冻融循环作用下路基结构水热汽迁移规律研究

Study on the law of water-heat-vapor migration in subgrade structureunder freeze-thaw cycles

在冻融循环环境下,路基结构内水分迁移是导致道路冻胀、融沉变形的主要原因。为了探究冻融循环条件下路基结构的水热汽迁移规律,模拟了实际工程的路基结构,基于半透膜材料开展了冻融循环条件下的水、汽分离的水分迁移试验,通过监测试样的水热变化、荧光素上升图像、补水量和集水量变化曲线,得到了水分迁移规律,进而分析了土的孔隙结构、升降温速率和温度梯度等因素对水汽迁移和液态水流出特征的影响。试验结果表明,在土水势作用下,马氏瓶的水分首先以水-汽混合的形式在土柱中向上迁移,达到一定高度后,转变为以水汽的形式向上迁移。非饱和土柱的水汽会透过半透膜向碎石层和控温板底迁移,在整个冻融循环过程中,水汽向碎石层和控温板底的迁移量呈现线性增加的趋势,表明水汽迁移在水分聚集过程中的作用不可忽视。碎石层和控温板底液态水的流出主要集中在每个冻融周期的两个阶段,第一阶段为降温阶段,以冷凝水为主;第二阶段为融化阶段,以融化水为主,融化水占一个冻融周期液态水流出量的70%以上。粉质黏土孔隙较小导致水汽迁移主要受体积含气率的影响,随着粉质黏土土柱含水率的增加,水汽迁移量呈减小趋势。而砂土的孔隙较大,水汽迁移主要受水汽扩散增强因子的影响,随砂土土柱含水率的增加,水汽迁移量呈增大趋势。降温速率减小使冻结锋面有一个缓慢下移的过程,减小土体冻结对水汽迁移通道的封闭影响。升降温速率的减小导致碎石层和控温板底的水分聚集量增大。较大的温度梯度导致水汽扩散系数和水汽密度梯度的增加,从而引起碎石层和控温板底的水汽迁移量增大。

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.