Experimental Study On The Water-vapor Transfer Characteristics Of Unsaturated Soil

There is not only liquid water migration in soil,but also water-vapor transfer,which is often ignored due to the limitation of testing technology.In fact,liquid water in soil is closely related to water-vapor transfer,which is an important factor affecting soil mechanical properties.Especially in arid and semi-arid areas,between semi-rigid base and loess subgrade,the water-vapor transfer will cause the thin layer of semi-rigid base layer to empty,and then cause the failure of the road surface.In this paper,aiming at the problem of water-vapor transfer in soil,experimental devices are developed and experimental schemes are designed to carry out laboratory tests.Considering different factors,the changes of liquid water and vapor water in soil are analyzed to reveal the mechanism of water-vapor transfer in soil.Paper by collecting and sorting the existing research materials,analyses the Dun-Huang airport and Shen-Yang taoxian airport pavement and G30 Lian-Huo high-speed permanent ancient period and Karamay area highway pavement disease causes,the results show that semi-rigid base and "the cover effect" caused by climate factors,led to significant watervapor transfer,is one of major factor in the production of disease.Soil-water characteristic curve is a curve reflecting the relationship between soil matrix suction and moisture content,which directly affects the characteristics of water transfer.Filter paper method was used to test the soil-water characteristic curve of silty clay and sandy soil,and VG model was used to fit the test results,and the fitting effect was good.An indoor unsaturated soil water-vapor transfer test device was developed to conduct moisture transfer tests on silty clay and sandy soil under high temperature conditions,and the influence mechanism of moisture content level,moisture content gradient,migration time and soil type on water-vapor transfer was analyzed.The results show that with the same moisture content gradient,the movement of sand increases first and then decreases with the increase of moisture content.The migration of silty clay increases with the increase of water content.When the initial moisture content of the dry end is the same,the migration increases continuously with the increase of the moisture content gradient.The longer the migration time,the larger the migration amount;The migration of silty clay is generally greater than that of sandy soil.In order to simulate the working state of subgrade under semi-rigid base,a soil column model test device for water-vapor transfer was developed.Under the condition of closed system,the "pot cover effect" makes the water vapor in the soil column model of silty clay move significantly,resulting in a significant increase in the water content between the soil column and the roof from about 10 cm to 45 cm.However,in the soil column model of sandy soil,the water-vapor transfer caused by the "pot cover effect" is not obvious,the effect of gravity potential is significant,and the water content in the soil column shows a trend of small upward and large downward.Under the condition of the open system,the surface of the silty clay column is subject to evaporation,resulting in the formation of a soil layer with high density and small water permeability,forming a "pot cover effect",resulting in an increase in the moisture content between 33 cm and 48 cm from the top of the soil column.However,in the soil column model of sandy soil,due to evaporation,the moisture content inside the soil presents a decreasing trend as a whole,indicating that the water-vapor transfer caused by the "pot cover effect" is not obvious.The mechanism of water-vapor transfer in unsaturated soil is analyzed and summarized.Then the diffusion coefficient calculated by fick’s law is used to quantitatively characterize the migration of silty clay and sandy soil under the action of high temperature.