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SECTION 2.WMU FU-03 BASE AND FINAL GRADING 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(Jang, et al., 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 completely collapses, creating a void directly below the lining system. This collapse 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(Jang, et al., 1993). The separation/elastic solution methodology was used to assess the magnitude of separation required between the lining system and a potential void for interface liners to be placed on slopes steeper than 6:1 (horizontal:vertical). For slopes flatter than 6:1, the magnitude of separation increases exponentially and it becomes cost prohibitive to place sufficient fill to maintain the required separation. For slopes flatter than 6:1, a design method based on soil arching and tension membrane theory was utilized (Sheriden, T.G.). The soil arching calculations allow the correlation of the shape of the soil arch to the strength of the soil. Once the shape and dimensions of the arch are known, the uniform normal pressure over the yielding area at the base of the liner system can be calculated. The tension membrane equation can then be used to calculate the required geogrid tension at the allowable strain limit. 2.2.1 Supported Lining Systems for Slopes Steeper than 6:1 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 1 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 1-ft thick. In addition, it is unlikely that refuse that would create a large void(i.e.rusted refrigerator)would be placed within 1 to 2-ft of the former Austin Road Landfill final cover slopes. Therefore,to provide a lining-to-collapse separation of 641 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. The WMU FU-04 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-04 interface lining system. 2.2.2 Supported Lining Systems for Slopes Flatter than 6:1 The grade in the portion of the landfill between the toe of WMU ART-03 and the southern top deck crest of the former Austin Road Landfill is approximately ten percent or less. Because this region IsBased on information provided by Butch Stefani,Site Manager at the Forward,Inc. Landfill. SF0199RPT.DOCI991130002 2-3