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Research Doctorate - Doctor of Philosophy (PhD) / Consolidation of clayey contaminant barriers, such as are employed as landfill liners, has been postulated as a cause of accelerated transit of contaminants and hence their earlier than expected appearance in secondary leachate. This proposition is theoretically investigated in this thesis using a novel large-deformation, one-dimensional continuum model of coupled mechanical consolidation and solute transport. The model is a generalization of existing coupled consolidation and solute transport models described in the literature. It takes into account both non-linearities in geometry as well as material constitutive relations. The latter relate the compressibility, hydraulic conductivity and effective diffusion coefficient to the deformation of the soil. In addition to providing details of the governing equations and constitutive relations, a detailed derivation of the three classical one-dimensional consolidation boundary conditions, i.e. undrained top and drained bottom, drained top and undrained bottom, drained top and bottom is also given. From the continuum model formulation, a numerical model was developed using proprietary finite element software – FEMLAB 2.3. The numerical model is verified by comparing results with those produced from other recently developed consolidation – transport models. In the course of conducting these comparisons, some results from a recent modelling investigation indicating that a twenty-fold reduction in the transit time of contaminants across a composite soil – geomembrane barrier may be possible were re-examined. This comparison reveals some apparent errors in the original analysis and indicates that the predicted large acceleration of contaminant transport induced by consolidation is probably unrealistic. The model is subsequently applied to a case study of a clay liner and geomembrane system. Results obtained are compared with those from various simplified models, including a “diffusion-only” (i.e. a rigid soil) model traditionally used in contaminant barrier design. For barriers incorporating low compressibility soils (such as well compacted clays), there is little difference between contaminant transit times predicted by the two models. However, for contaminant barriers incorporating more compressible soils, consolidation is shown to be capable of accelerating transport. These results indicate the potential importance of accounting for the effects of soil consolidation and they highlight some limitations of existing models when modelling solute transport through composite barriers utilizing soft soils. Based on these limited results, a way of taking into account soil consolidation using simplified models is suggested. In the penultimate chapter of this thesis, an extensive parametric sensitivity analysis of coupled consolidation and solute transport in composite contaminant barrier systems is presented. The analysis incorporates results of more than 3000 simulations for various combinations of barrier thickness, waste loading rate, initial void ratio, compression index, hydraulic conductivity and dispersion coefficient. Results are succinctly presented using dimensionless plots, which allow the comparison of results for a large number of parameter values, and hence, the clear identification of the most important factors affecting contaminant transport through a consolidating composite barrier system. The results demonstrate that there exists a pessimum rate of consolidation for which the contaminant transit time is minimised. In cases of extremely high barrier compressibility it is shown that an order of magnitude reduction in contaminant transit time may arise due to coupling between solute transport and consolidation. For barriers of low compressibility and porosity, such as well-engineered composite compacted clay landfill liners, it is found that the contaminant transit time is far less affected, though it may still be reduced by up to 30%. In general, the results suggest that the use of a coupled consolidation–contaminant transport model may be required for informed and conservative contamiant barrier design, especially if relatively compressible earthen components are utilised.

Research Doctorate - Doctor of Philosophy (PhD) / Consolidation of clayey contaminant barriers, such as are employed as landfill liners, has been postulated as a cause of accelerated transit of contaminants and hence their earlier than expected appearance in secondary leachate. This proposition is theoretically investigated in this thesis using a novel large-deformation, one-dimensional continuum model of coupled mechanical consolidation and solute transport. The model is a generalization of existing coupled consolidation and solute transport models described in the literature. It takes into account both non-linearities in geometry as well as material constitutive relations. The latter relate the compressibility, hydraulic conductivity and effective diffusion coefficient to the deformation of the soil. In addition to providing details of the governing equations and constitutive relations, a detailed derivation of the three classical one-dimensional consolidation boundary conditions, i.e. undrained top and drained bottom, drained top and undrained bottom, drained top and bottom is also given. From the continuum model formulation, a numerical model was developed using proprietary finite element software – FEMLAB 2.3. The numerical model is verified by comparing results with those produced from other recently developed consolidation – transport models. In the course of conducting these comparisons, some results from a recent modelling investigation indicating that a twenty-fold reduction in the transit time of contaminants across a composite soil – geomembrane barrier may be possible were re-examined. This comparison reveals some apparent errors in the original analysis and indicates that the predicted large acceleration of contaminant transport induced by consolidation is probably unrealistic. The model is subsequently applied to a case study of a clay liner and geomembrane system. Results obtained are compared with those from various simplified models, including a “diffusion-only” (i.e. a rigid soil) model traditionally used in contaminant barrier design. For barriers incorporating low compressibility soils (such as well compacted clays), there is little difference between contaminant transit times predicted by the two models. However, for contaminant barriers incorporating more compressible soils, consolidation is shown to be capable of accelerating transport. These results indicate the potential importance of accounting for the effects of soil consolidation and they highlight some limitations of existing models when modelling solute transport through composite barriers utilizing soft soils. Based on these limited results, a way of taking into account soil consolidation using simplified models is suggested. In the penultimate chapter of this thesis, an extensive parametric sensitivity analysis of coupled consolidation and solute transport in composite contaminant barrier systems is presented. The analysis incorporates results of more than 3000 simulations for various combinations of barrier thickness, waste loading rate, initial void ratio, compression index, hydraulic conductivity and dispersion coefficient. Results are succinctly presented using dimensionless plots, which allow the comparison of results for a large number of parameter values, and hence, the clear identification of the most important factors affecting contaminant transport through a consolidating composite barrier system. The results demonstrate that there exists a pessimum rate of consolidation for which the contaminant transit time is minimised. In cases of extremely high barrier compressibility it is shown that an order of magnitude reduction in contaminant transit time may arise due to coupling between solute transport and consolidation. For barriers of low compressibility and porosity, such as well-engineered composite compacted clay landfill liners, it is found that the contaminant transit time is far less affected, though it may still be reduced by up to 30%. In general, the results suggest that the use of a coupled consolidation–contaminant transport model may be required for informed and conservative contamiant barrier design, especially if relatively compressible earthen components are utilised.

In landfill design, the containment of solid and liquid contaminant is essential. Leachate is produced from the biodegradation of the waste with the migration of liquid including rain-water through the heap. This liquid can become a health hazard if it leaches into the groundwater. Liners are placed beneath leachate collection systems to prevent leachate from seeping into the soil underneath the landfill. Compacted clay liners, usually containing bentonite clay, are widely used. Bentonite can be characterized by its low hydraulic conductivity and high swell potential. With a low hydraulic conductivity, the liner can serve as a barrier. The high swell potential aids in the integrity of a liner when suffering from cracking or puncturing. The chemicals that can be found in leachate are capable of increasing the clays hydraulic conductivity due to chemical interactions. Chemical compatibility testing - laboratory hydraulic conductivity tests using specific chemical solutions as a permeant - are performed to determine the effects. Laboratory hydraulic conductivity tests, regardless of the permeant, can be time-consuming and expensive. In this study, pure Wyoming bentonite clay and Bentofix clay were used. Deionized water and 0.01M, 0.1M, 0.5M concentrations of four inorganic salt (NaCl, KCl, MgCl2, and CaCl2) solutions were the liquids to which both clays were exposed during testing. Plastic limit and liquid limit tests were run on both clays with all 13 liquids. Laboratory hydraulic conductivity testing with pure Wyoming benonite clay was done with 12 different permeants- all solutions except 0.01M CaCl2 and 0.5M CaCl2. The hydraulic conductivity testing on Bentofix clay was run with 3 permeants- de-ionized water, 0.1M CaCl2, and 0.1M NaCl. The purpose of this study was to determine if a correlation exists between the experimentally determined liquid limit and plastic limit of a specific clay and its hydraulic conductivity when exposed to a synthetic leachate. It was determined that a trend exists that will allow for less expensive and time-consuming determination for hydraulic conductivity of a clay liner when exposed to a specific chemical solution. However, more experimental data need to be collected before a definite trend is verified. The proposed procedure requires that a hydraulic conductivity test of the clay be run using deionized water as the permeant, and plasticity index tests be performed using the leachate.

Accurate modeling of contaminant transport and sorption processes in the soil and landfill liners is a prerequisite for realistic model simulations of contaminant fate and transport in the environment. These studies are also important for the remediation of soil and groundwater contamination. Modeling of contaminant transport through soils and landfill liners consists of either solving the direct/forward problem or the inverse problem. In this thesis, an automated time-stepping implicit procedure is developed from the convergence and error studies of explicit and implicit finite-difference solutions for the advection-dispersion transport of contaminants through soil with different sorption mechanisms. This study is further extended for transient through-diffusion (TTD) transport of contaminant in landfills by considering linear sorption mechanism. To validate the numerical solution and also to study the behavior of finite-difference numerical solutions for TTD transport problem, closed-form analytical solution is derived. Further, a new interface condition is proposed based on the finite-volume procedure for stratified soil or landfill liner system. Solvers are developed for the parameter estimation of inverse problem by integrating the developed procedures for the above forward problem with different optimization procedures. Solvers based on Simulated Annealing (SA) and Genetic Algorithm (GA) are developed for TTD transport in the landfill liners and verified with the existing methods of parameter estimation. Novel swarm intelligence based solver is developed for the first time for parameter estimation in contaminant transport inverse problem to overcome some of the limitations of the classical optimization methods and evolutionary methods such as GA. Additionally, the proposed swarm intelligence based algorithms and a new variant is applied to solve ill-posed problem of contaminant source characterization. The presented work in this dissertation can be unswervingly applied for modeling the contaminant transport in laboratory through-diffusion tests and contaminant transport through landfill liners where the transport is usually considered to be one-dimensional and also diffusion-dominated. Similarly, the advection-dispersion transport through laboratory soil columns can also be modeled with the developed, fast, automated, implicit numerical procedure with very good accuracy. The present study can be applied further for contaminant transport through stratified soil/liner system using fast converging numerical algorithms. Finally, the problems of design parameter estimation and source characterization can be handled accurately by the use of developed automated nature-inspired solvers.