On Mechanism And Consequences Of Water-vapor Migration And Phase Transition In Unsaturated Frozen Soils

Frozen soil is widely distributed in the world with characteristics of frost heave and thawing settlement,which brings great challenges to the construction and operation of infrastructures.The core issue of such characteristics induced disaster is the mechanism of water-vapor migration and phase transition.At present,more attention has been paid to water migration and water-ice phase transition in the literature,while studies on water-vapor migration and phase transition are still vary limited.This thesis is completed based on Key Projects of High speed Railway Joint Fund of NSFC(No.U1834206),Hunan High level Talent Gathering Project-Innovation Team(No.2019RS1008),National Natural Science Foundation of China(No.51878665),aiming at the problem of frost-induced damage of high-speed railway and airport engineering in cold regions of China.Specifically,this thesis focuses on water-vapor migration,phase transition and their disaster-causing mechanism in unsaturated frozen soils by using theoretical analysis,laboratory experiments and numerical simulation methods.The main conclusions are as follows:A series of experiments of water-vapor migration with liquid and gaseous water supply on different types of unsaturated soils were conducted using the self-developed one-dimensional unsaturated soil freezing instrument.The experimental results have reproduced the phenomenon of water-vapor migration,enrichment and phase transformation in unsaturated frozen soil.The influence of soil type,temperature gradient and initial water content on the evolution of water content along the soil column height during frost heaving is analyzed.This study verifies the rationality of the hypothesis that the water content of the sample increases due to the gaseous water supply,which leads to frost heave,and also undermines the traditional understanding that sand is not a frost heave sensitive material.The water-ice phase transition problem in unsaturated frozen soil is also tightly connected to the water-vapor migration,which is usually defined as soil freezing characteristic curve(SFCC).Based on the nuclear magnetic resonance(NMR)technology,two types of soil,i.e.,quartz sand and silt,are tested to analyze and determine the influence of soil properties and degree of saturation on SFCC.The freeze-thaw hysteresis SFCC model considering the free energy barrier and ink bottle effect is established with emphasis on the effect of pore structure shape.The proposed model is simple in form,has clear physical meaning,and validated with the experimental results,which demonstrates its satisfactory performance with the correlation coefficients being larger than 0.86.In order to better understand the mechanism of vapor migration and phase transition in unsaturated soil,a vapor-heat coupling model considersing vapor-ice formation is developed,which breaks the local equilibrium assumption of vapor-water-ice phase transition.The vapor-ice phase transition is directly described by the adsorption-desorption equilibrium relationship.The numerical instability of the water-heat coupling equation is solved by the finite element method.Finally,the proposed model is verified using the experimental results,and the influence of soil parameters is also discussed.Based on the premelting theory,the relationship between micro-forces and macro-forces is established with a newly proposed criterion for the initiation of ice lens.The mechanism of frozen soil ice formation and the law of frost heaving are revealed,and the effects of overlying pressure,surface temperature and inherent permeability on the formation and distribution of ice lens are analyzed.The results show that for the cohesionless porous media,the maximum overburden pressure is sdopted to judge whether a new ice lens will appear somewhere inside the freezing edge.On this basis,the boundary conditions for judging single ice lens and periodic ice lens are further proposed,and the relationship between ice formation,soil properties and environmental parameters is developed.A hydro-thermal-vapor-mechanical coupling model considering water-vapor-ice formation is finally developed based on the experimental data and previously proposed hydro-mechanical coupling equation.The model is validated and the frost heave phenomenon is simulated by using COMSOL Multiphysics platform.The results show that the model can simultaneously simulate the frost heave phenomenon dominated by vapor migration when the initial degree of saturation is low and the frost heave phenomenon dominated by liquid water migration when the initial water content is high.The model is also used to predict the frost heave phenomenon and analyze the freezing characteristics of the field subgrade in Maduo County,which fits well with the filed measured data.This study improves our understanding of frost damage happened in most coarse-grain filler subgrades.