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2e2 Interface Liner Because the WMU FU-03 interface lining system will be constructed upon the former Austin Road Landfill southern waste slope, settlement is an important element of the interface liner design. A lining system constructed over an existing landfill is potentially subject to differential settlements that may occur because of the variable compressibility of different wastes and/or the collapse or degradation of large objects within the landfill. Differential settlements in the subgrade of an interface liner could result in tensile strains at the surface of the liner system and could affect the drainage capacity of the lining system's LCRS. If the tensile strains within the liner exceed the tensile capacity of the lining material—whether a soil or geomembrane—tension cracks or tensile failure could develop. Under extreme conditions, tension cracks will reduce the effectiveness of the liner as a hydraulic barrier by providing a direct flow path through the lining system. Additionally, differential settlements could affect the slope at which the lining system had been constructed and could possibly result in the reversal of leachate drainage grades. If grade reversal takes place at the surface of a liner and LCRS, leachate will pond on the liner, and the potential for infiltration of the leachate into the underlying waste will increase. Typical designs of interface lining systems incorporate measures to bridge over potential voids occurring near the underlying waste surface, which may result from the collapse of a large object, such as a refrigerator. This type of localized collapse is generally considered the worst possible occurrence and therefore is typically the basis for design (fang,et A., 1993). The conservative "rusted refrigerator" design method was used to develop the requirements for the lining system subgrade. The "rusted refrigerator"design method assumes that a large object(3 feet x 3 feet x 6 feet)completely collapses, creating a void directly below the lining system. This collapse AMIL results in a local depression in the lining system. The magnitude of the depression is a function of the separation between the lining and the collapse and can be quantified using elastic solutions (fang, et al., 1993). According to calculations performed for this project(Appendix B), if a lining-to-collapse separation of approximately 6 feet is maintained,drainage grades upon the 5:1 (H:V)slope will be maintained and tensile strain in the lining system will be limited to less than l percent. Both the geomembrane and GCL components of the lining system have yield strains greater than 10 percent and are, therefore, flexible enough to withstand this conservatively calculated tensile strain. According to site operations personnel,I the former Austin Road Landfill waste slopes have an existing interim cover that is approximately I-ft thick. In addition, it is unlikely that refuse that would create a large void would be placed within I to 2-ft of the former Austin Road Landfill final cover slopes. Therefore, to provide a lining-to-collapse separation of 6-ft and given that at least 2-ft of separation already exists to any potential void, approximately 4-ft of additional compacted fill material will be placed on the former Austin Road Landfill slopes that are to receive the interface lining system. The WMU FU-03 grading plan (Appendix A)was developed to provide the necessary additional compacted subgrade fill. The combination of the interim cover already in place and the additional compacted subgrade fill will provide the required compacted fill material thickness below the WMU FU-03 interface lining system. 16ased on information provided by Butch 5tefani,Site Manager at the Forward,Inc.Landfill. FORWARD LANDFILL WMU F-03 AND F-WEST DESIGN REPORT 2-2