The Mechanism And Numerical Simulation Of Water Transfer In Seasonal Freezing Soil

Seasonal freezing soil lies on the northward area of north latitude 30o inour country, and its acreage is up to 513.7×104 square kilometer that occupiesabout 53.5 percent of the acreage of whole country. Seasonal freezing soil is akind of special soil-water system in which there is ice crystal, and it is geologyobject composed of soil grain, ice, pore water, and gas.Freezing in soil makes water transfer to frozen area, and it plays a veryimportant role in the research of agriculture,water resources,environment andengineering. Seasonal frozen damage is a very important problem in buildingroad in the north of our country. The problems of swelling and road mudding allcome of water translocation.At present, the research of water translocation in freezing soil alreadyobtains quite lots of achievement, but due to limited test condition, most ofsoil freezing tests still be carried through in the laboratory. So it is verydifferent with multivariate nature boundary condition. For example, it isdubious about the mechanism of water translocation, and if the expression ofdriving force is reasonable etc. Thus there are lots of aspects worth to go deepinto.On the basis of field monitoring of road formation at the orientation ofChangchu-Songyuan, Changchun-Siping, and the north to Changchun-Jilin,combining tests in the lab, this paper carries on the research of mechanism ofwater translocation in seasonal freezing soil. This paper sets up couple modelof water translocation in order to forecast the dynamic variation law ofunfrozen water, ice and temperature at different freezing time in the system offreezing soil.This paper carries through hereinafter research work:1 Seasonal frozen damage comes of water translocation in the processof freezing. Material composition,structure character,physical and chemicalproperties of soil are the base of the research on water translocation. Theproperties of particle face and pore structure can determine capability of waterholding and stress in soil. At the same time, it can also determine the phasechanging rule of water in soil at different temperature, and affect watertranslocation at different states and procedures.Firstly this paper studies material composition, physical and chemicalproperties of seasonal soil of the area of Changchun. There are so many siltand silty clay in the soil layer. Granularity composition has more than 70% siltand more than 20% clay. Mineralogical composition mostly are quartz andfeldspar. Structural connection is mainly bound water. All this characters makethe channels in soil free to water translocation, and some problems will occurdue to water translocation. There are so many Na+ in the soil which will makebound water more thicker in the pore of silty clay which has worse dispersity.This will help the outer bound water translocation. In addition, because themain mineralogical composition are quartz and feldspar which has few clayminerals, as a whole, specific area is small.In addition, through the research of the parameters of exchange of heatand hydraulic power, we find that it is a linear relation between conductivityfactor and negative temperature, besides, diffusion coefficient andtransmissibility coefficient have the similar change rule. Influenced by bulkvolume,moisture content,granularity composition,temperature, diffusioncoefficient and transmissibility coefficient will increased with volumetricwater content in power function.This paper carries on the analysis of microstructure with the scanningelectronic microscope, and finds that the seasonal soil of the area ofChangchun has cluster structure. It is mainly small and medium-sized porewhich lies in the soil in which has not bonding materials. The grains arebonded mainly by bound water. All of them are the base of the study on themechanism of water translocation.2 Unfrozen water translocation in freezing soil and frozen soil are veryimportant because that water translocation can cause redistribution of moisturecontent in freezing soil, however changing of temperature gradient is themoisture content moisture content trigger factor of water translocation. On theprocess of freezing of pore water in soil, freezing temperature is different atdifferent position of pore water. In soil , from far to close on the face of soilgrain, there are capillary water, weakly bound water, strongly bound water.The freezing temperature of capillary water is higher than the others, and thatof strongly bound water is the lowest that it can reach -78oC. On the base offield monitoring temperature and moisture content, the paper analyses theinfection of the temperature on moisture in soil , according to test in lab andset up power function of unfrozen water content and negative temperature.According to variation of moisture content in soil in freezing period ofNovember in 2004 to April in 2005, freezing period of freezing soil innortheast area can be divided to 3 stages: freezing and thawing alternationstage, freezing development stage, and thawing stage.For the analysis of the zone of water translocation, the paper carries onthe comparison of moisture content before and after freezing and finds thatincrease extent of moisture content is the largest in 1m depth beneath theterrene, and the increase extent of moisture content in the north toChangchun-Jilin is up to 20%. From the terrene to 2.5m depth, the generaltrend is that with the depth increasing, the increase extent of moisture contentreduces. In the depth within 2.5m~3m, the moisture content before and afterfreezing is basically equal. The author marks off three zones from terrenedownwards, A(depth 1m~1.5m),B(depth 0.5m~2m),C(depth 3m~4m), whichrepresent freezing zone, middle zone, capillary water zone. The A and B is themain zone the paper studies. There is a zero flux surface of the total potentialof soil and water at the 2.5m depth, and the potential of soil and waterdecreases up or down the surface. When freezing layer develops downwards,form the lower interface of freezing layer to 2.5m depth, the potential of soiland water increases, so the adhesive water and vapourous water translocationupwards from the B zone to the freezing frontal surface. Beneath 2.5m depth,the potential of soil and water decreases with the depth increase, so theadhesive water and vapourous water translocation downwards. As a result, themoisture content is increased above the 2.5m depth and decreased beneath2.5m depth. At the same time, in the A zone, because the temperature ofterrene is lower than the freezing frontal surface, the potential of soil and waterdecreases from the freezing frontal surface to the terrene, which cause theinner weak bound water translocation from the freezing frontal surface to theterrene. But due to the strong binding force of grain, plus the narrow channelsafter freezing and large viscidity beneath negative temperature, the transferamount is so little.3 the seasonal freezing soil of the area of Changchun has obviouscapillary phenomenon, and the arising height of capillary water is up to3m~4m. But the frozen depth is mostly about 1m~1.5m, and the groundwaterlevel depth is under 6m, so the distance between maximum arising height ofcapillary water and freezing layer is 0.5m to 2m. we can confirm that capillarywater do not transfer. When soil is freezing, the outer weak bound water isfrozen firstly. Due to the strong binding force of grain, the strong bound waterhas lost the character of fluid, just like solid, so it can not transfer. Under theeffect of outside force, it is only inner weak bound water that can transfer. Butdue to the still strong binding force of grain, plus the narrow channels afterfreezing and large viscidity beneath negative temperature, the transfer amountof inner weak bound water is so little.4 in order to simulating and forecasting the rule of water translocationin seasonal soil, and on principle of simplifying model and having not anyeffect on the rule of transfer, this paper gives 6 pieces of hypothesizesaccording to practical conditions. Basing on these hypothesizes, according toenergy conservation law, the author set up basic partial differential equation ofmoisture movement,heat transfer partial differential equation and solutemigration basic equation. Accordingly these equation make up of wholecoupling model of water translocation.5 Because some functions of unfrozen soil and freezing soil are notcontinuum at two spots of initial freezing temperature, this paper do somespecial treatment at inflexion. This can avoid the case of un-constringencywhen the model is calculated. Because the vapourous water content can not bemeasured accurately at present and the transfer content of vapourous water isso small, the paper ignore the item related to vapourous water. In order toforecast dynamic variations of unfrozen water,ice and temperature in thesystem of freezing soil, the model is calculated using mathod of weightedresidual based on one dimension and two dimension. With the contrast of calculatedresult and actual measurement, we find that calculated temperature is reasonable, andcalculated moisture content has the same tendency with actual moisture content,but there are difference of 10%~15% in amount. The reasons of difference arethat measure sites locate on two sides of road covering snow and the modeldoes not take into account the effect of snow.