The municipal solid waste landfills are a common disposal alternative in most areas. Covers are a practical tool to control water infiltration into waste containments. Traditional two-layer capillary barrier covers can be effective in impeding percolation into underlying waste or contaminated soil in semiarid and arid regions. However, it is found the two-layer capillary barrier cover system may not satisfy the requirement to limit rainwater percolation in areas with high precipitation intensity. Besides, the methane generated from a complex series of biological and chemical processes in landfill (i.e. biodegradability of organic content in the municipal solid waste) poses a risk to the climate as a strong greenhouse gas as well as an explosive gas.

Conventional approaches to ev...[ Read more ]

The municipal solid waste landfills are a common disposal alternative in most areas. Covers are a practical tool to control water infiltration into waste containments. Traditional two-layer capillary barrier covers can be effective in impeding percolation into underlying waste or contaminated soil in semiarid and arid regions. However, it is found the two-layer capillary barrier cover system may not satisfy the requirement to limit rainwater percolation in areas with high precipitation intensity. Besides, the methane generated from a complex series of biological and chemical processes in landfill (i.e. biodegradability of organic content in the municipal solid waste) poses a risk to the climate as a strong greenhouse gas as well as an explosive gas.

Conventional approaches to evaluating the performance of granular cover focus on limiting water percolation. Most infiltration analysis assumes that the pores in soil can be fully saturated with water. This assumption ignores the influence of gas entrapped in soil pores during infiltration. In principle a landfill cover is hydraulically unsaturated by the simultaneous presence of water and air in soil porous volume, which will result in a two-phase flow in that capillary zone during water infiltration or gas emission. The influence of gas phase in soil should not be ignored.

This thesis investigates a three-layer cover consisting of a surface fine-texture soil layer, a coarse-texture soil layer and a bottom fine-texture soil layer, as an alternative cover for use in Hong Kong where precipitation is high. To evaluate the performance of three-layer covers on impeding rainwater percolation and gas emission, a multiphase flow model is proposed considering the movements of the gas-phase and the water-phase simultaneously. A gas permeability prediction method is adopted as a tool to estimate the gas permeability based on soil properties.

The influence of soil material properties on the cover performance to limit rainwater percolation and impeding gas emission is fully investigated. The influences of layer thickness on the cover performance during rainfall and gas emission process are also studied. Based on these analyses, various types of covers with different surface fine-texture, coarse-texture and bottom fine-texture soil layer materials are proposed to optimize the cover performance. A three-layer capillary barrier cover comprising sandy silt (ML), well-graded gravel with silt (GW-GM) and lean clay with sand (CL) as its surface, middle and bottom layers is recommended for practical uses due to its relatively small amounts of rainwater percolation and gas emission. Increasing the thickness of the surface or bottom fine-texture soil layer will substantially enhance the cover performance.

A random field analysis method is proposed to investigate the effects of material heterogeneity on the three-layer cover performance in limiting rainwater percolation. By incorporating uncertainties of soil properties, reliability analyses are conducted to enhance engineering judgement and facilitate improved decision making. The spatial variation of soil permeability in the three-layer cover system results in the variability of rainwater percolation amount through the cover, which implies that spatial variation of soil properties may increase the risk of greater rainwater percolation in the three-layer cover system.