| Frost heave thawing is one of the most typical diseases of permafrost roadbed,which poses a huge threat to the safe construction and operation of traffic infrastructure in permafrost areas.The root cause of frost heave and thaw subsidence in frozen soil subgrade is the result of the evolution of its temperature field,moisture field and deformation field under the coupled action of heat,water and force.In winter,when the temperature drops,the pore water in the unfrozen soil freezes,which increases the volume of the subgrade soil,causing frost heave deformation such as frost cracking and distortion;in summer,when the temperature rises,the pore ice in the frozen soil melts,reducing the strength of the subgrade.,with the melting and infiltration,the pore water pressure of the soil decreases,and the subgrade produces uneven settlement,depression and other melting deformation due to drainage consolidation.Frost heave and thaw subsidence deformation have great inhomogeneity,which seriously affects the stability of frozen soil subgrade.Therefore,it is of great significance to carry out research on the occurrence mechanism of frost heave and thaw settlement of frozen soil subgrade and related theoretical models.In this paper,theoretical derivation and numerical simulation methods are used to study the frost-heave-thaw-settlement characteristics of unsaturated permafrost under the coupled action of heat,water and force,and to provide theoretical support for the prevention and control of permafrost roadbed diseases.The main research results are as follows:.(1)A thermo-elasto-plastic constitutive model of unsaturated frozen soil is established which can describe frost heave and thaw subsidence at the same time.Based on the critical state theory,the concept of pore ice content is introduced into the modified Cambridge model,the relationship between the tensile yield limit and the compressive yield limit of unsaturated permafrost with temperature is given,and a thermoelasticity of unsaturated permafrost is deduced and established.Plastic constitutive model,which can effectively describe the stress-strain relationship of unsaturated frozen soil under loading and temperature changes.(2)A thermal-water-mechanical coupling mathematical model for unsaturated frozen soil is established.Based on the established thermo-elasto-plastic constitutive model of unsaturated frozen soil,considering the ice-water phase transition and water migration of unsaturated frozen soil,by introducing the static equilibrium equation,water migration equation and heat conduction equation of frozen soil mixture,a mathematical model of thermal-water-mechanical coupling of unsaturated frozen soil with displacement,volumetric water content,volumetric ice content and temperature as variables is derived and established.(3)The numerical calculation function of the established unsaturated permafrost thermal-water-mechanical coupling mathematical model is realized.Using the solid heat transfer module,the PDEs interface custom coefficient partial differential equation module and the solid mechanics module in the COMSOL numerical simulation software,the calculation function of the unsaturated frozen soil heat-water-mechanical coupling mathematical model is realized.Based on the indoor one-dimensional melting and consolidation test of Yao et al.,the melting and consolidation process of the sample under different loads and different moisture contents was simulated.Rationality of forcecoupled mathematical model for simulating frost-heave-thaw-settlement deformation of unsaturated frozen soil.(4)Research on the application of unsaturated permafrost thermal-water-mechanical coupling mathematical model in permafrost subgrade.Based on the established thermalwater-mechanical coupling mathematical model of unsaturated frozen soil,numerical simulation analysis of typical frozen soil subgrade was carried out.The distribution of temperature field,moisture field and deformation field of frozen soil subgrade in a freezing and thawing cycle,and the change law of temperature field and deformation field at typical time of frozen soil subgrade under long-term action are studied.(5)A study on the thawing and settlement reliability of frozen soil subgrade was carried out.Based on the thermal-water-mechanical coupling numerical simulation analysis results of typical frozen soil subgrade,the response surface method is used to take the maximum depth of frozen soil of subgrade and the average temperature above the upper limit of frozen soil of subgrade as random variables.As the response value,the approximate functional function of the subgrade melting reliability analysis is solved;according to the design check point method,the MATLAB software was used to write a program,and the evolution law of the reliability index and failure probability of the frozen soil subgrade within 30 years after construction was calculated by the method of iterative calculation. |
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