Nonlinear analysis for the cooling effect of Qinghai-Tibetan railway embankment with different structures in permafrost regions

The thermal convection of fluid inside ballast layer and ripped-rock layer, which are regarded as porous media in railway embankment, is the process of heat and mass transfer. In this paper, in order to research the influence of different embankment structures and geometries on the underlying permafrost thermal regime along Qinghai-Tibetan Railway, a numerical representation of the unsteady two-dimensional continuity, momentum (non-Darcy) and energy equations of thermal convection for incompressible fluid in porous media is used to analyze temperature characteristics of a traditional ballast embankment, a horizontal ripped-rock embankment and two U-shaped ripped-rock embankments for the 50 years. The calculated results indicate: (1) the traditional ballast embankment will cause the great degradation of the underlying permafrost under the assumption that the air temperature will warm up 2.6 °C in the 50 years; (2) the U-shaped ripped-rock embankment with 150-cm-thick ripped-rock layer and 160-cm-wide ripped-rock revetment can efficiently protect the underlying permafrost. However, 120-cm-thick horizontal ripped-rock layer has weak cooling effect. Therefore, the horizontal ripped-rock layer thickness is a very important factor to the effect of ripped-rock embankment. These analyses indicate that reasonable ripped-rock embankment structure and embankment geometry can provide an effective mechanism for preserving permafrost under trend of global warming and avoiding large deformation and embankment failure due to thaw settlement in high-temperature permafrost regions along Qinghai-Tibetan Railway.