| Recently, with the development of economy and construction in our country, more and more constructions, such as energy, transportation and communications and so on will be built in frozen soil areas. The rock and soil which is at the temperature of 0℃or below 0℃and containing ice was called frozen soil. Freezing in soil causes water migrate to frozen area. As an important link in natural water cycle, the moisture migration in seasonal frozen soil plays a very important role in agriculture, water resources, environmental systems and the basic constructions. In north area of our country, frost heave and boiling are two typical road damages which have a serious impact on life expectancy, and both them occupy a fairly large proportion in various road damages. All the phenomena come to one reason that is moisture migration in frozen soil. Therefore, the study of moisture migration in seasonal frozen soil, not only contributes to the theory promotion of water infiltration and migration in unsaturated soil, but also makes the characteristics of soil studied quantitatively, so that it can be a lead role in all kinds of constructions. At present, the research of moisture migration in frozen soil already obtains quite lots of achievement, but due to limited test condition, most of soil freezing tests still be carried through in the laboratory. So it is very different with multivariate nature boundary condition. In addition, the mechanism of moisture migration still be a uncertain problem, especially there are many questions in the study of water-heat doubling model. For example, if the expression of driving force is reasonable; the model built was too simple; the parameters are less; the solution method is difficult etc. Thus there is lots of aspects worth to go deep into. So the properties of seasonal frozen soil in Changchun area is studied in this paper associated with the project of"study on the microscopic mechanism of moisture migration in roadbed of seasonal frozen area in northeast China"sponsored by the national natural scientific fund. In this paper the basic physical and chemical properties, thermodynamic parameters and hydromechanical characteristics of frozen soil were studied firstly to know about their influence on moisture content migration process. Then the scanning electron microscope was used to study the microstructure of seasonal frozen soil in Changchun area which was frozen under different temperatures, including being frozen and after frozen. Thirdly the coupling model of water and heat which is based on unsaturated soil porous medium theory was built up with the basic laboratory test data to forecast the dynamic variation law of unfrozen water, ice and temperature at different freezing time in frozen soil system. Finally, the Changchun1 and Changsong line soil sample were chosen as the example to compare the computing results to test results and field monitoring data respectively and satisfied conclusion was acquired. Through the study, the flowing conclusions were obtained:1. The moisture content migration in seasonal frozen soil is the reason why the frozen damage appears while the material composition, structure character, physical and chemical properties of soil is the base of the research on moisture content migration. The property of particle surface and pore structure can determine water-holding capacity, at the same time, it can also determine the phase changing rule of water in soil at different temperature, and affect moisture content migration at different states and procedures.Firstly, the granularity composition, mineral composition, soluble salt content, specific surface area and cation exchange capacity were introduced, and then the heat exchange parameters and hydromechanics characteristics of frozen soil were also studied. The study shows that there is lots of silt in the soil layer and the content will change when dispersant was added into, namely the content of silt decreases while clay increases. Mineralogical composition mainly is original mineral such as quartz and feldspar and hardly any clay minerals in soil, so the value of specific area is small. And when dealing with the moisture content migration problem the solute transport can be ignored because of the low content of salt. In addition, through the research of the parameters of exchange of heat and hydraulic, it is found that there is a linear relation between thermal conductivity and negative temperature, besides, influenced by density, moisturecontent, granularity composition and temperature, the diffusion coefficient will increase with volumetric moisturecontent in power function while the transmissibility coefficient have the similar variation law.Secondly, through the qualitative and quantitative study of microstructure of the soil under different temperature conditions the follow conclusion can be acquired: the seasonal soil of this area has large particle and many pores with loose structure. The grains are connected mainly by bound water. After frozen with the aggregate appear the small pores diminish and big pores increase, and the pore content of soil which is being freezing is less than that after frozen.After frozen, micro-structure elements with diameter between 10 and 20μm diminish and diameter between 1 and 2μm increase. At the mean time, the pore of a diameter of 1 to 2, 2 to 5 and 5to 10μm reduce, while less than 1μm increase a small amount. Also we can conclude that the water freezing to ice has an effect on the soil microstructure because that average diameter of both structure unit and pore reduced.2. Unfrozen moisture content migration in freezing and frozen soil are so important because that moisture content migration can cause redistribution of water in freezing soil, while the changing of temperature gradient is a factor of moisture content migration.Firstly, the seasonal freezing layer is about 1.7m deep below ground surface, so depth more than 1.7m is always at a temperature above 0℃according to field monitor data in this area. Therefore in the whole process of freezing and thawing, the freezing was only from ground surface to lower part while thawing from both upside and lower part to middle part. According to variation of moisturecontent in soil across freezing period of 2004 to 2008, the freezing period of this area can be divided to 4 stages: unstable freezing stage, one-way fast freezing stage, stable freezing stage and two-way thawing stage.Secondly, the arising height of capillary water in this area is about less than 2m according to the test results so that the research object in this paper is unsaturated soil. As for Changsi line, Changsong line and Changji line, whose groundwater level is nearly in depth of 7m, the distance between maximum arising height of capillary water and freezing layer is comparative large. So we can confirm that capillary water does not transfer in the process of moisture content migration. When soil is freezing, the outer weak bound-water is frozen firstly while the strong bound-water can not transfer because due to the strong binding force of grain, water has lost the character of fluid, just like solid. So under the effect of outside force, the water which can transfer is only inner weak bound-water. However, the binding force of grain on the weak bound-water layer is still so strong and the narrow pathway generated by freezing is unfavorable factor for moisture content migration, at the mean time the water which is beneath negative temperature has relatively large viscidity, so the transfer amount even of inner weak bound-water is so little. Therefore at last we can conclude that the migrated water mainly is the membrane water in unfrozen soil layer below freeing layer in the three road lines. To Changchun1and Changchun2 soil, their groundwater level is only about 2m below ground surface. So when water in upper soil layer was frozen, the ground water can be a water source to supply the icing. In other words, the capillary water is the main body of the process of moisture content migration in these two areas.Thirdly, in the test of moisture content migration, soil sample was freezing at the temperature of -10℃, -5℃, -2℃and room-temperature (25℃) from top to bottom. In this process, water part of which is in soil with lower temperature was frost firstly, while the water below this area migrates upward to supply the soil that with less water. We also conclude that after 3 months freezing, the final moisturecontent is smaller than initial value for the soil with higher compactness and larger for soil with lower compactness, and the higher of compactness the lower of strong capillary water height. When the compactness is relatively lower, basically from the study in this paper we can conclude that when compactness is less than 90%, the higher of compactness the higher of capillary water height. Because that with diameter of capillary reduce the characteristic of capillary increase, so the rising height of capillary water of the soil is large. But when compactness is increasing till more than 90%, we got the contrary trend.3. In order to simulating and forecasting the rule of moisture content migration in seasonal frozen soil, this paper gives five hypothesizes and illustrations according to practical conditions on the base of assuring precision of computing. Basing on these hypothesizes, according to mass balance theory, energy conservation law and heat engineering, considering the change in temperature and mass of water during the change of temperature, the coupling model between water and thermal of unsaturated soil was established in this paper. In this model diffuse and convection was also considered, then methods to determinate parameters were showed.We can not get analytical solutions but only numerical solution from the model established because it is composed of nonlinear differential equation. So in this paper the finite method was used to deal with the equations to find our answers. The Changchun1 and Changsong line were chosen as examples to get the solution of moisture content migration equation and temperature transmission equation. It shows that, under the affection of different temperature gradient, the temperature in soil is unstable. The velocity is higher and easier to get balance in area which is in larger temperature gradient. As for moisturecontent, due to the liquid water froze to ice, the pathway for moisture content migration was affected and then in turn the freezing progress was affected too. While moisturecontent was aggregating partly and the growth of water was uneven because of the mutual constraints. Compare the computing results of Changchun1 to test results we can conclude that both the two kinds of results'basic change tendency are almost same, but there is inaccuracy between them. The computing result is larger than test results on the top of the soil and smaller at the bottom basically because of the reason that the upper soil was air dried and the lower soil was put into water. As for Changsong line, both the computing tests of temperature and moisturecontent have a very small inaccuracy with monitor results at the time of about 5 months while other time have relatively high inaccuracy. Thereinto the moisturecontent of computing results was smaller than that of test results and the reason perhaps is that the snow infiltration was not considered in this model. So maybe the further study is how to include all kinds of factors affecting the model, such as snow infiltration, seepage flow, dynamic load, deformation of soil and so on.
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