Theoretical effects of consolidation on solute transport in soil barriers.

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