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■ Differential settlement due to variable composition of the refuse fill. ■ Differential settlement due to voids. Settlement during the post -closure period is primarily due to decomposition of the refuse. Settlement rates for southern California landfills have been measured in rates of approximately 117o per year(2) for approximately ten year old refuse. Generally landfill data shows that the rate of settlement decreases with time. For the post -closure settlement analysis, it is assumed that the landfill will exhibit an average settlement of 25% over the 30 -year post -closure period. The critical area for analysis is assumed to be the minimum grade closure slopes toward the center of the landfill. For the Austin Road Landfill Expansion, the proposed minimum closure grade is 596'. For this analysis, the minimum refuse depth is assumed to be 100 feet. The effects of the varying depth of waste on settlement and final post -closure land ill grade was evaluated by assuming that the landfill ha:s,,a.consistent total settlement of 25% in all areas and depths of the refuse fill. This results in a decrease in the final grade. For example, 25 0 settlement of 100 feet of refuse results in a final depth of 75 feet and 25% settlement of 110 feet of refuse results in a final depth of 82.5 feet. For a 5% slope, this means 'that' the 'elevation difference over 200 horizontal feet changes from 10 feet to 7.5 feet, resulting in a grade change from 57o' to 3.759o' (see Calculation No. l). Differential settlement due to variable composition of the refuse was analyzed by assuming a 3-dimensional array of landfill cells would have a range of random settlement values between 22% to 28% (see attached Calculation No. 2). Figure 3 shows a representative cross-section for differential settlement from the analysis.Basedoonn to this range of settlement, the original 5% constant slope would vary from approximately 1% If a greater range of settlement is assumed, then the localized ponding could occur. However, based on the analysis in Section 3.3below, the geomembrane is expected to maintain its integrity and continue to act as a moisture barrier between the cover soil and the refuse. Periodic inspections for ponding conditions during the post -closure period should be conducted. Areas showing evidence of ponding should be filled 'as required tomaintain the proper site drainage. The third analysis is differential settlement due to formation of a void within the refuse fill. A theoretical void condition is the decomposition of a large appliance such as a refrigerator and the subsequent local collapse of the fill under the cover. Jang and Montero (3) analyzed this condition using an elastic analysis method to predict the degree of subsidence and strain relative to the depth of the void. The analysis was to a final ed for a liner system used in a vertical expansion, but the results are also app WW-1811-CA1-AA/3110 Austin Road Landfill Expansion August 2,1995 ML.I033.DOC Landfill Settlement Page 3;'