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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. 2.3.2 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 5.5 feet is maintained, drainage grades upon the 4.5:1 (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,) 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 2-ft of the former Austin Road Landfill final cover slopes. Therefore, to provide a lining-to-collapse separation of 5.5-ft and given that at least 2-ft of separation already exists to any potential void and there is an approximate 1-ft thickness of existing soil cover, approximately 3-ft of additional compacted fill material will be placed on the former Austin Road Landfill slopes. The WMU FU-13 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-13 interface lining system. 2.3.3 Supported Lining Systems for Slopes Flatter than 6:1 The grade in the portion of the landfill that connects WMU FU-08 North and WMU FU-13 is approximately ten percent or less. Because this region overlies the former Austin Road Landfill and is relatively flat, a geogrid will be placed to support the overlying liners and LCRS. The geogrid calculations are based on both the short-term tensile strength of the geogrid and the long term allowable tensile strength. Because the multiaxial yield strength of geomembranes is about 7 to 10 percent, the specified short term tensile strength of the geogrid is measured at five percent rather than using the ultimate geogrid tensile strength. This requirement maintains strain compatibility by allowing the geogrid to achieve the required tensile strength before the overlying geomembrane deforms beyond its yield strength. Because the tensile strength of geogrids can degrade over time due to factors such as creep installation damage, and other environmental factors, the short-term tensile strength of the geogrid is reduced to calculate a suitable long-term allowable load. The long-term allowable load is compared to the required tensile load in specifying the geogrid properties. The final lifts of refuse on the top deck of the former Austin Road Landfill were placed in 2002. According to site operations personnel, no large metal objects such as refrigerators were placed in the final refuse lifts. The type of refuse placed was municipal solid waste and some construction and demolition debris. It was estimated that the largest void that may collapse in this type of waste has a ® 1 Based on information provided by Butch Stefani,former Site Manager at the Forward,Inc.Landfill. WMU FU-13 REPORT.DOC 2-3