A Constitutive Model Of Unsaturated Soils With Consideration To Arbitrary Wetting/Drying Histories

Unsaturated soils are widely distributed earth materials, which display complex behaviours in engineering practice. A constitutive theory of unsaturated soils addresses the soil-water characteristics curve (SWCC) and the stress-strain relationship as well as their coupling effects. In this dissertation, a SWCC model is first developed and improved so that it can be used to describe arbitrary wetting/drying histories. An elasto-plastic constitutive model of unsaturated soils is then proposed, which is capable of addressing the coupling of SWCC and stress-strain relationship. This research is summarized as follows:(1) Based on an internal variable-based theory of capillary hysteresis, a SWCC model is developed, which can be used to simulate the soil-water characteristics of unsaturated soils subjected to arbitrary wetting/drying histories. A new material parameter is introduced to describe the reversible changes of water content and to ensure the slope of scanning curve not to become infinite when scanning curves approach the boundary of SWCC. The modified model retains the advantage of the original internal variable-based model, while can improve the performance of the proposaed model. Comparisons with experimental data show that the new model is able to simulate the soil-water characteristics of unsaturated soils under arbitrary wetting/dring cycles. Determination method of the new parameter is also discussed.(2) When a porou medium experiences drying/wetting cycles, part of air phase will be trapped as air bubbles in the pore space. Such air entrapment has significant influences on the soil-water characteristics and the permeability of theporous media. Full SWCC can be represented by 3 parts, i.e., the initial drying curve (IDC), the main drying curve (MDC) and the main wetting curve (MWC). Traditionally a hydraulic hesteresis test is very time-consuming. Based on the data from the literature, an empirical model is developed。Provided IDC and MWC have been determined, the proposed model has only one parameter and can be used to determine MDC. We show that the simulation results agree very well with the measured data.(3) By combining the proposed MDC relationship with the new SWCC model, a mathematical model is developed that can be used to describe the soil water characteristics of unsaturated soils under arbitrary drying-wetting paths. In particular, the effect of air entrapment is fully taken into account. Provided that the hysteresis loop is given, the proposed model includes only one new material parameter, which can be determined by using any first-order scanning curve (or one datum point on the curve). The model is numerically solved using the forward Euler iterative scheme. Numerical simulations are compared with the measurements of the soil-water characteristic curves for four different types of geotechnical media. It is found that the computational results agree very well with the measured data.(4) A comprehensive approach to modeling the constitutive behavior of unsaturated soils is presented. The new framework can be used to describe elasto-plastic deformation and capillary hysteresis of unsaturated soils in a unified way. A simple model of capillary hysteresis coupled with plastic deformation is developed. It is shown that the model can capture the main features of unsaturated soil behavior. Particularly, it can describe the effects of any wetting/drying history on the the elasto-plastic deformation of unsaturated soils.