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 ERDC TN-DOER-R6
December 2004
General design guidance for use of GCLs in liner systems can be found in EPA 625/4-89/022 (USEPA
1989), EPA 530/SW-91/054 (USEPA 1991a), and TM 5-818-8 (HQDA 1995). Design guidance
considerations for employment of GCLs in CDFs are similar to those discussed for geomembranes.
Composite Liner. Composite liners consist of a combination of geomembranes with compacted clay or
geosynthetic clay liners. While each composite component possesses particular strengths, including low
permeabilities, their resistance to permeation is enhanced when used in a composite manner. For
example, geomembranes function best when they are used as part of a composite liner system. Leachate
seepage can occur throughout the entire cross-sectional area of a single soil liner. However, employment
of a geomembrane with potential defects (due to rips, tears, or improper welding of seams) only
marginally improves leachate collection and the resistance to percolation, unless the geomembrane is
coupled with a low-permeability underlying layer such as a compacted clay liner or fine-grained
foundation soil. In this case, the pathway for leachate transport is limited to only the area provided by the
original geomembrane defect (Qian 1995).
Detailed procedures for design of composite liner systems for solid and hazardous waste landfills are
provided in USEPA (1988a, 1988b, 1991a, 1992a). Current design specifications for landfill liners
include use of double composite systems. The first (upper) composite layer acts as the primary leachate
collection system underlain by the primary hydraulic barrier. The second (lower) composite layer serves
as the secondary leachate collection system, and the bottom portion of the lower composite system serves
as a secondary hydraulic barrier.
No similar design requirement exists for CDFs; in fact, employment of a double composite liner for CDFs
would likely be unnecessary and cost-prohibitive. It is more likely that CDF designs contemplating the
use of engineered composite liner systems will only consider single composite liner systems. In such
cases, design procedures similar to those noted in USEPA (1988a, 1992a, 1993a, 1993b) should be
employed, with special regard to use of the dewatering systems described below.
CDFs result in the production of leachate. While vertical wick drains remove water throughout a CDF,
leachate collection systems (although rarely employed in CDFs) target leachate accumulated at the top of
bottom liners. This leachate is then collected and transported via gravity through drainage layers to
centralized locations for subsequent removal and treatment. Vertical wick drains and leachate collection
systems are important for their ability to assist in reducing the accumulation of leachate overlying liner
systems. Reduction of the hydraulic head results in a direct reduction in the primary driving force for
contaminant movement through CDF liner systems advective transport.
General design criteria for installation of leachate collection systems in municipal solid waste and
hazardous waste landfills are detailed in USEPA (1992a). Such leachate collection systems employ a
highly permeable drainage layer (e.g., gravel with perforated pipe or geocomposite drain) overlying a
very low-permeability compacted clay liner or geomembrane. The drainage layer is then overlain by a
filter soil or synthetic geotextile to separate the contaminated dredged material from the drainage layer
and to filter the leachate to prevent clogging of the drainage layer.
Leachate collection systems for CDFs, if employed, will likely differ significantly from those employed
in solid and hazardous waste landfills in that CDFs do not normally employ double composite liners with
primary and secondary leachate collection systems. Additional differences include:
Quantity of leachate Dredged material, depending on the method of dredging, contains between 50
and 80 percent water (USEPA 1994), which is significantly higher than the 20-percent water content
typically found in solid waste in municipal and hazardous waste landfills (Tchobanoglous et al.
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