Study On Mechanical Properties And Freezing Deformation Characteristics Of Unsaturated Seasonal Frozen Soil In Changchun Region

Unsaturated seasonal frozen soil affected by seasonal temperature is widely distributed in China.In order to study its mechanical properties and freezing deformation characteristics,it is necessary to consider not only the seasonal temperature but also the unsaturated characteristics of soil.More than half of Chinese land area is seasonally frozen ground.Due to the needs of national policies and construction plans,such as the "One Belt One Road" initiative,there will inevitably be a lot of projects such as new construction,reconstruction and expansion.In practical engineering,the soil is in a multi-field coupled environment of water field,temperature field and stress field.Therefore,it is necessary to study the mechanical characteristics and freezing deformation characteristics of unsaturated seasonal frozen soil under the coupled effect of water-heat-mechanics,so as to improve the theoretical research system of unsaturated frozen soil and provide reference and guidance for the design and construction of unsaturated soil engineering in seasonal frozen area.This paper takes the silty clay of Changchun,a typical seasonal frozen area and the node city of the "China-Mongolia-Russia Economic Corridor" in the "One Belt One Road" construction plan,as the research object,while taking into account temperature and soil unsaturated characteristics,in terms of water-heat-mechanics three-field coupled conditions,the mechanical properties and freezing deformation characteristics are systematically studied.Based on macroscopic physical and mechanical tests,the soil water characteristic curve,freezing characteristic curve,thermophysical parameters,strength parameters and freezing deformation characteristics of soil under different matric suction and temperature conditions are obtained;the mechanism of macroscopic properties is explored based on microscopic tests;based on theories of continuum mechanics and thermodynamics,a mathematical model of three-field coupled of water,heat and mechanics considering soil temperature and unsaturated characteristics is derived.The mechanical properties and freezing deformation characteristics of unsaturated seasonal frozen soil are analyzed and simulated.The main work includes the following aspects:(1)Based on the theories of interface mechanics and granular matter mechanics,the physical,chemical and mechanical processes of liquid-solid and liquid-air interface interaction are analyzed,and the essence of the contribution of matric suction to the mechanical properties of unsaturated soil is clarified.Through soil water characteristic curve test and freezing characteristic curve test,the prediction models of matric suction and volumetric water content,temperature and volumetric unfrozen water content were established,and the functional relationship between temperature and matric suction was further established.(2)Based on the relationship between temperature and matric suction,the shear strength prediction formula of unsaturated soil considering temperature is proposed.The mechanical properties of unsaturated silty clay at different temperatures are studied by GDS triaxial tests.It is pointed out that the matric suction,unfrozen water content and ice cohesive force are the main controlling factors of strength and deformation of unsaturated silty clay under the influence of seasonal temperature.Meanwhile,the composition characteristics of total cohesive force and effective internal friction angle are explained.The shear strength prediction formula of unsaturated soil considering temperature is verified by the data obtained from laboratory tests.(3)Through the isotropic isothermal freezing tests,the influence of different negative temperature conditions on the freezing deformation of unsaturated soil is studied,and the variation laws of axial deformation and radial deformation of soil samples with different initial matric suction are discussed.It is pointed out that the degree of frost heaving and shrinkage of unsaturated frozen soil under different negative temperature and initial matric suction conditions is determined by the volume expansion degree of water-ice phase transition in soil pores and the degree of volume reduction under the action of matric suction.When the initial matric suction is low at low temperature,the volume expansion degree of water-ice phase change has a great influence on the deformation of soil,and the soil is prone to frost heaving.When the initial matric suction is high at high temperature,the volume reduction degree under matric suction has a great influence on the deformation of soil,and the soil is prone to freeze shrinkage.(4)GDS unsaturated soil triaxial test system was used to control the matric suction of undisturbed samples,and undisturbed samples with different initial matric suction were prepared.Zeiss lsm700 laser confocal microscope was used to obtain the frozen structural pictures of unsaturated soil with high and low initial matric suction under negative temperature conditions.The morphological characteristics of porous ice in samples under different conditions were clarified.The article explains the related mechanism of the change of pore structure characteristics during the freezing process of unsaturated soils,the growth and development characteristics of ice crystals,and their effects on soil mechanical properties and freezing deformation characteristics.Based on this,the concept of critical frost heaving saturation is proposed.(5)Based on the theories of continuum mechanics and thermodynamics,the coupled mathematical model of water,heat and mechanics is derived,which takes into account the temperature and unsaturated characteristics of soil.The reliability of the model is tested based on the indoor test results.The model can well reflect the mechanical properties and freezing deformation characteristics of unsaturated frozen(thawed)soil under the action of temperature and matric suction.