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4.0 DIFFERENTIAL SETTLEMENT EFFECTS ON THE VERTICAL EXPANSION LINER SYSTEM Vertical expansion for the Austin Road Landfill consists of constructing a landfill liner system over the final cover of the existing landfill. The concerns for this type of landfill expansion is potential damage to the integrity of the liner system and loss of drainage slope due to differential settlement of the underlying refuse. These are similar to the problems with final closure settlement. However, the primary settlement mechanism in the existing refuse fill below the vertical expansion liner is anticipated to be mechanical compression from: the weight of the vertical expansion refuse fill instead of decomposition of the existing refuse; This is because the age of the existing refuse will be approximately 25 to 50 years at the time of vertical expansion and is expected to have completed most of its biological decomposition phase. 4.1 COMPRESSIVE SETTLEMENT UNDER THE VERTICAL EXPANSION LINER The method of analysis for this settlement type is to determine the settlement of the existing refuse due to the additional weight (i.e., surcharge) of the vertical expansion,zefuse. and mal cover system. The settlement calculation calculated both the primary (short term) consolidation and the secondary (long term) consolidation (Calculation No. 5) based on the following assumptions: ■ The primary consolidation coefficient (Cc) was assumed to be 0.5. Landva(6) reported test values for Cc in the range of 0.2 to 0.5. with older refuse in the lower range. The selection of the higher value should provide a conservative over estimate of the landfill settlement. ■ In-place refuse/daily cover density was,assumed .to be 70 lbs/ft ^ 3 ■ The secondary consolidation coefficient (Cs) was assumed to be .02 based on values reported for southern California Landfills('). Time period of settlementwas 30 years. Settlement of the existing landfill refuse under the vertical expansion liner from compressive settlement is expected to range from approximately 2 to 4 feet. However, the general grade of the landfill slope will not appreciably change. This is because, generally, the depth of the vertical expansion refuse overburden decreases as the depth of the existing landfill refuse increases. The resulting effect is that while the percentage of settlement changes in the existing landfill refuse, the elevation settlement in feet only shows a minor change. ,For example the 5% minimum design grade after settlement varied from 4.9% to 5.6%. Overall strain on the liner system is expected to be negligible. However, this analysis does not try to account for possible settlement in the geologic subgrade under the existing landfill and/or possible variation in the compressive settlement of the waste fill. Also, the parameters for the analysis were based on literature values and may not accurately represent the characteristics for the Austin Road Landfill refuse. It is recommended that additional geotechnical isinvestigation work be conducted prior to the final design of the vertical expansion to define the compaction characteristics and variability of the existing landfill refuse. It is also recommended WW-1811-CAl-AA/3110 Austin Road Landfill Expansion August 2,1995 ML1033.DOC Landfill Settlement Page 5