Numerical analysis for critical thickness of crushed rock revetment layer on Qinghai-Tibet Railway

Under the warm and ice-rich nature of permafrost and the scenarios of climate warming on the Qinghai-Tibet Plateau, the construction of Qinghai-Tibet Railway has to utilize new techniques by cooling down the ground temperature. The non-dimensional governing equations for natural convection in variable permeability porous media are derived. Onset and development of winter-time natural convection in the crushed rock revetment of Qinghai-Tibet Railway are investigated by an airflow function. Numerical results indicate that the natural convection pattern occurring within the crushed rock revetment will be unicellular, thus forming the chimney effect to cool down the roadbed during winter months. In order to evaluate the cooling effectiveness of winter-time natural convection in the crushed rock revetment embankment, the overall Nusselt number and relevant Nusselt percentage representing heat transfer through unit length at the embankment base were defined. The dependences of the overall Nusselt number and relevant Nusselt percentage on the thickness of crushed rock revetment layer and the amplitude of harmonic surface temperature are analyzed using a numerical representation of the non-dimensional governing equations for convection. The approach to evaluation of critical thicknesses of crushed rock revetment in the railway embankment was developed on the basis of the dependence of the overall Nusselt percentage on the thickness of crushed rock revetment and the amplitude of harmonic surface temperature. Validity of evaluating thicknesses of crushed rock revetment filled in the embankment was demonstrated both numerically and experimentally. Thus, this evaluation approach is available for the construction of Qinghai-Tibet Railway.