Test And Numerical Analysis On The Problem Of Temperature And Moisture Migration Of Loess Subgrade

According to the existing reference and data, the problems of temperature and moisture migration in loess subgrade are studied in this thesis. Using self-made test equipment, the moisture migration regulation of unsaturated loess under the effect of the gravitational gradient is studied by test, and the calculation method of gravitational potential is discussed. Based on the existing reference and data, a numerical model for loess subgrade thermal field is put forward after consideration of subgrade surface features and natural elements such as air humidity, radiation and atmospheric temperature, and parameters of the model are got. Taking the soil density and water content into account, a numerical model of moisture migration in unsaturated loess subgrade is given, and parameters of the model are got. It is pointed that both the coefficient of water volume change m₂^w and the coefficient of permeability k_w are all relating to not only water content but also soil density. Then the thesis puts forward the calculation model of coupled temperature and moisture migration of loess subgrade, and gives the calculation scheme.The test and calculated results of the subgrade surface temperature in Xi'an and Yan'an area verify the reliability of the numerical model and the parameters. After analyzing calculated results, it is pointed that obvious transverse thermal difference occurs in transmeridional loess subgrade, and the transverse thermal difference is resulted from the transverse difference of the external factors and the surface material properties. The results reflect the surface thermal difference between the sunny slope and the shady slope of the subgrade. The thermal distribution along the depth of the subgrade in January and July are obtained, it is pointed that the subgrade soil is endothermic in July whereas radiative in January. And the thermal change in the central area of the subgrade is unconspicuous. Furthermore, the thermal difference of the subgrade day and night is studied, it is indicated that the daily changing of temperature in subgrade layer may be ignored when the layer depth is over than 30cm. By simulating unfavorable rainfall, moisture migration in loess subgrade is studied. The slaking scope in subgrade by seepage is gradually enlarged. The area of the slaking scope is relating to loess density, less density of subgrade loess result in bigger slaking scope, more quickly seepage and bigger change of soil water content. It is also pointed that hard shoulder is more benefit than soft shoulder in keeping water from seepage into the loess subgrade. Therefore, building subgrade in bigger soil density or using hard shoulder is advantage to avoid seepage endanger.