An experimental study on the cooling mechanism of a shading board in permafrost engineering

Abstract

Shading boards (SB) were tested on an embankment slope as one of the measures for actively cooling permafrost in roadbed engineering along the Qinghai-Tibet Highway on the Tibet Plateau, China. A series of tests, evaluating the cooling mechanism of SBs, also were conducted in the laboratory that showed that the SB cooled the permafrost by significantly reducing the input of solar radiation onto the side slope surfaces. In these laboratory tests, the radiation intensity was lowered by 85.5%. In addition, the enhanced air convection between the SB and the slope surface removed the heat produced by the secondary radiation from the back side of the board, as well as heat released from the soil. Laboratory tests indicate that the temperature on the slope surface lowered in more degree with the SB can be explained by the “stack effect” triggered between the SB and the slope surface. The accelerated air flow under the SB brings the temperature of the slope surface to ambient air temperature. There was an optimal height of 15 cm for the SB, where the air flow reached its maximum amount and velocity, which means that, in practice, the installed height of the SB should be taken into account. Also, the free flow of the air under the SB should be assured by leaving the space between the SB and the protected surface unblocked. Practical applications of the SB along the Qinghai-Tibet Highway from November 2003 to November 2004 showed that the SB could reduce the annual average surface temperature of the entire embankment side slope by about 3.2 °C compared with the unshaded embankment slope. The maximum reduction was 4 °C. The annual average slope surface temperature under the SB was about 1.5 °C lower than the nearby natural ground surface.