Liner Systems for MSw. The objective in the design of landfill liners is to minimize the infiltration of leachate into the subsurface soils below the landfill thus eliminating the potential for groundwater contamination. A number of liner designs have been developed to minimize the movement of leachate into the subsurface below the landfill. The sand or gravel layer serves as a collection and drainage layer osor any leachate that may be generated within the landfill. The geotextile layer is used to minimize intermixing of the soil and sand or gravel layers. The final soil layer is used to protect the drainage and the leachate collection layer. Composite liner designs employing a geomembrane and clay layer provide more protection and are hydraulically more effective than either type of liner alone. A specifically designed open weave plastic mesh(geonet) and geotextile filter cloth are placed over the geomembrane which, in turn, is placed over compacted clay layer. A protective soil layer is placed above the geotextile. The geonet and the geotextile function together as the drainage layer to convey leachate to the leachate collection system. The permeability of the liner system that is composed of a drainage layer and a filter layer is equivalent to that of coarse sand. Because of the potential for the geotextile filter cloth to cle many designers favor the use of a sand or gravel layer as the drainage layer TABLE Guidelines for leachate control facilities Comments Synthetic flexible Liners must be designed and constructed to contain fluids, which membrane liners include wastes and leachates For Msw waste management units, (FMLs) synthetic liners are not required. However, if this alternative is selected synthetic liners must have a minimum thickness of 40 mils. These liners must be installed to cover all natural geologic materials that are likely to be in contact with waste or leachate at a waste management ttom seals No specific regulations exist governing the application of bottom seals at MSw waste management units. Design, construction, and installation of bottom seals are subject to the approval of the local enforcement agencies Artificial earthen Clay liners are o al for Msw landfills, If re d by site conditio clay liners for Msw waste management units must be a minimum of 1 ft thick and must be installed at a relative compaction of at least 90 percent. a clay liner must exhibit a maximum permeability of 1 x 10-6 cm/s. Clay liners, if installed, must cover all natural geologic materials that are likely to be in contact with waste or leachate at a waste management unit Subsurface barriers geologic materials to assure that lateral permeability standards are A subsurface barrier is intended to be used in conjunction with natu satisified Barriers may be required by regional agencies at MSw waste man- agement units where there is potential for lateral movement of fluid. cluding waste and leachate, and the permeability of natural geologic laterals is used for waste containment in lieu of a liner Barriers must be a minimum of 2 ft thick for clay material or a minimum of 40 mils for synthetic materials. These structures are required to be keyed a minimum of 5 ft into natural geologic materials that satisfy permeability requirements of1×106to1×10-cm/s.‖ walls are used, excavations for waste management units must be keyed into natural geologic materials exhibiting permeabilities of no greater than 1 x 106 cm/s Barriers are required to have fluid collection systems upgradient of the structure. The systems must be designed, constructed, operated, and maintained to prevent the buildup of hydraulic head against the structure. The collection system must be inspected regularly and12 Liner Systems for MSW. The objective in the design of landfill liners is to minimize the infiltration of leachate into the subsurface soils below the landfill thus eliminating the potential for groundwater contamination. A number of liner designs have been developed to minimize the movement of leachate into the subsurface below the landfill. The sand or gravel layer serves as a collection and drainage layer for any leachate that may be generated within the landfill. The geotextile layer is used to minimize the intermixing of the soil and sand or gravel layers. The final soil layer is used to protect the drainage and barrier layers. A modification of the liner design involves the installation of leachate collection pipes in the leachate collection layer. Composite liner designs employing a geomembrane and clay layer provide more protection and are hydraulically more effective than either type of liner alone. A specifically designed open weave plastic mesh (geonet) and geotextile filter cloth are placed over the geomembrane which, in turn, is placed over compacted clay layer. A protective soil layer is placed above the geotextile. The geonet and the geotextile function together as the drainage layer to convey leachate to the leachate collection system. The permeability of the liner system that is composed of a drainage layer and a filter layer is equivalent to that of coarse sand. Because of the potential for the geotextile filter cloth to clog, many designers favor the use of a sand or gravel layer as the drainage layer. 15-3