Wind field and thermal performances of an expressway constructed with two separated crushed-rock embankments in high-altitude permafrost zones

The crushed rock layers (CRLs) with a satisfactory cooling effect has been widely used to mitigate permafrost thaw beneath road embankments in cold regions. However, whether the CRLs can still work as well for an expressway constructed with an integral embankment is still under discussion, it is generally accepted that the expressway with wide asphalt pavement should be constructed with two separated embankments rather than one integral embankment in permafrost zones. In that case, wind field around the two embankments would be of great importance as it would impact the work conditions of the CRLs considerably. Here, a coupled computational fluid dynamics and heat transfer model was developed to numerically investigate and quantify the characteristics of wind field and thermal performances of an expressway constructed with two separated crushed-rock embankments (SCREs) with different separating strip widths (SSWs). The model was validated by the field observed data of wind flows and ground temperatures around an experimental expressway. The simulated results indicated that the two SCREs can ensure the long-term thermal stability of permafrost subgrade in the projected climate warming scenario. However, due to different distribution of wind field around the two SCREs, permafrost thermal regimes beneath them differed considerably under different SSWs. The SSW exerted a significant impact on the cooling effect of the rear embankment, while less significant to that of the front embankment. To ensure that the two SCREs have close thermal performances, the SSW between them should be not less than 15 times of the embankment height. In addition, the local warming effect, which is caused by low wind speed, can result in severe permafrost degradation when the separating strips were narrow.