Numerical investigation on gas emission from three landfill soil covers under dry weather conditions

The design of existing soil landfill final covers, e.g., a single clay layer cover or a cover with capillary barrier effect (CCBE), aims at limiting water infiltration rather than minimizing landfill gas emissions. We propose an alternative three-layer capillary barrier landfill cover with an added clay layer underneath a CCBE to improve the effectiveness of limiting gas emission. Finite element analyses of coupled water and gas flow were performed to investigate and compare the performance of these three types of soil covers regarding gas emission under a continuous duration of 34 dry d. Due to evaporation, suction in the single clay layer cover increased larger than its air-entry value, causing a significant increase in the air coefficient of permeability. For the CCBE, evaporation also led to water loss in the silt layer, which cannot serve as a gas barrier. On the contrary, the bottom clay in the alternative three-layer capillary barrier cover system was protected by a relatively dry sand layer, which acts as a barrier, minimizing water loss from the clay layer. This is because the dry sand layer has a very low water permeability. As a result, the gas emission rate through the alternative three-layer capillary barrier cover was the lowest among the three types of covers. This rate was lower than the criterion set by the Carbon Farming Initiative, indicating that the alternative three-layer capillary barrier cover is a possible barrier to minimize gas emission even under dry weather conditions.