Hydraulic Conductivity Of Frozen Soil And Separate Ice Frost Heave Model For Coupled Hydro-Thermal-Salt Transfer

Frost heave induced by separation ice is a hotspot and difficult point in frozen soil engineering.At present,the existing theoretical models and experimental studies of frost heaving mainly focus on freezing non-saline soils,but saline soils are often encountered in actual frozen soil engineering,which is significantly different from the frost heaving mechanism of freezing non-saline soils.Meanwhile,most theoretical models cannot explain and predict the appearance and growth of segregated ice in freezing saline soils.Therefore,the mechanism of frost heaving for saline soils is of great significance and scientific value to guide in saline soil engineering with one-dimensional freezing.In this paper,based on the one-dimensional freezing problem of saline soil engineering in permafrost or seasonally frozen soil area,the physicochemical characteristics,water and salt distribution,evolution of ice lens and deformation mechanism of saline frozen soils are systematically studied by means of theoretical analysis,numerical calculation and laboratory test.The theoretical model for hydraulic conductivity in frozen soils under the straight capillary with a clear physical meaning was established.Meanwhile,considering the changes in the pore structure of the freezing process,the hydraulic conductivity model is further modified.Then,the evolution model of a single ice lens with salt is established,and separated ice frost heave model for coupled hydro-thermal-salt transfer is established.The above-mentioned studies jointly revealed the mechanism of frost-salt heaving,crystalline salt distribution,ice lens growth and distribution under one-dimensional freezing.Finally,a new method for dynamically measuring salt expansion during freezing is studied.The main research results are as follows:(1)Considering the adsorption of soil particles on the unfrozen water film and the effect of salt on pore water,the equivalent water pressure is optimized based on the thermodynamic theory of water film at ice-water interface.Meanwhile,the hydraulic conductivity model of freezing non-saline/saline soils is obtained based on the straight capillary theory,and compared with the experimental data and empirical methods to prove that this model is reasonable and reliable.Moreover,considering that the macroscopic deformation of the soil is the result of changes in the pore shape during the freezing process,the pore radius correction factor is introduced into acorrection factors for partial correlation and the eccentricity of the flow path,which further improves hydraulic conductivity model of frozen soil under the consideration of deformation.Finally,the determination method of m is given,and the optimal value of m should be 3.8(2)Considering the effects of concentration gradient,temperature gradient and pressure gradient in frozen fringe,the driving force of water migration is studied,and the growth mechanism of ice lens is revealed in freezing saline soil.The results show that the growth thickness of the ice lens is proportional to the absolute value of the temperature gradient,while the pressure gradient and the concentration gradient inhibit the growth of the lens body.Three gradients essentially affect the equivalent water pressure gradient.The effect of concentration gradient on lens growth is higher than the other two factors.Therefore,the concentration gradient and pressure gradient in the evolution of lens need to be considered.(3)The water film pressure on the surface of the soil particles with the influence of salinity is derived,and the separation criterion for ice separation in frozen soils is developed.Considering the salt crystallization and the distribution of solute in ice-water two phases,combined with the solute kinetics equation of solute,a solute migration equation for freezing soil was established.Based on this,coupled with the water and heat equations,active zone consolidation,separation criteria,and freezing characteristic curves,separate ice frost heave model for coupled hydro-thermal-salt transfer was developed.This model is comprehensively considered by in-situ frost heave,segregational heave,salt expand and soil skeleton deformation,and can well predict the development of ice lens and crystalline salt.If the salt is not considered,the model degenerates into the traditional separate ice frost heave model for coupled hydro-thermal.(4)For soils with low salt content,crystalline salts tend to appear near the cold end of the warmest ice lens during freezing.For soils with high salt content,salt crystallization occurs during the whole freezing process,and the salt crystallization occurs in the position near the cold end of the warmest ice lens and the last frozen fringe.The appearance and distribution of crystalline salts are the result of the influence of the effective solute distribution coefficient on the concentration at the ice-water interface.The micro-layered distribution characteristics of ice lens in high-salinity soils are related to water film pressure and equivalent water pressure.(5)Based on the strong dependence of soil resistance and pore concentration,the mathematical relationship between the resistivity of freezing saline soil and the unfrozen water content,temperature,and pore solution concentration is derived theoretically,and then the amount of salt expansion and deformation is calculated.This method provides a new idea for the measurement of salt expansion in freezing soil,and also theoretical support for studying the evolution mechanism of frost heaving and salt expand.