Microbial aerobic methane oxidation in unsaturated landfill cover involves coupled water, gas and heat reactive transfer. The coupled process is complex and its influence on methane oxidation efficiency is not clear, especially in steep covers where spatial variations of water, gas and heat are significant. In this study, two-dimensional finite element numerical simulations were carried out to evaluate the performance of unsaturated sloping cover. The numerical model was calibrated using a set of flume model test data, and was then subsequently used for parametric study. A new method that considers transient changes of methane concentration during the estimation of the methane oxidation efficiency was proposed and compared against existing methods. It was found that a steeper cover had a lower oxidation efficiency due to enhanced downslope water flow, during which desaturation of soil promoted gas transport and hence landfill gas emission. This effect was magnified as the cover angle and landfill gas generation rate at the bottom of the cover increased. Assuming the steady-state methane concentration in a cover would result in a non-conservative overestimation of oxidation efficiency, especially when a steep cover was subjected to rainfall infiltration. By considering the transient methane concentration, the newly-modified method can give a more accurate oxidation efficiency.

Vegetation reduces soil pore-water pressure (PWP) through root water uptake. There are limited studies about the effects of vegetation on PWP distributions in layered soil, such as multilayered earthen landfill cover. New analytical solutions for vertical water flow in vegetated layered soil at steady state are derived considering various root architectures, including exponential, parabolic, triangular, and uniform roots. The solutions are verified through (1) hypothetical vegetated layered soil using numerical seepage analyses and (2) a soil column test investigating PWP distributions in vegetated layered soil. The new solutions are capable of determining PWP distributions for both inside and outside root zone in vegetated layered soil. These solutions provide a relatively simple and reliable approach for estimation of PWP distributions in vegetated layered soil.

Although the working principles of capillary barrier effects (CBEs) have been well explained based on unsaturated soil mechanics, the selection of materials for engineering structures with CBEs mostly relies on designers’ experience or previously reported data, largely due to the lack of understanding of the microscale behavior of CBEs. This study explores the impact of pore sizes on CBEs based on pore network modeling. The results indicate that the pore size variability in either the fine or coarse layer has a remarkable influence on CBEs, with the latter having a dominating influence. A larger coarse-to-fine mean pore size ratio results in more effective CBEs. In addition to selecting materials with more uniform pore sizes, the coarse-to-fine mean pore size ratio is recommended to be >6 (median particle size ratio of 30) to ensure the performance of CBEs.