Numerical Simulation for Solute Transport in a Deformable Porous Medium

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Jeng, Dong Sheng
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Zhang, Hong
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2020-01-15
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Abstract

Landfill is the main method of waste disposal in large and medium-sized cities. The system of landfill liners plays a crucial role in keeping the surrounding environment free from the waste contaminants and averting the appearance of secondary pollution. The composite landfill liner, composed of geomembrane and compacted clay, is the primary impervious material. To prevent the breakthrough of volatile organic contaminants is a core concern when designing an effective barrier. During the serving time of landfill, waste is disposed into the center void until it reaches the landfill’s capacity. In this model a time-dependent load is placed on the top of the clay liner, the load gradually rises over a period of time and then stabilizes. Although research has been conducted to study the coupled consolidation and solute transport model to simulate the contaminant condition under a landfill, most investigations assume a fully saturated condition. However, a partially saturated condition is more likely to exist in practice. Based on the most recent studies, several improvements for the coupled model have been addressed in this study. Firstly, the effect of soil stratification was studied through 1D numerical investigation based on the coupled solute transport model in deformable unsaturated layered soil. The theoretical model implied two-way coupling between excess pore pressure and soil defor-mation, and a one-way coupled volatile pollutant concentration field developed from the advection-diffusion theory. Embedded in the model, the degree of saturation, fluid compress-ibility, self-weight of the soil matrix, porosity variation, longitudinal dispersion, and linear adsorption were computed. Based on simulation results, soil parameters of the top layer are more critical than the lower layers but controlling soil thicknesses will alter the results. Also, a simplified modelling method of assuming the averaged properties across the soil profile was proved to be inaccurate for a stratified soil environment. Secondly, the effects of hydraulic conductivity and degree of saturation that depend on pore pressures on the solute transport in deformable unsaturated soils were investigated. The storage equation and solute transport equations were revised to account for the dependence of hydraulic conductivity and degree of saturation on pore pressures. Compared with the conventional model, the simulation results for the dynamic models showed that both hydraulic conductivity and degree of saturation increased in the upper layer due to the increase of pore pressure within a certain period. Dynamic hydraulic conductivity results in a slightly slower solute transport while dynamic degree of saturation accelerates the migration of contaminants. Including both dynamic effects produced limited differences in solute concentration, while consolidation results were affected significantly. This study provides new methods to incorporate inhomogeneous soil parameters that are not only spatially and temporally variable, but also dynamically changed by pore pressures. Thirdly, the current 1D model was extended to 2D and 3D with newly derived governing equations. The application of the multi-dimensional model on landfill show the horizontal spreading of contaminants. A case study of corner loading with varying loading rate reveals that external loading accelerates solute transport, mainly on the loading direction. Also an proposed landfill case with anisotropic hydraulic conductivity was presented. The multi-dimensional model can be applied in a partially saturated soil environment and takes into account the fluid compessibility, temporal and spatial variation of void ratio, adsorption and desoprtion etc. Ramp load can be placed on the loading surface to mimic landfill operation from the beginning of use. In addition to the case studies presented in Chapter 5,the proposed model provides more guidance to the landfill industry for damage control such as simulating the contaminant leakage in inhomogeneous soil environment. Furthermore, by changing some boundary conditions, the proposed models, necessary but not limited to the application of landfill, have the potential to be applied in any case where contaminant soil undergoes loading. Application could include for example, the pollution from industrial activity, pollution under ground or at seabed, gasoline leaking underneath an underground storage tank or even nuclear waster disposal issues.

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Thesis (PhD Doctorate)
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Doctor of Philosophy (PhD)
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School of Eng & Built Env
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Landfill
waste disposal
volatile organic contaminants
soil stratification
hydraulic conductivity
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