Convective Heat Transfer in Crushed Rock Aggregates: The Effects of Grain Size Distribution and Moisture Content

This study deals with the susceptibility to convective heat transfer of the materials used in the Finnish rail network. Previous studies have found that convection clearly influences the thermal conductivity of coarse aggregates in certain conditions. The occurrence of convection may cause subsoil frost heave. This study investigated the susceptibility of three subballast materials, which were made of different crushed rock aggregates, to convection: railway ballast (31.5/63 mm), subballast layers of crushed rock, and 5/16 mm crushed rock. Convection was found to increase the thermal conductivity of railway ballast severalfold, while the thermal conductivity of the currently used subballast material was also noted to increase clearly when the moving medium contained water. However, no significant increase in thermal conductivity was found in the case of the 5/16 mm crushed rock. Based on these results, it is clear that it is not possible to use tremendously coarse materials in thick structure layers in the northern area. The results of this study were one of the most important factors when the grading recommendation for subballast material used in Finland was changed to include finer material, which clearly reduces the possibility of the onset of convection.