Study On Thermal Parameters And Temperature Field Evolution Of Clay During Freezing

The frozen soil is a composite material composed of four phases,namely solid,liquid,gas and ice.The ice phase in frozen soil has a decisive influence on its physical,mechanical and engineering properties.Therefore,the study on the existent form and formation of ice phase is the foundation and condition for revealing the complex properties of frozen soil.But understanding the thermal parameters and thermal conductivity of frozen soil is essential to reveal the formation law of frozen soil.In this paper,the heat constitution in the freezing process was studied based on the material composition of frozen soil,the new thermal physical parameters model of frozen soil is established,a test device model is developed,and a model test is carried out;the new thermal physical parameters model of frozen soil is verified by the analysis of model test and numerical simulation.It provides the design theoretical basis for the engineering construction in cold regions and freezing method construction.The results obtained included the following aspects.(1)The heat constitution was revealed in the freezing process.The unfrozen soil was a composite of particles,pore water and pore gas.Then the freezing process was essentially the combination of four physical processes,namely,the freezing process of pore water,the cooling process of pore water,the cooling process of pore ice,the cooling process of soil particles.Therefore,the heat consumption in the freezing process was mainly used for the temperature change of particles,the temperature change of water(ice)and the phase change between water and ice.(2)The evolution characteristics of specific heat and thermal conductivity of soil in the freezing process were revealed.Because the content of water and ice in the freezing stage was constantly changing,and the specific heat and thermal conductivity of water was completely different from that of ice.So the specific heat and thermal conductivity of soil were dynamic.Therefore,the study on the thermal conductivity and specific heat of soil in the freezing stage must be related to the phase transformation of pore water and the corresponding latent heat.(3)Based on the existence of phase change between water and ice or latent heat,a dividing method of freezing process was given.With the decrease of temperature,the stage was called the unfrozen phase that water in the soil has been in liquid form.The stage was called the freezing phase that solid ice existed and liquid water gradually reduced.The stage was called the frozen phase that solid ice existed but the liquid water no longer reduced.(4)The specific heat calculation models of three stages in the freezing process were established.Based on the specific heat theoretical calculation of composite materials,the specific heat calculation models of soil were established in the unfrozen and frozen phases.Considering the physical meaning of the nature of phase change and the latent heat,the specific heat calculation model of the freezing phase was established.Thus,a complete specific heat calculation model of the unfrozen phase,the freezing phase and the frozen phase was given.(5)The thermal conductivity calculation model of three stages in the freezing process was established.Based on the experimental phenomena,the relationship between thermal conductivity and parameters such as density,moisture content,and temperature was studied.Based on the ideal model,a thermal conductivity theoretical model with temperature also considering the phase transition was established.Finally,the thermal conductivity calculation model of soil was established in the unfrozen phase,the freezing phase and the frozen phase.(6)The freezing model test system was developed,also a three-dimensional stress state testing device and a three-dimensional strain testing device were developed,and the model test was carried out successfully.According to similarity criteria of freezing model test,a freezing model test system was developed with freezing function,temperature testing function,loading and testing function,hydrating function,three-dimensional stress and three-dimensional strain testing function.And the corresponding single pipe freezing and double pipe freezing model test were carried out.(7)A method to judge the freezing process based on stress and strain test data is presented.Through the variation monitor of temperature,three-dimensional strain and three-dimensional stress in the soil freezing process,the starting and stopping time and location of freezing are determined according to the sudden change point of the three dimensional strain and three-dimensional stress in the soil model.(8)A finite element method considering latent heat was established.Based on the established thermal parameter model and the sensible heat method considering latent heat,the changing temperature field process of frozen clay model test was simulated.(9)Theoretical model validation.The temperature field was simulated by whether considering the latent heat of phase change,and the reliability of the theoretical model is verified by the comparison between the numerical simulation results and the measured results.