Permeability effects on winter-time natural convection in gravel embankments

Abstract

Winter-time natural convection in open-graded gravel embankments has been suggested as a technique which can be used to provide passive cooling and thereby avoid thaw- settlement of roadways located in permafrost areas (Goering and Kumar [1]). The present paper examines the ability of these embankments to maintain the structural integrity of thaw-unstable permafrost which 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 6°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.