Numerical analysis for cooling effect of open boundary ripped-rock embankment on Qinghai-Tibetan railway

The heat convection of fluid inside the ballast layer and ripped-rock layer, which are regarded as porous media in railway embankment, is a process of heat and mass transfer. At present, the ripped-rock embankment, as a new type of embankment structure, has widely been used in the construction of Qinghai-Tibetan railway. However, because its ripped-rock layer is almost open in two bilateral boundaries and closed at top and bottom, and air can flow into/out of the ballast layer and ripped-rock layer, the convection and transfer heat patterns are very complicated in the embankment. Therefore, based on the temperature and geology conditions of the Qinghai-Tibetan Plateau, a numerical approach of the unsteady two-dimensional continuity, momentum (non-Darcy flow) and energy equations of heat convection for incompressible fluid in porous media is provided to analyze the velocity and temperature characteristics of the ripped-rock embankment with different embankment heights under open boundary condition for the coming 50 years in this paper. The calculated results indicate that, due to the influence of the outside wind, the convective heat transfer mainly relies on the forced convection in the open ripped-rock embankment. Even if the air temperature will be warmed up by 2.6 degrees C in the coming 50 years, it still has a better cooling effect on the underlying soils and a low temperature frozen-soil core is formed in the permafrost below it if the embankment is constructed in the regions whose present mean annual air temperature is -4.0 degrees C. Furthermore, the cooling effect of high ripped-rock embankment is better than that of low embankment. This is because the wider bottom of high embankment has a more influence dimension on the underlying frozen soil. However, cardinal winds on the Qinghai-Tibetan Plateau disturb its convection pattern, so that an asymmetric temperature distribution occurs under high embankment and it is possible to induce a transverse uneven deformation of embankment, but no similar situation occurs under low embankment. This asymmetric temperature field problem should be considered when ripped-rock embankment is designed and constructed.