Research On Coupling Of Multifield In Frozen Soil Based On Hybrid Mixture Theory

China is one of the major countries with large area of cold regions,where the area of permafrost and seasonally cold regions account approximately 75% of the total land area.Frost heave is the trickiest problem of all in cold regions,and accidents caused by frost heave are also common.At present,the description of frost heave mechanism is mainly based on the multi-physical field coupling theory under the framework of continuum media mechanics.As a research hotspot and academic frontier in recent years,this method has attracted the attention of scholars and governments in various countries.However,due to the limitation of computing capacity and poor understanding of the mechanisms of frost heave of soil,there are still a lot of problems perplexing scholars and engineers.Considering the existing problems,it is thus necessary to develop a more general and comprehensive multi-field coupling model for unsaturated frozen soil,which is a new development and new field of unsaturated frozen soil,and will promote the application of multi-field coupling theory in practical projects in cold regions.Therefore,the aim of this article is for engineering constructions and protection of the practical problems in cold regions,the traditional multi-field coupling method is improved and established by integrating the background of mathematics,mechanics and actual physics,which can provide the corresponding theoretical basis for engineering construction and protection in cold regions.Through basic research and experimental verification,the research results are listed as follows:(1)Based on the framework of continuum mechanics theory,the conservation equation of freezing porous multiphase media under macroscopic state was derived by combining the mixture theory and the average method.The constitutive equation of freezing porous multiphase media at macroscopic state was also derived on the basis of extended entropy inequality,equilibrium state limitation,thermodynamic law and near-equilibrium state conditions.By combining the conservation equation and the constitutive equation,a multi-field coupling framework of porous multiphase media based on the hybrid mixture theory was established.(2)The four-field coupling model of water,heat,air and stress in unsaturated frozen soil was established by simplifying the theoretical framework.The proposed coupled model is different from the traditional coupled model in that it takes into account dry air transfer,water vapor migration,and pressure-induced liquid/vapor fluxes,ice separation condensation criteria,ice pressure and elastoplastic damage constitutive equations.Moreover,the coupled equations were discretized by the weighted residual method,and the finite element method discrete matrix was obtained by the triangular element function.(3)Through triaxial tests of frozen soil,the variation in mechanical parameters such as shear strength,elastic modulus,cohesion and internal friction angle of soil under different test conditions were studied.The elastoplastic damage constitutive equation was derived and the parameters of the mechanical constitutive equation in the coupling model were fitted.Besides,the soil freezing characteristic curves under different test conditions were studied by pulse nuclear magnetic resonance(P-NMR)tests.Based on the capillary theory and Gibbs-Thomson equation,the soil freezing characteristic curve with physical significance was deduced,and the soil thawing characteristic curve was inverted.(4)The validity of the proposed model was verified by comparing with the uni-directional freezing test of silty clay in open and closed system s,Mizoguchi sand soil test,two kinds of "pot cover effect" tests,and numerical simulation of saturated frozen soil.At the same time,the variation in temperature,hydraulic,stress and displacement fields in the freezing process were studied.(5)The hydrothermal coupling model of saturated frozen soil considering seepage was established by simplifying the theoretical framework,which was used to predict the formation of artificial freezing curtain during freezing in the presence of water seepage.The validity of the coupled model was verified by the model test conduted by Pimentel et al.In addition,the coupling model was applied to the freezing reinforcement project of the end well of Harbin Metro Line 2 and the freezing construction of the cross passage of Harbin Metro Line 3,and the time of the freezing curtain formation is calculated.And these results can provide some references and theoretical basis for the construction of the artificial ground freezing method.