Study On Hydro-Thermal Characteristics And Thermo-Hydro-Mechanical Coupling Model Of Turfy Soil In Seasonally Frozen Regions

Turfy soil is a special humus soil formed in the swampy wetland environment with sufficient water,gentle topography and rich surface vegetation,which is widely distributed in the Daxinganling,Xiaoxinganling and Changbai Mountains in the northeast of China,the Ruoerge Plateau in western Sichuan,the Sanjiang Plain,most mountainous areas in Xinjiang and the Qinghai-Tibet Plateau,most of these areas are seasonally frozen regions at high latitudes or high altitudes.Studies have shown that turfy soil is a special kind of soil with high water content,high organic matter content,high permeability,high compressibility and poor engineering geological properties.At present,as a kind of special soil widely distributed in seasonally frozen regions,there are few studies on the frozen soil properties of turfy soil.There is a lack of systematic research on the hydro-thermal characteristics,frost heave and thaw settlement properties,microstructure changes,multi-field coupling model and other aspects of turfy soil in seasonally frozen area during freeze-thaw.The design of engineering construction and the prevention and control of frost damage in the distribution area of seasonal frozen turfy soil lack the parameter basis of frozen soil properties.The subgrade in seasonally frozen soil area is often damaged by pavement cracking,subsidence and subgrade frost boiling.Other geotechnical engineering structures also have the frost damage risk of uneven settlement and foundation pit instability.These frost damage problems are severe threats to engineering construction in seasonally frozen areas,causing economic losses and damage to regional infrastructure and road traffic.Carrying out frozen soil research,identifying the fundamental physical properties of frozen soil,providing theoretical and empirical basis for engineering construction in frozen soil distribution areas,is of great importance for engineering construction and disaster prevention in the permanently and seasonally frozen regions.This study is based on the National Natural Science Foundation of China（NSFC）project"Research on the environmental effects of line engineering on the wetland of turfy soil in the seasonally frozen regions"and"Research on the freezing and swelling mechanism of turfy soil in the seasonally frozen regions".Taking the turfy soil in the seasonally frozen regions as the research object,field investigation and stratified sampling were carried out.The experimental methodology and research methods for this study were designed with regard to the characteristics of turfy soil and the meteorological data of the study area.The main studies include the following:Firstly,by means of physical and chemical tests,nuclear magnetic resonance（NMR）technology and steady-state comparison method,the fundamental physical properties of turfy soil,the unfrozen water content of soil during freeze-thaw and the thermal conductivity of soil at different temperatures were obtained.The hydrothermal characteristics of turfy soil during freeze-thaw and its correlation with the fundamental properties were explored,and the parameterization effect of hydrothermal characteristics of freezing and thawing turfy soil was optimized.Secondly,the frost heave force,frost heave displacement and thaw settlement of turfy soil were obtained by unidirectional indoor freezing and thawing test.The development law of frost heave and thaw settlement properties was analyzed.Comparison of microstructural changes in grass-tar soils before and after freeze-thaw based on CT scanning techniques and 3D reconstruction.Finally,the hydro-thermo-mechanical coupling numerical model of turfy soil was established,and the accuracy of was verified by simulating frost heave and thaw settlement tests.The main findings of this study are as follows:（1）The meteorological,topography,hydrogeology and other natural conditions of the study area were investigated,and the favorable conditions for the formation of turfy soil were analyzed.The moisture content,density,organic matter content,decomposition degree,particle density,particle size distribution and other fundamental physical and chemical properties of undisturbed turfy soil in different layers were determined.From the perspective of soil organic matter accumulation,decomposition and migration,combined with the causes of turfy soil,the specific properties,difference law of turfy soil in different depths were analyzed.（2）The unfrozen water content of turfy soil during freeze-thaw process was determined by temperature-controlled nuclear magnetic resonance（NMR）test equipment,and the soil pore water was classified based on the threshold setting of transverse relaxation time T₂.The soil freezing characteristic curves（SFCCs）were drawn,and the variation characteristics of soil unfrozen water content were discussed.The migration law of soil pore water before and after freeze-thaw and the hysteresis effect of unfrozen water content were analyzed.A parametric model of unfrozen water content considering the organic matter content and decomposition degree of turfy soil was established.（3）The evolution law of thermal conductivity of turfy soil at different temperatures was revealed by steady-state comparison method,and the correlation between fundamental physical properties and thermal conductivity was analyzed by combining the material composition of turfy soil.The applicability of various empirical models for calculating soil thermal conductivity to frozen and unfrozen turfy soil was compared,and the empirical model for calculating the thermal conductivity of turfy soil was optimized.（4）Through laboratory tests,combined with the hydrothermal characteristics of turfy soil during freeze-thaw process,the evolution law of frost heave force,frost heave displacement and thaw settlement of turfy soil was analyzed.According to the frozen soil physics and national standards,the frost heave and thaw settlement properties of turfy soil in the study area were classified.（5）By using CT tomography technology,the CT tomography image sequence of turfy soil before and after freeze-thaw was obtained and the three-dimensional reconstruction of turfy soil was carried out,which intuitively showed and described the microstructure of turfy soil.The microstructure and pore characteristics of turfy soil before and after freeze-thaw were analyzed qualitatively and quantitatively.（6）Based on the Fourier heat transfer law and Richard equation,the hydro-thermal-mechanical coupling numerical model of turfy soil was established by taking the unfrozen water-temperature parametric model of turfy soil as the hydrothermal coupling factor.The unfrozen water content and thermal conductivity of turfy soil were introduced as hydrothermal parameters.The test of frost heave and thaw settlement of turfy soil was simulated to verify the accuracy of the model.The simulation results can reveal the evolution of the temperature,moisture and stress-strain fields of the soil during freeze-thaw.The conclusions and results of this study can provide parameters and theoretical basis for scientific research and engineering construction in the distribution area of seasonally frozen turfy soil,and provide experience and reference for the study of frozen soil properties,hydrothermal characteristics,microstructure and multi-field coupling of special soil with high organic matter content in cold regions.