According to xu xi ci in the "frozen soil physics" in the description of the underlying layer under the principle of [48]: soil surface involved in the sun to teach radiation, ground turbulence exchange and evaporation, solidification and other comprehensive heat transfer process, so the surface temperature cannot be the upper boundary of soil heat conduction. The temperature of the soil under the bottom of the surface of the soil directly affect the internal temperature distribution and heat transfer, should be as the upper boundary Conditions. According to the literature [47] to take the surface layer thickness of natural position 0.6m, roadbed surface 0.5m. The results show that the average temperature of the surface of the original surface is 6 ℃ and the ground temperature 21.9 ℃. Due to the sunshine slope effect, the surface temperature of the global warming is increasing year by year, according to the literature Data [47], take it’s temperature rate of 0.04 per year, which can be modeled on the upper boundary conditions Subgrade top and slope.
Natural surface