Study On The Determination And Application Of The Nonlinear Thermal Parameters For Frozen Soils

The stability prediction of permafrost,the freeze-thaw disaster of seasonal frozen soil,and the balance between the strength and excavation efficiency of artificial frozen soil,are important aspects affecting the construction efficiency of the cold region and underground space engineering.The stability,freeze-thaw deformation,and strength of frozen soils depend on their temperature fields.Due to the nonlinear freezing behaviors of water in the soil,the thermal parameters such as thermal conductivity,specific heat,and latent heat of the frozen soil change nonlinearly with temperatures.Effected by the soil ice melted by the heat of the heating-temperature measurement method,the conventional test methods of specific heat and thermal conductivity can not be directly used for frozen soils.Meanwhile,it is very difficult to input the nonlinear thermal parameters into the calculation programs and predict the accuracy of the frozen soil temperatures.Hence,it is of great engineering value to study the test methods of thermal parameters in a frozen soil and the applicability of nonlinear thermal parameters in the prediction of temperature field,to improve the prediction accuracy of temperature field in the cryosphere engineering,and to serve the prediction of strength,deformation,and stability in a frozen soil for engineering construction and resource exploitation.In this dissertation,the influences of phase change on the test process of thermal parameters in a frozen soil are considered,the determination methods of specific heat and thermal conductivity for frozen soils,and the thermal conductivity of soil solid materials are proposed based on the multi-component calorimetry method,the inverse method,etc.On this basis,a precise method is proposed to transform the nonlinear phase change latent heat to specific heat,the sensitivities of thermal parameters and boundary conditions on the formation of temperature field are analyzed by comparing the test temperature with that calculated from the nonlinear thermal parameters.Furthermore,considering the dual effects of nonlinear thermal parameters and boundary conditions,the similarity criterion of the soil freezing process is deduced,and a quantitative prediction method of frozen soil temperature field for the trans-dimension analysis is proposed.The main works are as follows:1)The initial freezing temperature,electrical conductivity,and nonlinear variation of unfrozen water content with temperatures are studied,and the influence of heating power in the transient method on the test results of frozen soil thermal conductivity is analyzed.The variation law of physical parameters with the phase composition in the frozen soil is clarified from the point of the material model.Based on the calorimetry theory and recursive computation method,calorimetiy of a multi-component system for the analysis of specific heat test in a frozen soil is proposed and the unfrozen water content,apparent specific heat,and specific heat of the frozen soil are determined systematically.The results show that,there is a certain relationship between the physical parameters and the phase compositions of frozen soil,and it is feasible to retrieve the compositions of frozen soil based on the physical parameters.The specific heat of frozen soil considering the phase change latent heat can reflect the physical compositions of the soil,which is more consistent with the definition of specific heat in the heat equation,and the retrieved value of unfrozen water content can also correspond to the NMR test results.2)Based on the finite element analysis and the statistical analysis of the thermal behaviors of the composite element,the relationship between the effective thermal conductivity of the composite and the thermal conductivity of each element in the composite is obtained,and the determination method for the thermal conductivity of the soil solid material is realized.The relative error and absolute error of the inversion method of the statistical model,the inversion method of Johansen,the inversion method of Maxwell-Eucken are verified by the laboratory test of soils and model soils,respectively.The results show that,on the basis of Johansen method,when the range of thermal conductivity of soil solid material can be roughly determined,its thermal conductivity can be inversed by testing the soil solid material filled with the fluid with a similar thermal conductivity with the soil solid material.The inversion method of the statistical model proposed herein is suitable for the prediction of thermal conductivity of solid material when the volume ratio of soil to water is 1:1.3)Considering the nonlinear change of latent heat with temperatures,a precise method is proposed to transform the phase change latent heat to specific heat,and a segment processing method of the frozen soil latent heat is also proposed,and all the nonlinear thermal parameters can be applied to the calculation of temperature field in the general finite element software.On this basis,the sensitivities of 13 thermal factors on the formation of temperature field in frozen soil are quantitatively analyzed using the comparison method of orthogonal simulation and physical model test,and the evolution laws of the sensitivity for the thermal parameters and boundary conditions with the freezing process are classified.The results show that,the precise method reduces the errors of the average equivalent method and interval equivalent method,improves the accuracy and reduces the dispersion of temperature at the measuring point.The influence of boundary conditions and thermal parameters on the temperature field can be ranked as follows:the first kind of boundary conditions>the third kind of boundary conditions>frozen soil thermal parameters(latent heat>thermal conductivity>specific heat).The stability of the cold source should be stable in the construction of the artificial freezing method,the uncertain factors of the external heat exchange should be quantified,the initial temperature and the thermal parameters of soil should be determined accurately,to ensure the construction stability and the prediction accuracy of the temperature field.4)To realize the quantitative reflection of physical model test on trans-dimension engineering practice,considering the dual effects of nonlinear thermal parameters and boundary conditions,the similarity criterion of soil freezing process is deduced based on similarity transformation method,the similarity relationship between the physical model test and engineering practice is analyzed by similarity transformation method,and a trans-dimension analysis method for the model test is constructed by combining the numerical simulation and physical model test,which transforms the macro prediction of "law to law"into the quantitative prediction of "value to value" for the prediction of frozen soil temperature field.The results show that,the similarity criterion considering the influence of nonlinear thermal parameters and thermal boundary conditions is more suitable for engineering practice,and the trans-dimension prediction of frozen soil temperature field in the thermal-opening system should be implemented by the model soils.