The Soil-water Characteristic Curve Model Based On Thermodynamics And Soil Microstructure Evolution

The soil water characteristic curve is the relationship between degree of saturation(or the volumetric water content)and suction in an unsaturated soil.Many research fields,such as the strength theory,seepage theory,consolidation theory and constitutive theory,use SWCC to reflect the influence of water content on the mechanical and hydraulic behavior of soil.Therefore the soil water characteristic curve is a key factor in determining the behaviors of unsaturated soils.The main achievements of this thesis are listed as follows:(1)The evolution laws of microstructure under the hydraulic path are first summarized based on experimental results.Subsequently,a novel model is developed by relating the pore-size distribution to the void ratio.Based on these findings,it is assumed that the pore size distribution in a deformed state can be transformed from the initial PSD through three steps:translation,scaling and dispersion.The quantities of displacement and scaling are linear with void ratio,and the dispersion is exponential decay with void ratio.Finally,the model predictions are compared with experimental measurements in the literature,showing that the new model is capable of describing the microstructure evolution of unsaturated soil during drying process.(2)Soil fabrics is great importance in geotechnical,geoenvironmental and agricultural engineering as it influences many soil characteristics,such as compressibility,hydraulic conductivity or the soil-water characteristic curve.Several models have been suggested to link a soil's pore-size distribution of a soil to its retention properties.However,most models ignored the effects of particles of different sizes,shapes,separation distances and adsorbed water.Based on pore-size distributions,a SWCC model incorporating porosity,particle geometry and adsorbed water was established to predict retention properties.The proposed model is finally validated against a number of laboratory tests,showing good agreements between the model predictions and the experimental measurements.(3)A thermodynamically consistent SWCC framework is proposed for modeling the hysteresis of capillarity and the effects of deformation in unsaturated soil.It is derived from the second law of thermodynamics:capillary hysteresis and plastic deformation is an intrinsic dissipation;the suction is not only related to the changes of fluid phases,but also controlled by the deformation of solid phase;the changes of solid phase and fluid phases are coupling.Capillary hysteresis and the effects of solid deformation are demonstrated at microscopic scale.By virtue of the notion of the bounding surface plasticity,a model of SWCC is developed,which adds only two parameters.Finally,the model predictions are compared with experimental measurements found in the literature,showing that the new model is capable of describing the capillary hysteretic phenomena and the effects of soil deformation in a variety of unsaturated soils.(4)Based on the model derived in the chapter 5 which can incoprate the capillary hysteresis and soil deformation,the constitutive framework for unsaturated soils can be established using the thermodynamical method.A constitutive model coupled stress-strain and hydraulic behaviors of unsaturated soils is established.The constitutive equations is then used to predict the exiting experiment data,and results shows the model can agree well with test points in both low degree of saturation and high degree of saturation situation.