A Two-Phase Flow Model Of Unsaturated Soil And Its Application

Two-phase flow problem in unsaturated soils is one of the key problems which need to be solved in protecting the underground environment and geological disasters, exploiting fossil energy source effectively, and evaluating groundwater resource. Because of heterogeneous of material and the influence of capillary effect, unsaturated soils always show complex hydraulic properties, which behaves the effect of bulk and variation history of water content on the process of seepage. Classical seepage models, taking the Richard equation for example, have obvious limitation on describing two-phase flow problem in unsaturated soils under complex condition. As a result, researching on two-phase flow problem in unsaturated geological material has important scientific significance and practical application value.Centralling on two-phase flow problem in unsaturated soils, theoretical simulating and numerical analysis have been performed, mainly including four parts as follows:(1) The soil-water characteristic curve (SWCC) models concerned hysteretic behaviors which reported in literatures are summarized and analyzed in this part. Based on the internal-variable theory of capillary hysteresis,a mathematical model is developed that can be used to describe the soil-water characteristics of common geological materials experiencing arbitrary drying-wetting paths, particularly,the effect of air entrapment is fully taken into account. The scanning curves which exist in main hysteresis loops and exist between initial dry boundaries and main dry boundaries could be characterized by this model, and the air entrapment could be presented. The establishment of the new model provides an effective and reliable constitute model for seepage analysis.(2) Based on the theory of thermodynamics in multiphase porous media,a coupling model is proposed which contains about poroelastic stress-strain relationship and capillary hysteresis SWCC. In this model, the effect of water content variation history on SWCC is reflected, the property of two-phase (water-air or water-contaminant) flow in unsaturated soils is characterized, and the deformation of soil skeleton is simulated.(3) Based on above-mentioned two-phase flow model, a simplified governing equation is established. Galerkin method is used to discrete the simplified governing equation in space,and finite element equations are obtained. Hilber,Hughes and Taylor-αmethod are used to discrete in time domain, the numerical form is presented, and a finite element code, U-DYSAC2,is developed. Seepage and deformation property of unsaturated soils under complex external condition could be analyzed by this code.(4) Two-phase flow problems in unsaturated soils, for example, Air-water flow and water-contaminant seepage, are simulated and analyzed by U-DYSAC2 procedure. For the first section, the classical two-phase flow problem, Liakopoulos test, and two centrifugal model tests which concerns about silt embank experiencing flooding and the contaminant transporting through saturated porous media are simulated by this code. It is found that the predicted values agree very well with experimental record in the literature,showing that the proposed model is capable of simulating the seepage and deformation in unsaturated soils. The non-aqueous phase liquids (NAPL) migrating in soils are simulated in the second section, the influences of heterogeneous of soil layers and sensitivity of seepage parameters are analyzed. At last, the embank seepage problem which under arising water lever condition is simulated and the problems of slope seepage and stability under rainfall condition are analyzed, the hysteresis effect on seepage and stability is emphasized.