In new waste containment facilities, it is usually necessary to provide a liner system beneath the waste disposal area. The major function of a liner is to prevent leachate or waste from migrating downward and entering the groundwater flow regime. The barrier layer for liner systems can consist of native clays, processed clays, or geosynthetic membranes as previously discussed for covers. It is important to note that under the present U.S. regulations, the use of a synthetic membrane liner is considered the best technology to "prevent" migration of wastes, whereas a clay liner will "minimize" migration of wastes.
The compatibility between the compacted natural clays and the waste is an important design consideration for the use of natural clays as liners. It is important to ascertain the volume change and permeability characteristics of the proposed clay liner material. The bulk transport of liquid waste through cracks as discussed in Section II must be precluded. Bulk transport of liquid through clay liners could occur due to differential settlement of the foundation base ma terials. Compacted natural clay liners are often used as the final barrier layer in a liner system composed of multiple leachate collection and geosynthetic barrier layers. In this configuration the capacity of natural clays to sorb contaminants is acknowledged.
Design and construction considerations for the use of processed clay for liners must include waste-liner compatibility as well as the considerations previously discussed. The volume change characteristics of the processed clay are especially important. Generally, a bentonitic processed clay is mixed with the subgrade material to form the low-permeability liner. The impedance to groundwater flow is primarily due to the processed clays, especially when the matrix soil is relatively free of natural fines. Hence, if the processed clay shrinks upon exposure to the waste or leachate, large increases in hydraulic conductivity can occur. The hydration of a processed clay liner with uncontaminated water prior to waste disposal is recommended . Triaxial permeability tests using the actual proposed clay subgrade material, groundwater, and leachate should be conducted as part of the design studies.
As with other liner types, waste compatibility is a major design consideration. However, the permeability of a polymeric liner can also increase due to liner stretching. Thus, total and differential foundation settlement can impact the liner design. Close construction control is essential to the overall system performance. The "permeability" of an installed membrane liner system is generally a function of bulk transport through seams, joints, tears, holes, and pinholes. The long-term durability of geomembranes is discussed by Koerner et al. , Studies include ultraviolet radiation and chemical degradation as well as swelling, oxidation, and temperature. In addition, predictive methods for the evaluation of long-term durability are proposed and discussed.
A double-composite liner system proposed by Daniel and Koerner  is based on the combination of clay and geotextile structures. This system is the minimum standard of care to which future landfills should be held. It requires careful design, testing, and construction. As shown in Figure 12, the double-composite liner system consists of four major parts:
Leachate collection systems (layers 1,2, and 3) Primary liners (layers 4, 5, and 6) Leak detection system (layer 7) Secondary liners (layers 8 and 9)
These major parts form nine layers. A tenth layer, the soil or rock subgrade, completes the double-composite liner system.
The leachate collection system is the uppermost segment, as shown in Figure 12. The system is composed of a highly permeable granular material on the base of the disposal unit and a high-transmissivity geocomposite material for side slopes, provided timely cover and adequate protection are provided.
Layer I: Filters (Soil or Geotextile). A filter (soil or geotextile) to separate the lowest portion of select waste or initial operations layer of soil from the leachate collection and drainage medium is essential. If suspended particles from the leachate travel into the drainage system, it
Barrier ^^ (geosynthetic clay liner)
Barrier (compacted soil iinerj
Soil or rock subgrade
Waste Filter (soil or geotextile)
Protector (geotextile or other)
Major system segments
Leachate collection system Primary composite liner Leak detection system
—1 Secondary composite liner
Figure 12 Double-composite liner system. (After Daniel and Koerner .)
may clog. To minimize this problem, it is suggested that (1) filters be used only when they are truly needed and that (2) high-permeability filters be employed.
A geotextile filter is part of the geocomposite drainage layer along side slopes. Geotextiles are sensitive to ultraviolet light degradation if left exposed and therefore require protection to avoid this failure mode.
Layer 2: Drain (Gravel for Base, Geocomposite for Side Slopes). The leachate collection layer requires high in-plane transmissivity and pores sized to resist plugging. In this way the hydraulic head on the underlying barrier layer can be minimized.
Layer 3: Protector (Geotextile or Other). This layer prevents materials in the drainage layer from puncturing the primary geomembrane liner. Protectors are usually thick, needle-punched, nonwoven geotextiles.
Layer 4: Barrier (Geomembrane). This component of the primary liner can be made from polymeric materials, including polyvinyl chloride (PVC), chlorinated polyethylene (CPE), chlorosulfonated polyethylene (CSPE), ethylene interpolymer alloy (EIA), high-density polyethylene (HDPE), and very low density polyethylene (VLDPE). At present, HDPE geosynthet-ics offer the most versatility with respect to contaminant resistance and overall engineering properties such as strength and permeability.
Layer 5: Barriers (Geosynthetic Clay Liner). For the soil component of the primary liner, the geosynthetic clay liner (GCL) is recommended. GCLs, previously called prefabricated clay blankets or referred to by other terms, are factory-manufactured dry bentonite clay layers sandwiched between geotextiles or attached to a geomembrane. This barrier offers low hydraulic conductivity along with adsorption capacity and reduces the rate of contaminant transport, particularly transport due to diffusion.
Layer 6: Separator (Geotextile or Other). To avoid migration of clay particles from the GCL into the underlying geonet, adequate separation is needed. Best results in experimental tests have been obtained with (1) nonwoven, needle-punched geotextile with small apparent openings; (2) nonwoven, heat-bonded geotextile; and (3) geomembranes.
This leak detection system is the third major part of the system. It is also known as the secondary leachate collection system. It identifies leakage from the primary lining system and enables it to be collected and removed. Its design is similar to that of the primary leachate collection system.
Layer 7: Drain (Geonet). A geonet is preferable to granular materials for the leak detection layer because it is much easier to place on side slopes and can be placed with lightweight equipment, and because granular materials can puncture the underlying geomembrane. Geonets also offer faster detection of leaks than most granular materials. Hazardous waste facilities typically must detect leaks within 24 hr, and often a geonet is the only material than can do this.
Layer 8: Barriers (Geomembrane). Technical requirements for the secondary geomembrane liner are generally the same as for the primary layer (layer 5), so the same type and thickness of material is usually used.
Layer 9: Barriers (Compacted Soil Liner). A compacted soil liner can be constructed without risk of damaging any underlying liner system components.
5. Subgrade (Soil or Rock) (Layer 10)
For large-scale facilities, a complete subsurface soil investigation is necessary. The large areal extent of landfills results in a significant depth of stress influence. The strength and compressibility of the underlying soils must be fully investigated to ensure foundation stability. To correct any disturbance due to construction, the site should be proofrolled after final grading.
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