| With the increasing requirement of traffic to the development of flourishing economy in our country, the construction of highway and railroad were enlarged, with the result that such the Fine-grained Soils(including silty soil and clayey soil),expansive soil,loess,collapsible soil,and so on,were gradually used as subgrade materials. The engineering property of unsaturated soil and moisture transfer had been attracted much attention by practical engineering workers and researchers more and more .The roadbed is belong to unsaturated soil, which is greatly influenced by water content of soil. The subgrade constructed was exposed to environment,strongly suffering the load of road surface and seepage effect of rainfall and capillary effect of under-ground water and seasonal temperature exchange,etc.,which straightly provided the motility of water transfer in the subgrades and the water content of subgrades changed a lot. The variety of water content in the subgrade would decrease the engineering property such as the decrease of rebound modulus in subgrade. Therefore,it is essential to study the rule of moisture transfer in the roadbed with the various natural factors in order to improve the roadbed design in future. The moisture effect of engineering property of unsaturated-soil subgrades and the water transfer in the sungrades with capillary effect and the seepage of rainfall and the coupled thermal and water effect was studied with theory analysis and laboratory experiment and numerical simulation in the paper. First , the essential characteristics of matrix suction of unsaturated soil was discussed in the paper. The characteristics of water film of soil grains,which is the mass of matrix suction,was studied. theory of water film was applied in the petrochemical science and soil science. The theory of water film is used and amplified based on the origin, namely, the water film composes of absorbed bound water and free water induced by capillary effect and matrix suction and the total suction was analyzed,meanwhile,the formula of total suction was come into being step by step. In addition,With the unsaturated triaxial equipment for remold red clay in Changsha test , we analyze systematically the characteristics of soil-water characteristic curve (SWCC) and the shear strength under the condition of various suction.Second,the characteristics of moisture transfer with the capillary effect was studied. The capillary mechanism in subgrade and the relationship between the height of capillary rising and time and soil size were discussed in the paper for the purpose of investigating the capillary effect on the subgrade. The porous medium model of capillarity in subgrade was analyzed on the basement of the theory of porous medium. The formula,which describe the variation of water content in unit soil because of capillary effect,was deduced based on the simple model of capillarity and soil-water characteristic curve. The capillary mechanism in subgrade and the relationship between the height of capillary rising and time and soil size were discussed in the paper for the purpose of investigating the capillary effect on the subgrade. The porous medium model of capillarity in subgrade was analyzed on the basement of the theory of porous medium. The formulas, which describe the variation of water content in unit soil because of capillary effect,were deduced respectively based on the simple model of capillarity and soil-water characteristic curve and the theory of porous medium.Third,the characteristics of moisture transfer with the seepage effect of rainfall was studied. The seepage of the subgrade in the process of rainfall and the impaction of rainfall on the water content of the subgrade were discussed in the following paper. The soakage effect of rainfall to the subgrade was studied by the movement mechanism of interior water in subgrade;Second, the mechanism of rainfall's seepage of IEM and FDEM in the subgrade was discussed and the formulae of seepage depth in the subgrade with different rainfall models were derived; In addition, saturated-unsaturated seepage flow in the subgrade was discussed and the formulae of saturated-unsaturated seepage flow were deduced based on the general seepage theory. Meanwhile,the paper discovered the characteristics of seepage of subgrade's fissures and the effect of seepage to subgrade engineering and the characteristics of subgrade's fissures and the models of subgrade's fissures were studied. For the main source of seepage of subgrade is rainfall,the formula of seepage of subgrade with the effect of rainfall were derived.Finally,the characteristics of moisture transfer with the effect of coupled moisture and thermal.The characteristics of moisture transfer in the subgrade with the process of temperature field and the coupled moisture and thermal field were discussed,respectively. Meanwhile,with the boundary of different initial dry intensity and initial water content,the change characteristics of temperature field with the time in the subgrade of sandy soil and the subgrade of clay soil were numerical modified, based on the thermal parameters of subgrade soil tested. Then,the formula of the coupled moisture and thermal was educed in the light of the thermo of mass conservation and thermodynamics. The coupled effect of matrix potential and thermal potential and the coupled effect of matrix potential and thermal potential and seepage potential were discussed in details,meanwhile,the formula of the coupled effect of matrix potential and thermal potential and seepage potential was educed,accordingly. The change characteristics of moisture field with the time in the subgrade with he coupled effect of matrix potential and thermal potential and seepage potential were numerical modified in the Matlab of computer language.
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