| Frozen soil is a geological phenomenon formed by the exchange of mass and energy between the earth and the atmosphere.The global frozen soil area accounts for about 70% of the land area,and the frozen soil area of our country accounts for about 75% of the land area,which ranks the third largest in the world.In recent years,an increasing number of large-scale projects in China will be launched in frozen soil areas,such as QinghaiTibet Railway,Sichuan-Tibet Railway,Sino-Russian oil pipeline and so on.The frozen soil is very sensitive to temperature,the periodic change of temperature makes the surface soil in the state of alternating frost heave and thaw,which can cause geological disasters such as frost heave,thaw settlement and frost-thaw boiling of roadbed soil,thus bringing great technical difficulties to the construction and maintenance of engineering projects.Soil freezing process is a coupling process accompanied by many physical and chemical phenomena,among which the most important is the process of water-heat transfer and phase transformation,and soil frost heave is the coupling result of soil water-heat transfer and phase transformation.Therefore,the establishment of a numerical model of pore water migration law in soil freezing process is helpful to effectively predict the geological hazards caused by soil freezing.The experimental study of pore water migration law in soil freezing process is helpful to understand the influencing factors of pore water migration and to reduce the construction difficulties of engineering projects in frozen soil areas.Firstly,based on lattice Boltzmann method,SC multiphase flow model and enthalpybased phase change model were introduced into the numerical method of porous media.The ranges of cohesive and adsorptive force parameters suitable for the numerical model of pore water transport in frozen soil were determined,and the validity of enthalpy-based model in describing numerical simulation of phase change in porous media was confirmed.The particle shape characteristics of real soil were observed,and the distribution pattern of soil particle size was analyzed.A pore-scale stochastic spherical particle soil structure model describing the characteristics of soil particle size distribution was established;a pore-scale stochastic growth soil structure model describing the diversity of soil particle shape and the roughness of soil particle surface was established;and a macro-scale stochastic volume fractional soil structure model describing the characteristics of soil porosity distribution was established.The permeability of porescale soil structure under different particle size distribution,porosity,specific surface area and particle shape was numerically simulated.The relative errors between numerical simulation results and theoretical values for stochastic spherical particle soil structure of different particle size distributions are within ± 25%,for stochastic growth soil structure of different porosity and specific surface area are within ± 15%.For the same porosity and specific surface area,the permeabilities and pore curvatures of stochastic growth soil structure are higher than those of stochastic spherical particle soil structure at low porosity(0.25 ? 0.45 m3/m).Secondly,based on SC multiphase flow model and pore scale stochastic growth soil structure,a multiphase model on pore water distribution in soils was established,and the water-air distribution in unsaturated soils with different porosity,hydrophobicity and saturation were simulated.The decrease of soil porosity will lead to the formation of dispersed small bubbles;when the surface of soil particles changes from hydrophilicity to hydrophobicity,the bubbles will gradually migrate from the middle to one side of pore;the increase of soil saturation promotes the formation of small bubbles in pore channels.Based on the pore-scale stochastic spherical particle soil structure model,a geometric calculation method on pore water phase change temperature distribution was established,and the content of unfrozen pore water during soil freezing at different temperatures was predicted,the correlation coefficients between the simulated and experimental results are as high as 0.97(sandy soil)and 0.97(fine soil),respectively;A model for the permeability of frozen soil was developed,and the permeability of frozen soil at different temperatures were predicted,the numerical results were compared with the corresponding experimental results,the correlation coefficients between the simulated and experimental values are as high as 0.96(sandy soil)and 0.95(fine soil),respectively.Finally,combined with the model of frozen soil infiltration in pore-scale,a pore water migration model during soil freezing was established by introducing frozen suction driving force in frozen frontier.The pore water distributions in soil freezing process at different freezing times and the changes of soil pore water at different temperatures were simulated.For freezing time of 12 h,24 h and 50 h,the correlation coefficients between numerical simulation results of soil pore water distribution and experimental results are as high as 0.92,0.83 and 0.92,respectively.For soil with water saturation of 0.33,the correlation coefficients of soil water content between the numerical results and the experimental results at different temperatures were as high as 0.97(layer 1)and 0.95(layer 2),respectively.For soil with saturation of 0.16,the correlation coefficients of water content between the numerical results and the experimental results in layer 1 and 2 were both 0.97,which indicates the accuracy of the established model in predicting the water content distribution and variation during soil freezing process.In addition,the pore water migration in foot-scale soil freeze-thaw and the frost heave of small-scale soil freezing were experimentally studied.In foot-scale soil freezing,it was found that the water content in the unfrozen area increased gradually when the temperature decreased from high temperature to the phase change temperature,and drops abruptly near the phase change point.In foot-scale soil thawing,pore water in frozen soil decreased with the increase of soil temperature from low temperature to phase change point.Based on the dynamic equilibrium hypothesis between the soil pore water migration and the soil pore water evaporation,combined with SWRC and initial pore water potential fitting formula,the variations of pore water content with temperature in layer 1 and 2 in unfrozen soil freezing-thawing process were calculated,respectively.The correlation coefficients between calculated and experimental values are as high as 0.95 and 0.98 in freezing process,and 0.94 and 0.99 in melting process,respectively.Based on the existing formulas for calculating the frost heaving rate,the fitting formula of the direct suction distance DzTf in the unfrozen region due to the formation of ice len in the rapid frost heaving stage was introduced.Combining with the numerical simulation results of pore water permeability in the unfrozen region and the thickness of water film in the frozen region,the frost heave for soil samples with the average particle size of 6.25 ?m and 1.25 ?m was predicted.The error between the predicted and experimental values was within ± 15%,and the correlation coefficient was 0.99. |
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