| Seasonal frozen soil covers a wide area in China,whose periodic freezing and thawing is the main factor to induce the water migration in subgrade.The redistribution of water will directly change the unsaturated state of subgrade soil,thus reducing the strength of subgrade,and even cause frost heave and thaw settlement of subgrade,which will affect the service performance and service life of the road.So it is a hot engineering problem that domestic and foreign scholars pay close attention to.In this paper,the common silty clay in the northeastern seasonal frozen soil area is taken as the research object,and the freezing and thawing cycle is studied emphatically.Model test in laboratory,and the theoretical and numerical analysis are used to study the freeze-thaw cycle of subgrade.A model test system for the freeze-thaw cycle of unsaturated subgrade soil was developed,and the water migration test of silty clay under freeze-thaw cycles was carried out.A numerical model of water heat coupling for unsaturated subgrade soil was established,and the law of water migration for unsaturated subgrade soil under freeze-thaw cycles was analyzed.It has important theoretical significance and practical value for improving the design system of Subgrade in cold region and improving the service period of Subgrade.The main contents and achievements of this paper are as follows:A model test system for freeze-thaw cycle of unsaturated subgrade soil was developed.In order to simulate the freezing and thawing process of the subgrade in the frozen soil area,considering temperature,humidity and load boundary,a complex system including the circular test cylinder,the temperature control subsystem,the replenishment subsystem,the overburden pressure subsystem,the temperature acquisition subsystem,the humidity acquisition subsystem and thermal insulation subsystem was designed,which can realize real-time acquisition and record of the distribution of the internal temperature and volume water content of the model.At the same time,the test method of unsaturated subgrade model was put forward through several experiments.A model test of repeated freezing and thawing of unsaturated silty clay was carried out.Based on the reliable and stable equipment,the model test of water migration under no water replenishment and replenishment was carried out respectively to simulate the water migration of unsaturated soils under different groundwater levels.The change of water content and temperature with time in the freezing and thawing sta ges of the model was analyzed.The change of water content with temperature in the temperature affected area and the migration of water in the non-temperature affected area were compared.A numerical model of water-heat coupling for unsaturated subgrade soil was established,and the law of water migration under freeze-thaw cycle was analyzed.According to the differential unit of mass and heat conservation theory,the coupled equation of water and heat under the test conditions was developed,and the supplementary equation based on the relationship between unfrozen water and temperature shown by the model test also was developed.With the coefficient type partial differential equation of COMSOL,realizing the numerical solution of the coupled model,and the reliability of the model is verified by the test data;what's more,the impact of water replenishing condition,freezing temperature and cooling gradient on the law of migration of subgrade moisture was analyzed. |
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