Experimental and theoretical studies on the solar reflectance of crushed-rock layers

摘要

Crushed-rock layers have been widely used to cool subgrades in permafrost regions. The convective cooling effect of crushed-rock layers depends not only on the cavities of the layers but also the outmost skin temperature, which is mainly regulated by the solar absorption. Here we experimentally studied the solar reflectance of crushed-rock layers with mean particle sizes of 0.9 cm, 1.5 cm, 2.2 cm, 3.5 cm, 5.4 cm, 8.4 cm, 11.8 cm, 17.3 cm and 26.4 cm, respectively. The experimental results indicate that the solar reflectance of the crushed-rock layers firstly decreases, and then increases with the mean particle size. It is found that the crushed-rock layer with a mean particle size of 5.4 cm reflects the least sunlight. The theoretical analysis indicates that a crushed rock layer absorbs more sunlight as the cavity ratio increases and as the mean intersection angle of crushed-rock surfaces decreases. Our findings imply that the solar reflectance of a crushed-rock layer can be raised by reducing the surface cavity ratio of the crushed-rock layer and/or flattening the crushed-rock layer surface; however, the surface cavity ratio of the crushed-rock layers and the mean intersection angle of the crushed-rock surfaces are uncertain because the crushed-rocks with different shapes were randomly poured into the wood trays; therefore, it is expected that the trend, the macro-reflectance of the crushed-rock layers changes with mean particle sizes, is validated by further refined experiments.