Numerical study of solute transport in deformable soil liners
For reasons of simplicity and economics, landfills are the main methods of disposing the solid waste (either household or industrial) and the highly contaminated aqueous sediments due to toxic substances. To protect the surrounding environment and groudwater from pollution, liner system is usually constructed beneath the landfill. In a modern composite basal liner system, preventing the breakthrough of volatile organic contaminants (VOCs) is a core concern to design a effective barrier. Conventional methods of analysis assume that the soil is fully saturated. However, throughout much of the world unsaturation exists in landfill basal liner. Although a few investigations have treated the migration of VOCs in unsaturated soil liner, no deformation of liner due to mechanical consolidation was included and VOCs only move in liquid phase. In reality, landfill compacted clay liner (CCL) is compacted on the optimum water content, where the air phase exists in the form of occluded air bubbles. However, the air phase becomes continuous when the temperature increases and then VOCs will be transported in the gaseous phase. Therefore, it is clear that the traditional approaches to assessing VOCs transport are inadequate to enable reliable assessment of VOCs break through landfill basal soil liner. This thesis attempts to make an investigation into migration of VOCs in partially saturated landfill soil liner using numerical modeling techniques. Firstly, pore fluid storage and solute transport equations suitable for quasi-saturated porous medium were developed. In the frame of small strain, a one-dimension coupling model was non-dimensionlized, whereby relative importance of the terms related to consolidation advection were compared. Based on the non-dimensional analysis, a simplified model was proposed and applied to a hypothetical landfill CCL. Numerical results demonstrated that the longitudinal dispersivity and compressibility of the pore fluid can be significant. Furthermore, the degree of soil saturation and loading rate of the waste surcharge affect significantly the contamination advective emission, namely the cumulative contaminant mass outflow per unit area from compacted clay liner (CCL) due to advective flow. Secondly, the coupled model was extended to include finite strain and geometric and material nonlinearity. Using the finite strain model, a parametric study was carried out to examine the influences of consolidation and several other parameters on the process of VOCs solute transport in quasi-saturated soil liner. Consolidation-induced advection was found to have a lasting effect on solute transport during and after the deformation for relatively compressible soil regardless of the sorption level, though the sorption could dramatically slow the solute transport process rate. A lower degree of saturation leads to a slower pore fluid flow and solute transport due to a narrower channel. Effective diffusion decreases during consolidation and consequently the relative importance of mechanical dispersion becomes profound. In general, reducing soil compressibility and improving sorption levels of clay are the most effective ways to retard contaminant migration. Thirdly, a fully coupled thermal-hydraulic-mechanical-chemical (THMC) model was proposed to describe the migration of VOCs in unsaturated landfill liners with continuous air phase. In the formulation, vertical soil stress, capillary pressure, air pressure, temperature increase and dissolved solute concentration were selected as primary variables. The finite deformation was addressed by use of Lagrangian coordinate. The non-isothermal moisture transport was dependent on both temperature gradient and VOCs concentration. VOCs were assumed to reside and be transported by three phases, i.e., solid, liquid and gas phases in soil. Based on the model, an illustrative example of unsaturated compacted clay liner (CCL) was presented. For the case considered, transport of gaseous phase VOCs was found to dominate the migration progress. Moreover, the temperature gradient could accelerate the breakthrough of VOC in unsaturated liner, whilst the mechanical consolidation slowed down the motion of VOCs due to soil contraction. The theoretical models established in this study encompass several important situations in landfill basal soil liner, which can facilitate understanding of the VOCs transport process and assessment of soil liner performance. In addition, some areas where further work is required are identified.
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