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Construction |
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University of Padua IMAGE Department |
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Top cover The engineering of top cover changes according to different local situations (climatology, morphology, etc.) Erosion control and vegetation support, coupled with a channel system for water run-off, in the presence of steep slopes A vegetative soil layer with a minimum thickness of 60-80 cm to support vegetation A sub-surface drain (gravel, sand or geosynthetics) to prevent ponding and allow rapid drainage of water A low permeability liner (clay, bentonite mat or geomembrane) for controlling of water infiltration An underlying drain for collection and removal of biogas Traditional top cover |
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Capillary barriers Two-layer system in which a layer of fine sand overlies a layer of gravel. In an unsaturated medium, the interface between a fine material and the coarser material functions as a capillary barrier and water cannot pass from the fine medium to the underlying coarse medium due to the difference in capillarity, which is stronger than gravity. Thus the water is in the first stage retained in the upper layer of fine sand. |
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As long as the capillary attraction at the interface remains sufficiently strong, the hydraulic conductivity of the fine sand will be higher than that of the underlying gravel, thus limiting infiltration; if the fine sand is sufficiently well drained, the sand retains enough capillary to maintain this effect. If the interface is horizontal, the infiltred water will accumulate just above the interface without penetrating into the underlying gravel, until the force of gravity becomes greater than the forces of capillary pressure; under this condition the water will pass the interface and flow rapidly through the coarse material. If the interface is an inclined plane, with a slope between 5° and 10°, less water will accumulate above the interface as it will flow laterally through the fine sand, and the penetration of water into the underlying layer will thus be limited. |
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Advantages A capillary barrier can be constructed at very reasonable costs by using ordinary techniques and ordinary standard materials The capillary barrier, due to the retension capacities of the fine sand, acts as a buffer and regulator during rainy periods The capillary barrier incorporated into multilayer systems acts as a safety layer, in case problems occur with the first hydraulic barrier immediately above it It is an ideal alarm system, thereby invoking the possibility of incorporating an effective alarm system to ensure the adequate functioning of the first upper liner by placing pressure sensors (tensiometers) in the fine sand or by monitoring the flow rate from the drains It enables gases to be extracted. The underlying gravel layer that is in direct contact with the waste enables any gas generated by the waste to migrate to the surface where they can be collected It is an excellent base for certain artificial hydraulic barriers; in fact the fine sand comprising the upper part of the capillary barrier can be used as an appropriate basis layer for a plastic liner, etc. Multilayered landfill caps Landfill caps are usually designed as multilayered systems with a combination of topsoil, drainage layers and barrier layers Three types of barrier layers are commonly used and can be combined: flexible membrane liners (usually HDPE), compacted soil liners and/or capillary barriers Methane oxidation in landfill top cover Biogas production in new landfills for pretreated municipal solid waste might be reduced by up to 90% of what expected in traditional landfills (Leikam and Stegmann, 1997) Results of lab-acale and pilot-scale tests have proven that methane oxidation and degradation of trace organic compounds can occur in landfill cover soil In particular, ripe compost from municipal solid waste provides excellent properties as a carrier for methane oxidation processes and it could be used as cover soil (Humer and Lechner, 1999) |