Study on the law of water-heat-vapor migration in subgrade structure under freeze-thaw cycles
Under freeze-thaw cycle conditions,the water migration in subgrade structure was a pivotal factor contributing to the deformations associated with frost heave and thaw settlement in road structures.Many schol-ars had confirmed that the phenomenon of water migration and accumulation under the pavement structure layer under freeze-thaw action was widespread,and the action of water vapor migration cannot be ignored.However,there was a lack of quantitative understanding of the study of only water vapor migration,and there was a lack of experimental methods for studying only water vapor migration.At the same time,the outflow of liquid water un-der the pavement covering layer during the freeze-thaw process was an important reason for the sharp increase of water content in the gravel layer or unsaturated subgrade soil layer at the bottom of the covering layer.At pres-ent,there were relatively few studies on the outflow law of liquid water during freeze-thaw cycle.In order to ex-plore the water-heat-vapor migration law of the subgrade structure under the freeze-thaw cycle,the subgrade structure of the actual project was simulated,including pavement structure layer,gravel layer and unsaturated subgrade soil layer,and the water migration test of liquid water and water vapor separation under the freeze-thaw cycle was carried out based on the semi-permeable membrane material.The water migration law under the freeze-thaw cycle was obtained by monitoring the hydrothermal change of the sample,the rise image of fluores-cence,the change curve of water replantation and water collection.And the pore structure of soil column,the rate of rise and fall and the temperature gradient and other factors on the water vapor migration and liquid water outflow characteristics.The experimental results showed that the temperature change of a freeze-thaw cycle can be divided into five stages:rapid cooling stage,slow cooling stage,stable freezing stage,rapid heating stage and slow heating stage.Under the action of soil water potential,the water in the Maanobis bottle migrated up-ward in the soil column in the form of water-vapor mixture,and then changed to the form of water vapor when it reached a certain height.The amount of water vapor migrated at the bottom of the temperature-controlled plate and the crushed rock layer increased linearly during the whole freeze-thaw cycle.The amount of water vapor mi-gration gradually increased with time,indicating that the action of water vapor migration in the process of water accumulation cannot be ignored.The outflow of liquid water from the bottom of the temperature control plate and the gravel layer was mainly concentrated in two stages of the freeze-thaw cycle:the first stage was the cool-ing stage,mainly condensate water,and the second stage was the melting stage,mainly melt water,which ac-counts for more than 70%of the outflow of liquid water in a freeze-thaw cycle.The small pore structure of silty clay resulted in water vapor migration mainly affected by volumetric gas holdup.With the increase of moisture content,the water vapor migration tended to decrease.However,sand had large pores,and the water vapor mi-gration was mainly affected by the water vapor diffusion enhancement factor.With the increase of moisture con-tent,the water vapor migration showed an increasing trend.The decrease of cooling rate led to a slow decline of the freezing front and reduced the closed effect of soil freezing on the water vapor migration channel.With the decrease of rising and cooling rate,the water accumulation at the bottom of the temperature control plate and the gravel layer increased.The larger temperature gradient led to the increase of water vapor diffusion coefficient and water vapor density gradient,which will lead to the increase of water vapor migration at the bottom of the temperature control plate and the gravel layer.Based on the above results,it was of great significance to reveal the mechanism of the increase of water content of subgrade soil under the condition of freeze-thaw cycle and the causes of road diseases.
freeze-thaw cyclesubgrade structuresemi-permeable membranewater vapor migrationpore structurerising and cooling ratemelt water
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