Numerical Analysis for Temperature Characteristics of Open Boundary Crushed-rock Embankment on Qinghai-Tibet Railway

At present, in order to protect the stability of permafrost beneath emban kment, the crushed-rock emban kment, as a new type of emban kment structure, has widely been used in the construction of Qinghai-Tibet Railway. Its crushed-rock layer is almost open in tow bilateral boundaries and closed at top and bottom, and air can flow into/out of the ballast layer and crushed-rock layer. Therefore, the convection and transfer heat patterns are very complicated in the ballast layer and crushed-rock layer of the emban kment, which are regarded as porous media. In this paper, based on the wind, temperature and geology conditions of Qinghai-Tibet 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 provide to analyse the velocity and temperature characteristics of the crushed-rock emban kment with different emban kment heights under open boundary condition for the coming 50 years. The calculated results indicate that, due to the influence of the external wind, the convective heat transfer mainly relies on the forced convection in the open crushed-rock emban kment. Even if the air temperature will be warmed up by 2.6℃ 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 emban kment is constructed in the regions whose present mean annual air temperature is -4.0℃. Furthermore, the cooling effect of high crushed-rock emban kment is better than that of low emban kment. This results from the fact that the wider bottom of high emban kment has a more influence dimension on the underlying frozen soil. However, cardinal winds on Qinghai-Tibet Plateau disturb its convection pattern, so that an asymmetric temperature distribution occurs under high emban kment and it is possible to induce a transverse uneven deformation of emban kment, but no similar situation occurs under low emban kment. This asymmetric temperature field problem should be considered when crushed-rock emban kment is designed and constructed.

At present, in order to protect the stability of permafrost beneath embankment, the crushed-rock embankment, as a new type of embankment structure, has widely been used in the construction of Qinghai- Tibet Railway. Its crushed-rock layer is almost open in tow bilateral boundaries and closed at top and bottom, and air can flow into/out of the ballast layer and crushed-rock layer. Therefore, the convection and transfer heat patterns are very complicated in the ballast layer and crushed-rock layer of the embankment, which are regarded as porous media. In this paper, based on the wind, temperature and geology conditions of Qinghai- Tibet 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 provide to analyse the velocity and temperature characteristics of the crushed-rock embankment with different embankment heights under open boundary condition for the coming 50 years. The calculated results indicate that, due to the influence of the external wind, the convective heat transfer mainly relies on the forced convection in the open crushed-rock emban kment. Even if the air temperature will be wanned up by 2.6℃ 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℃. Furthermore, the cooling effect of high crushed-rock embankment is better than that of low embankment. This results from the fact that the wider bottom of high emban kment has a more influence dimension on the underlying frozen soil. However, cardinal winds on Qinghai-Tibet 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 emban kment, but no similar situation occurs under low embankment. This asymmetric temperature field problem should be considered when crushed-rock emban kment is designed and constructed.