Heat transfer characteristics resembling thermal semiconductor of expressway embankment with ventilation and open-block layer in warm and high-altitude permafrost regions

Abstract

Active cooling methods have been widely used to protect the underlying permafrost and maintain subgrade thermal stability. Based on the monitoring data of test expressway embankment in Beiluhe area on the Qinghai-Tibet Plateau, the thermal controlling mechanism of expressway embankment with ventilation duct and open block layer has been revealed by analyzing the heat transfer processes and ground temperature responses. The heat is mainly transferred by forced convection in the horizontal direction and weakly by natural convection in the vertical direction, and the horizontally and vertically equivalent thermal conductivity in cold period is approximately 6.25 and 3 times of that in warm season, respectively, exhibiting thermal semiconductor effect in both directions. The net heat released state is provided for the underlying frozen soil foundation, and the total heat release is approximately 1.2 times of the total heat absorption. A thick and wide cold permafrost layer (T < -1.0 °C) has been generated gradually with its permafrost table rising and ground temperature decreasing. Otherwise, it has been found that the decline of wind speed from shady side to sunny side in the block layer contributes to the asymmetrical temperature field of subgrade like the “sunny-shady slope effect”. This achievement has deepened the understanding of the cooling methods, and it will contribute to developing the cooling efficiency and optimizing the embankment structure to protect the underlying frozen soil foundation in the warm and high-altitude permafrost regions.