The Progress of Study on Heat and Mass Transfer in Freezing Soils
HE Ping, CHENG Guo dong, ZHU Yuan lin (State Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou Gansu 730000, China)
The study on heat and mass transfer in freezing soil has made progress in recent years. Some research results are reviewed in this paper. Based on the tests, the thickness of frozen fringe, temperature of ice segregation, formation conditions of ice lens and frost heave models have been studied. It is found that ice segregation temperature decreases with the frost depth quickly in the initial stage of test, and then increases, finally becomes stable as a result of temperature gradient reduce with increasing the length of frozen section.The generation and growth of ice lens are dependent on freezing rate.Some researchers thought water is partly frozen in frozen fringe owing to different water migration forces and different frozen temperatures in different sites. The relationship between water migration forces and frozen temperature meets the demand of phase change in the fringe.In the tests on the formation of water-methane gas, it is revealed that only a part of free pore water is converted into hydrate during cooling of sediment, the other is converted into ice during freezing. A frost heave model is developed for practical engineering application in cold regions and for scientific purpose to understand the frost phenomena and mass migration mechanisms. A thermodynamic model, taking into account thermal and mass transfer, stress, and deformation of soil skeleton and ice, is put forward. In the framework of continuum mechanics and mixture theory, the influence of void ration on frost heaving is studied after proposing a criterion of formation of layered ice; and the results obtained coincide with experimental data. Considering the complex natural environment, the convection of air, snow cover, rain falling, vegetation, and seepage of surface water were introduced into some models. It is revealed that natural convection of the ballast pore air have a large impact on heat transfer rate during the period when unstable pore-air density gradient exists.This may lead to enhanced frost penetration and frost heave in foundation soils beneath an embankment, and have an adverse impact on the operation of high-speed rail lines during winter.In practical application, some models with simple form and few parameters have been widely used to analyze heat and water field of road embankment, but others is only in a numerical simulation stage due to the difficulty of determining parameters.Therfeore, it is necessary to improve the models in order to reflect the natural and physic properties, to test in fields and to search the methods of measuring the parameters in the models.