Winter-time convection in open-graded embankments

Winter-time natural convection in open-graded gravel embankments is studied using a numerical representation of the unsteady two-dimensional momentum and energy equations. The analysis examines the ability of these embankments to maintain the structural integrity of thaw-unstable permafrost that often underlies roadway or airport embankments in northern climates. As a result of low ambient temperatures acting on the embankment surface during winter months, an unstable density stratification develops in the embankment. Buoyancy-driven convection of the pore air occurs in reaction to the density gradient. The convection enhances the upward transport of heat out of the embankment during winter months, thus cooling the lower portions of the embankment and underlying foundation soil. During summer months the density stratification is stable and convection does not occur. Consequently, summer-time heat transfer is dominated by thermal conduction which transports heat less effectively. The results of the present study show that the winter-time convection can lower foundation soil temperatures beneath open-graded embankments by as much as 5 degrees C on an annual average basis compared to standard sand and gravel embankments. Time varying temperatures and pore air velocities are calculated for different embankment permeabilities and the results are visualized in the form of isotherm and velocity vector plots for different times of the year.