields within the traditional embankment (TE) and the crushed-rock basement embankment (CRBE). Measurements indicate that the TE and CRBE are not conducive to maintaining thermal stability.  

In contrast, the ground temperature fields of both the crushed-rock sloped embankment (CRSE) and the U-shaped crushed-rock embankment (UCRE) were symmetrical.  

However, the UCRE gave better thermal stability than the CRSE because slow warming of deep permafrost was observed under the CRSE. Therefore, the UCRE has the best long-term effect of decreasing ground temperature and improving the symmetry of the temperature field. 

This work was supported by the National Basic Research Program of China (Grant No. KZCX2-XB3-19), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 41121061), and the National Sci-Tech Support Plan (Grant No. 2014BAG05B05). This research achievement was published in Science China: Earth Sciences, under the title “Long-term Thermal Regimes of the Qinghai-Tibet Railway Embankments in Plateau Permafrost Regions”.