土中水分的蒸发过程试验研究
EXPERIMENTAL INVESTIGATION ON EVAPORATION PROCESS OF WATER IN SOIL DURING DRYING
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摘要: 土中水分的蒸发是含水量动态减小的过程,伴随着土结构、应力和应变状态的演化,对土体的工程性质有重要影响,是一些工程问题的直接诱因。以初始饱和的黏土试样为研究对象,在控制环境温度(25~45℃)和试样初始厚度(5~11mm)条件下,开展了一系列室内干燥试验。通过监测试样在干燥过程中的失水量变化,获得了试样的蒸发曲线。结果表明,土中水分的蒸发过程基本上分3个阶段:常速率、减速率和残余阶段。通过分析各阶段中水分蒸发的内在机制和蒸发特征,发现常速率阶段主要发生蒸发初期,对应试样的含水量较高,且基本为饱和状态,剖面水分迁移以毛细水作用为主,蒸发速率大小主要受外部环境因素的控制; 蒸发面上的蒸汽压梯度减小、剖面水分的迁移速率降低、剖面吸力梯度降低、土中可供蒸发的总水量减小和孔隙气体增加等是导致土体水分蒸发过程从常速率阶段过渡到减速率阶段的主要原因; 减速率阶段剖面水分迁移受毛细水和蒸汽扩散作用的共同支配,且后者逐渐在蒸发过程中起主导作用,蒸发速率的大小主要取决于土质条件和孔隙结构特征。环境温度越高,试样的初始蒸发速率越大,结束干燥所需时间越短; 试样厚度变化对初始蒸发速率的影响不明显,但干燥时间随厚度的增加而增加。Abstract: Evaporation of water saturated in soil is a time-dependent process of decreasing water content.It is always accompanied by the evolution of soil fabric,stress-strain state as well as soil engineering properties.It is also an initiator of many geotechnical engineering problems.In this investigation,desiccation tests under different temperatures(25~45℃)were carried on initially saturated clayey soil with various layer thicknesses(5~11mm).The water loss of the specimen during drying was monitored and the corresponding evaporation curve was obtained.The results show that the evaporation process of soil water occurs in three fairly distinct stages: constant rate stage,falling rate stage and residual stage.After analyzing the evaporation characteristics and the intrinsic mechanism involved in each stage,it is found that the constant rate stage generally occurs at the initial drying period,where the water content is relative high and the specimen is still saturate.During the constant rate stage,the profile moisture transfer is dominated by liquid flow and mainly controlled by capillary force,and the evaporation rate significantly depends on environmental factors or limited by the amount of energy available to vaporize soil moisture in the upper layer of the soil.The decrease of vapor pressure gradient on the evaporation surface,transfer velocity of profile moisture,profile suction gradient,availability of water amount and the increase of pore air are the primary factors that result in the evaporation transition from constant rate stage to falling rate stage.During the falling rate stage,the profile moisture moves in both liquid and vapor forms and the later one gradually dominates the evaporation process,the evaporation rate is mainly limited by the soil conditions and pore structure.In addition,it is also found that higher temperature corresponds to higher initial evaporation rate and shorter drying period,while the initial evaporation rate is less sensitive to specimen thickness changes; thicker specimen needs longer drying period.
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