Study On Soil-Water Characteristic Curve (SWCC) And Constitutive Model For Unsaturated Natural Soils

The natural soils existing in nature are usually structural and unsaturated. Different from saturated soils, unsaturated soils consist of four phase system:soil parti-cle (solid phase), pore water (liquid phase), pore air (gas phase) and interface between water and air. Variation of the above four phase system is the intrinsic factors of soil structure degradation, while extrinsic factors (e. g. external forces or wetting) could in-fluence the degradation of soil structure by changing intrinsic factors. As a result, mechanical properties of natural soils are much more complex than the remolded soils. It will cause safety hazards or unnecessary economic waste if the experimental results of remolded soils instead of natural soils are adopted to solve practical engineering problems. Therefore, taking the influence of soil structure on the mechanical properties of soils into account in establishing constitutive model has important academic and practical value.By theoretical and laboratory experimental studies, some research results about natu-ral unsaturated soils are obtained as follows:(1) Based on Fredlund SWCC pressiometer, laboratory experimental study of soil-water characteristic curve (SWCC) for natural silty clay from different depths in Pinggu New Town of Beijing is made to show the effect of initial void ratio on the SWCC. The SWCC equation proposed by Brooks and Corey is adopted to fit and ana-lyze the experimental results. It is shown that the void ratio has a great impact on the air entry value sa and the pore distribution indexδof SWCC for the natural silty clay, while less on the residual water contentθr(2) Relationships between parametersζ,υof Assouline SWCC equation and porosity n are proposed, whose validity and effectiveness are verified by existed ex-perimental data. It is shown that this formulation can reflect the variation rules of above parameters with the variation of porosity. Combining the derived formulation and the Assouline SWCC equation, comparisons between the predictive and measured results for seven different soils (including natural silty clay) are made, which show that the prediction results with this method are satisfactory.(3) On the basis of disturbed state concept for saturated soils proposed by Desai, a new expression of coupled disturbing variable for unsaturated soils is defined, in which both of disturbance factors that external force and wetting are considered. Evolution equations of disturbing variables with deformation are proposed, in which two new structural parameters that attenuation indexαand peak facto7βare defined. Based on homogenization theory for composites materials and the BBM model proposed by Alonso (1990), a new structural constitutive model for unsaturated intact loess is estab-lished. Comparing the calculation results of BBM and the new model with the experimental results respectively, it can be seen clearly that the new model can better simulate the influence of soil structure on the main mechanical properties of unsaturated intact loess, and the proposed evolution laws of the disturbing variables that describing the deterioration of soil structure are satisfactory.(4) Based on the above studies and the homogenization theory of composite mate-rials, a new constitutive model for unsaturated natural soils considering the coupling of hydraulic hysteresis and stress-strain behavior is formulated, in which average soil skeleton stressσand suction s are adopted as stress state variables. The solid phase deformation characteristics of the relative intact states part and fully adjusted states part are described by elastic and elasto-plastic models respectively. According to the above study in part (2) and the bounding surface model of Wei, the constitutive model of liq-uid considering the influence of soil deformation on SWCC is formulated. By making comparisons of predictions with experimental data of unsaturated natural expansive soil and by conducting the parametric study of structural parametersα,βandξ, it is demonstrated that the new model captures the essential behaviors of the unsaturated natural soils, especially the strain-softening behaviour.