تجزیه و تحلیل حساسیت پارامتری تثبیت مکانیکی توأم و انتقال املاح از طریق موانع خاک رس

In this paper, an extensive parametric sensitivity analysis of coupled consolidation and solute transport in composite landfill liner systems has been undertaken. 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. However, it is noted that to limit the extent of the study a constant coefficient of consolidation is assumed in the analysis presented here, though this assumption is easily relaxed. Results of the parametric sensitivity analysis are succinctly presented using dimensionless plots, which allow the comparison of results for a large number of parameter values, and so the clear identification of the most important determinants on contaminant transport through the liner system. The dimensionless plots demonstrate a pessimum (for which the ‘breakthrough time’ is minimised). Numerical results reveal that in cases of extreme liner compressibility an order of magnitude reduction in contaminant transit time may arise due to coupling between solute transport and consolidation, while for barriers of low compressibility and porosity (such as well-engineered composite compacted clay landfill liners), it is found that the contaminant transit time may still be reduced by more than 30%. The numerical results suggest that the use of coupled consolidation–contaminant transport models are sometimes required for informed and conservative landfill liner design.

Modern engineered waste-disposal facilities such as municipal landfills usually employ composite contaminant barrier systems (see Fig. 1). These typically consist of a low hydraulic conductivity clay layer (or equivalent) and an overlying geomembrane. A well-constructed composite barrier limits the migration of pollutants from the waste into surrounding groundwater largely by restricting the passage (leakage) of leachate. This can only occur through defects in the geomembrane and even here is restricted by the low hydraulic conductivity of the underlying clay. Low leachate leakage rates through well-constructed composite barrier systems mean that the advective transport of contaminants is kept to a minimum. As a consequence, diffusion is often considered to be the dominant mode of transport. Ionic contaminants are essentially incapable of diffusing through the organic polymer structure of most geomembrane materials (because of very low diffusion coefficients). However, the diffusion of small non-ionic molecules such as volatile organic compounds (VOCs) can be quite rapid. For this reason, it is the diffusion of small (and often toxic) VOCs that become the main focus of contaminant transport modelling in composite contaminant barriers [10]. Full-size image (16 K) Fig. 1. Schematic diagram of a composite contaminant barrier. Figure options Modelling of VOC transport through composite barriers is commonly based upon a relatively simple diffusion analysis. However, results from some field studies involving composite landfill liners have indicated that contaminant transit times may be significantly smaller than those expected from a “diffusion only” contaminant transport analysis. It has also been hypothesized that “consolidation water”, expelled from a porous clay liner upon mechanical loading, may lead to advective transport through the clay liner, and higher than expected secondary leachate production beneath the liner. These observations have led to the hypothesis that “consolidation induced advection” may be the cause of the accelerated transit of contaminants [20]. Recently, a number of theoretical investigations of coupled consolidation and contaminant transport in composite barriers have been carried out [2], [15], [16], [18] and [22]. These investigations have mainly focussed on the development and comparison of different model formulations and constitutive relations. Some of the investigations have incorporated case studies of landfill liners. These have shown that the coupling of consolidation and transport processes can be significant; resulting in contaminant transit times which are substantially lower than those predicted by a traditional “diffusion only” analysis. However, since the coupled consolidation–contaminant transport models generally require knowledge of various soil and contaminant transport parameters that are either not available or not easy to measure under field conditions, there is some uncertainty regarding the relevance of the results obtained to practical circumstances. For this reason a more extensive investigation of the effects of consolidation on the transport of contaminants through composite barriers such as landfill liners is warranted. The aim of this paper is to investigate the influence of the choice of model parameters and liner thickness on breakthrough times in composite liner systems. For this purpose an extensive parametric study consisting of over three thousand numerical simulations using the coupled, large-deformation consolidation–transport model of Lewis et al. [15] has been undertaken. Six key design variables are considered in the parametric study: the liner thickness, loading rate of waste in the landfill, and clay soils properties including the initial void ratio, the hydraulic conductivity, the compression index and the dispersion coefficient. Use of non-dimensional time variables allows representation of a vast amount of numerical data in concise form. These plots reveal the regions of the ‘parameter space’ for which the coupling is most influential in terms of reducing the transit (or breakthrough) time of contaminants across the barrier, so allowing ‘worst case’ or ‘pessimum’ scenarios to be identified. In addition, the investigation yields insight into the mechanisms through which consolidation affects contaminant transport in composite barrier systems. These insights contribute to a better understanding of composite liner system behaviour, and may help improve current engineering design.

This paper employs a coupled consolidation–contaminant transport analysis to numerical simulate the transport of contaminants through engineered composite liner systems. In order to capture a large range of possible soil property and liner loading conditions encountered in practical situations, an extensive parametric study has been performed. Numerical results of this parametric sensitivity analysis are concisely presented using dimensionless quantities: a breakthrough time ratio (BTR) versus a process timescale ratio (PTR). The BTR is defined as the ratio of breakthrough time obtained from the coupled consolidation–transport model to that of a diffusion only (i.e., no advective transport due to consolidation) model. On the other hand, the PTR is the ratio of the time required to achieve 90% of the theoretical maximum settlement to the characteristic diffusion time (L2/D). From the extensive parametric study the following conclusions can be drawn: • BTR–PTR plots are significantly influenced by the (initial) void ratio and compression index, while being quite insensitive to variations in loading rate, (initial) hydraulic conductivity, dispersion coefficient and barrier thickness. A remarkably consistent BTR–PTR curve is thus obtained for each combination of (initial) void ratio and compression index despite variation of other parameters. • Variation of BTR with PTR is generally non-monotonic characterized by a minimum (turning point) BTR, corresponding to a pessimum condition for contaminant transport through the barrier. The non-monotonicity is due to the contribution of advective flux to the total flux, which peaks at a PTR value slightly above the pessimum. When advection is neglected, effects of geometric and void ratio variation yield a monotonic reduction in BTR with decreasing PTR. • Insight into understanding the pessimum (i.e., minimisation of the BTR) at an intermediate PTR is gained by noting that for significant advective transport to occur, a relatively strong interaction is required between the concentration profile and fluid velocity profile in the liner system. It is noted that both are monotonic functions of x with their maximum values on opposite sides of the barrier. Relatively fast consolidation (low PTR) leads to fluid velocities that are high but short-lived. In contrast, slow consolidation (high PTR) produces a longer period of advective flow, but with low velocity. Advection is thus limited at low or high PTR and maximised at intermediate values. It is this effect, in combination with the effects of geometric and void ratio variation that leads to the minimisation of the BTR at the pessimum PTR. • Reduction of breakthrough time strongly depends on the compressibility of the contaminant barrier. The BTR values increase with increasing compressibility. For moderate compressibility BTR values are generally higher than 0.6 (i.e., a 40% reduction in breakthrough time). For well-constructed composite compacted clay landfill liners, which are typically of low compressibility and void ratio, results show that contaminant transit time can still be reduced by up to 30% by taking into account coupling between consolidation and solute transport. • Minimum BTR values are found to be strongly correlated with the proportion of pore water expelled from the barrier. This may provide a useful practical criterion for determining if consolidation is likely to have significant influence on transport in a composite barrier. The numerical results presented here suggest that employment of coupled consolidation–contaminant transport models are required in cases of soft clays in order to guarantee conservative contaminant barrier design. In future research, we plan to extend the parametric study to variable coefficients of consolidation and investigate the influence of different solute transport boundary conditions on contaminant breakthrough time.