Passively cooled railway embankments for use in permafrost areas

Permafrost (permanently frozen ground) underlies, approximately 25% of the world's land surface. Construction of surface facilities in these regions presents unique engineering challenges due to the alteration of the thermal regime at the ground surface. Even, moderate disturbance of the preexisting ground surface,energy balance can induce permafrost thawing with consequent settlement and damage to roadway or railway embankments. Railway embankments are particularly susceptible to thaw settlement damage because of the need to maintain the alignment and even grade of the rails. The present work examines the heat transfer and thermal characteristics of railway embankments constructed of unconventional, highly porous materials. It is possible to produce a passive cooling effect with such embankments because of the unstable density stratification and resulting natural convection that can occur during winter months. The convection enhances the upward transport of heat out of the embankment during winter, thus cooling the lower portions of the embankment and underlying foundation soil. Numerical results have been obtained with an unsteady two-dimensional finite-element model that is,capable of solving the coupled governing equations of pore air flow and energy transport. The numerical results are obtained for conditions typical of those found in railway configurations which allow open exchange of air between the embankment structure and the surrounding ambient air mass.