THESIS
2016
xvii, 209 pages : illustrations ; 30 cm
Abstract
Uncontrolled landfill gas (LFG) emission from landfills generates serious social and environmental concerns. A widely used approach to isolate municipal solid wastes from the environment is to apply an engineered cover over landfills. Compacted clay, due to its relatively low saturated water permeability, has been widely used in landfill final cover systems. Many studies demonstrated that desiccation cracks induced by drying and wetting cycles jeopardize the integrity of the compacted clay cover system. Most of these studies, however, focused on the evaluation of the performance of compacted clay cover in reducing water infiltration since water permeability of 1×10
-9 m/s is the only prescribed design parameter in most current design guidelines for landfill final covers. The performance...[
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Uncontrolled landfill gas (LFG) emission from landfills generates serious social and environmental concerns. A widely used approach to isolate municipal solid wastes from the environment is to apply an engineered cover over landfills. Compacted clay, due to its relatively low saturated water permeability, has been widely used in landfill final cover systems. Many studies demonstrated that desiccation cracks induced by drying and wetting cycles jeopardize the integrity of the compacted clay cover system. Most of these studies, however, focused on the evaluation of the performance of compacted clay cover in reducing water infiltration since water permeability of 1×10
-9 m/s is the only prescribed design parameter in most current design guidelines for landfill final covers. The performance of compacted clay cover in preventing LFG emission is rarely studied.
The principal objective of this research is to study gas breakthrough and emission in compacted clay considering cracks. One dimensional (1D) soil column tests were carried out on unsaturated compacted clay to determine gas breakthrough pressures and gas emission rates. Possible controlling factors such as soil thickness and on gas breakthrough pressure and emission rate were considered. Moreover, numerical simulations were carried out using a coupled gas-water flow finite element program (CODE-BRIGHT) to better understand the experimental results. Desiccation crack test was conducted in a large soil box (1m×1m×0.6m) under four drying-wetting cycles. Crack characteristics of cracked soil specimens taken from the soil box after the first drying process were quantified using a CT scanner and then utilized in following gas flow tests. Effects of desiccation cracks on gas breakthrough and emission were analysed.
The results of experimental study and numerical simulation reveal that as the degree of saturation and thickness of clay increase, the gas breakthrough pressure increases but the gas emission rate decreases significantly. Under a gas pressure of 10 kPa (the upper bound limit of typical landfill gas pressure), a 0.6 m or thicker compacted clay is able to prevent gas breakthrough at degree of saturation of 60% or above (in humid regions). Furthermore, to meet the limit of gas emission rate set by the Australian guideline, a 0.6 m-thick clay layer may be sufficient even at low degree of saturation (i.e., 10% like in arid regions).
Desiccation crack tests reveal that propagation of vertical cracks can be intercepted by the interface between successive sublayers. Results from the gas flow tests on cracked clay revealed that gas emission rate is governed by effective thickness of cracked soil samples. Gas permeability increases almost one order of magnitude when crack occurs in the soil specimen with an effective thickness of 0.3 m. Furthermore, the effect of crack depth on gas emission rate depends on the injected gas pressure. Gas emission rate increases almost 3 times when crack depth increases from 0.1 m to 0.15 m. A minimum clay thickness of 0.75 was suggested for landfill final cover system in preventing landfill gas breakthrough and emission below the gas emission limit set by Australian guideline under typical landfill gas pressure (less than 10 kPa).
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