Study on the formation mechanism and preventive measure of pot cover effect for subgrade in seasonal frozen soil area under freeze–thaw cycles

Abstract The presence of an impervious cover layer inhibits the free evaporation of moisture in the soil during seasonal freeze–thaw cycles, leading to a phenomenon known as the pot cover effect. This can result in severe frost heave issues in airport runways, highway subgrades, railway subgrades, and other similar infrastructure. In this study, a disease investigation was conducted at a gas transmission station situated in an area with seasonally frozen soil. Meanwhile, the causes of moisture accumulation and frost heave in the lower part of concrete pavement slabs were analyzed. A coupled water–vapor–heat model for unsaturated frozen soil is then established to analyze the formation mechanism of the pot cover effect in the subgrade. Finally, the preventive effect of the impervious layer on moisture migration in the pot cover effect was analyzed. The results indicate that the pot cover effect causes a significant accumulation of moisture beneath the concrete pavement at the gas transmission station, resulting in a moisture content increase of 5%–35%. During the freezing period, the vapor of shallow soil migrates upward, while the liquid water remains unchanged. During the melting period, both vapor and liquid water migrate downward. The laying of the impervious layer can prevent moisture migration of the pot cover effect. When the impervious layer is placed on the freezing front (0°C), the prevention effect of the pot cover effect is the best.