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| Resource Library > Technology Transfer > Programs and Initiatives > Landfill Covers > Design |
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Conventional Landfill Covers
Conventional landfill covers are intended to work at any site because they employ a variety of barriers. At wet sites, two barrier materials may be
used together with complete drainage and gas control layers. At arid and semi-arid sites a single barrier of greater hydraulic conductivity, without
drainage layers, may be permitted by regulations.
The design should compensate for the known limitations including:
- Compacted clay barriers may dry, crack and become more permeable in arid or semi-arid sites. Plastic membranes placed over the clay may reduce
drying. Geomembrane barriers may be used in place of clay barriers.
- Clay with adequate properties for clay barrier layers may not be available near the site, thus incurring large hauling costs. In that situation,
alternative materials such as geomembrane barriers or clay barriers should be considered.
Additional detail regarding design of conventional covers may be found in Koerner and Daniel(1997) and in the "Landfill Remediation Project Manager's
Handbook," available from the AFCEE.
The Evapotranspiration (ET) Landfill Cover
The ET cover will work at about 93% of Air Force sites; however, the concept should be evaluated for every site. Knowledge of locally available soils,
plants native to the site, and local climatic conditions is essential to create a satisfactory ET landfill cover. The following are important design
considerations:
- The soil used in the cover must have adequate water holding capacity.
- The soil must support robust root growth to allow plants to rapidly consume soil water.
- Long growing seasons are preferred for ET covers.
- Mixtures of warm and cool season grasses are preferred, if applicable.
- Soil for the cover is a major part of the cost of an ET cover; it should be located near the site.
| Plant Roots Critical to Success |
The function of plant roots in the soil of the cover is critical to success. High soil density is a major constraint on root growth;
however, it can easily be controlled during construction and should be a part of the design and specifications for an ET landfill cover.
Soil properties that affect plant root growth are often overlooked during design and construction of vegetated landfill covers; however, less than optimum
soil properties may cause failure. Several soil properties affect root growth, but one of the most important is soil bulk density. Soil
density is relatively easy to control in a landfill cover.
Soil bulk density is the mass of dry soil per unit bulk volume and is expressed as Mg m-3. Desirable soil bulk densities are in the
range of 1.1 to 1.4 Mg m-3. Bulk densities above 1.55 Mg m-3 may either reduce root growth rate and root mass or stop root
growth. Soil density above 1.7 Mg m-3 is likely to stop root growth completely.
Generally the roots of different plant species respond similarly to soil bulk density. The moving pictures of root growth for cotton show the effect
of soil density on cotton root growth; the response of grasses growing on a landfill cover will be similar.
Credit:
- Basic research -- the U.S. Dept. of Agriculture, Agricultural Research Service and the Alabama Agricultural Experiment Station.
- Video -- produced by the Auburn University Television Department.
- Available from -- American Society of Agronomy/Crop Science Soc. Of Am./Soil Science Soc. Of Am.
677 South Segoe Road, Madison, WI 53711 (608)273-8080. Order from the American
Society of Agronomy web site.
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Additional details regarding ET landfill cover design are available in a set of documents published by the AFCEE:
The links below access design documents developed by the AFCEE for a landfill at Lowry AFB, Denver, CO:
References
Koerner, R. M. and D. E. Daniel. 1997. Final covers for solid waste landfills and abandoned dumps. ASCE Press, Am. Soc.
of Civil Engineers, Reston VA.
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