Research On Water-Vapor Migration In Soils Under The Double Pot Cover Effect

The pot cover effect refers to the phenomenon of moisture accumulation beneath the pavement under condensation or desublimation because the vapor transfer in the soil is blocked by the pavement.The pot cover effect may result in subgrade strength degradation and cause engineering disasters such as deformation and cracks in the pavement,which affects the airport runway serviceability and road traffic safety.Therefore,it is of great significance to conduct investigations on the prevention and control of the pot cover effect.The method of laying an impervious layer in the soil can impede the water migration from the deep soil to the shallow soil.The impervious layer in the soil together with the cover forms a double pot cover structure,in which the moisture migration phenomenon can be summarized as the double pot cover effect.In this thesis,the water-vapor migration mechanism of the double pot cover effect is investigated.On this basis,an effective measure to prevent the water increment under the cover is proposed.The main work includes:1.The water-vapor migration mechanism of the pot cover effect is numerically revealed.First,a numerical model,which can depict the water-heat migration in the unsaturated soil,is established based on the existing research results.Then,parametric analyses are carried out to reveal the influence of the initial water content and the boundary low-temperature on the water-vapor migration of the pot cover effect.This work lays the foundation for further research on the mechanism of the double pot cover effect and the prevention measure.2.The water-vapor migration of the double pot cover effect is numerically investigated.The numerical model which can depict the water migration of the double pot cover effect is established.The influence of the impervious layer depth on the moisture migration is investigated.Results shows that the moisture content increases under the impervious layer when the depth of the layer is low and the moisture content increases under the impervious cover when the depth of the layer is deep.Preliminarily,the optimal position of the impervious layer to eliminate the water increase in the shallow soil is confirmed.Then,the influence of the boundary low-temperature on the moisture migration is discussed.It demonstrates that the optimal position for the impervious layer is slightly lower than the freezing front.Finally,a comparison of two special cases of the double pot cover effect and the double pot cover effect with an impervious layer at optimal position shows that laying the impervious layer at the optimal position can effectively prevent the water increase in the shallow soil.3.Laboratory tests are conducted to investigate the moisture distribution of the pot cover effect and the double pot cover effect.The one-dimensional laboratory tests are conducted.The low-temperature source is prepared using the NaCl solution,which has a freezing point lower than 0℃.The lab tests consider the influence of the initial water content and the existence of the impervious layer on the water-vapor migration in the covered soils.Test results show that the impervious layer at the position with a positive temperature close to the freezing front can effectively eliminate the water increment under the cover.The conclusions obtained by the laboratory tests coincide with the numerical results.4.A combined dredging and blocking measure is proposed to prevent engineering disasters.Based on the theoretical and experimental studies,a combined dredging and blocking measure is proposed to solve the water increment problem under the impervious cover.That is,an impervious layer is laid at a reasonable position in the soil to prevent the water increment fundamentally in the shallow soil.Meanwhile,a hydrophobic layer is laid above the impervious layer to dredge the potential accumulated water.To summarize,numerical simulation and laboratory tests are carried out to investigate the water-vapor migration of the pot cover effect and the double pot cover effect.A corresponding prevention measure to solve the water increase problem under the cover is proposed,which contributes to the disaster prevention in practical engineering.