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 measures has been widely used to protect the underlying permafrost and maintain the embankment stability. Based on the measured data of high-grade highway experimental demonstration engineering in the Beiluhe area on the Qinghai-Tibet Plateau, the heat transfer process and ground temperature response have been analyzed to reveal the thermal control mechanism of the expressway embankment with ventilation and open block layer. (1) Heat was transferred through the block layer in both horizontal and vertical directions by different driving modes. In the horizontal direction, heat was transferred by forced convection under the driving of the local prevailing wind. In the vertical direction, heat was transferred upward by natural convection in the cold season and downward by heat conduction in the warm season under the driving of temperature gradient between the upper and lower boundaries of the block layer. (2) The expressway embankment structure showed the thermal semiconductor effect in both directions. The horizontal and vertical equivalent heat conductivity in the cold period was approximately 6.25 times and 3.5 times of that in the warm period. (3) The underlying frozen soil foundation was provided a net heat released state, and the total released heat was approximately 1.2 times of the total heat absorption. As the result, thick and wide cold permafrost layer (T<-1.0℃) was generated gradually, permafrost table increased and ground temperature decreased generally. This expressway embankment with ventilation duct and block layer contributed to protect the underlying permafrost layer and improve the stability of the frozen soil foundation in the warm and high-altitude permafrost regions.