Abstract: An experimental technique to study the cooling effect of natural convection in the embankment ballast is introduced. Various testing samples for embankment materials of ballast, pebbly gravel and sandy gravel are studied both experimentally and theoretically. The results show that, under periodically fluctuating temperature of surface, the mean Rayleigh number may represent effectively triggering natural convection in the fractured rock layer. The fractured rock layers with various grain sizes can trigger various natural convection patterns. The winter-time cooling effects of natural convection in the ballast layer increase with the increase of ballast grain sizes and are accumulated with time. Meanwhile, the heat conduction in the fractured rock layer must be considered synthetically so as to determine the optimal grain sizes of embankment ballast causing the maximum winter-time cooling effect.