Studies On One-Dimensional Consolidation Theory Of Unsaturated Soils By Analytical And Numerical Methods

For the complication of consolidation problem of unsaturated soils, up to now there hasn't been a theory for the consolidation of unsaturated soils like Terzaghi's theory, which has been well recognized and widely used in the analysis of one-dimension consolidation of saturated soil. Therefore it is of theoretical and practical importance to carry out the study on the consolidation theory of unsaturated soils to propose a simplified and practical consolidation theory of unsaturated soil. In this dissertation, the consolidation theory of unsaturated soil was critically reviewed, and the new study was made on one-dimensional consolidation theory of unsaturated soil by analytical and numerical methods. The main original works were made as follows:1. Governing equations for one-dimensional consolidation of unsaturated soils were proposed with the combination of the Bishop effective stress principle, the soil-water characteristic curve equation of Fredlund-Xing, and the coefficient of permeability functions of Brooks-Corey.2. Two stages in consolidation process of unsaturated soil were assumed. At the first stage, neither water nor air was expelled from unsaturated soil, and the deformation of soil was only caused by compression of air. At the second stage, the deformation of soil was induced by pore-water and pore-air expelling. The computation methods for soil parameters and state variable components at these two stages were developed respectively. An implicit finite difference technique was used to solve the consolidation partial differential equations for the soil with drainage of water and air at the top or at both the top and the bottom, and corresponding computer codes were developed. Comparisons between the theoretical analyses and laboratory results (Fredlund, 1986) were made. The results indicate a close agreement between the theoretical analyses and the results from the laboratory test.3. The influences of nonlinear change of the soil-water characteristic curve, effective stress coefficient, the coefficient of permeability of fluid on the one-dimensional consolidation of unsaturated soil were studied. It is shown that all these parameters have effects on the one-dimensional consolidation of unsaturated soil, but not very obviously in the range of change of parameters. And the effect varies in different stages and different depths.4. Exploitation of simplified computation of one-dimensional consolidation for unsaturated soil was made, and a new simplified computation method and the formula were proposed. The effects of parameters of soil on one-dimensional consolidation of unsaturated soil were discussed based on the above simplified formula. The comparison with the laboratory results (Fredlund, 1986) shows the new simplified computation method proposed is reasonable.5. Analytical solution for equations of one-dimensional consolidation of unsaturated soil developed based on mixed compressible fluid method was deduced. The analysis show the mixed compressible fluid method is clear in concept and decouples the system of differential equations, hence makes it easier to solve the consolidation problems of unsaturated soil.6. A semi-analytical solution for one-dimensional consolidation equations of unsaturated soil was presented and a corresponding computer program was developed. The reliability of the programs was verified through comparisons with available finite difference solutions. It was concluded that semi-analytical method can convert the complicated nonlinear problems into linear problems and has the advantage of simplicity, clear logic, and less computation, therefore is good at solving consolidation problem of unsaturated soil than other numerical methods. On the other hand, it is shown that semi-analytical method depends on the analytical solution, which restricts the range of its application to some extent.The present work develops study of one-dimensional consolidation theory of unsaturated soils further, and provides a simplified computation method for practical unsaturated soil consolidation analysis.