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that the existing landfill post-closure settlement monitoring program be planned to provide background data for future design of the vertical expansion. 4.2 VOID SETTLEMENT The elastic differential settlement model discussed in Subsections 3.1 and 3.2 above is applicable to the analysis of liner strains and potential ponding conditions due to voids forming under the vertical expansion liner. This is because the liner actually has less stress for the equivalent deformation due to compression by the overlying refuse weight"'. For this assessment of the expansion liner geomembrane, the same assumptions have been made as for the cover geomembrane: • Geomembrane is 60 mil HDPE • Maximum allowable strain in geomembrane is 1% • Minimum slope for the vertical expansion liner is 5% Based on these assumptions, the minimum depth for a large void to create ponding conditions would be 10 feet, and the minimum depth to endanger the geomembrane would be 3.3 feet. it is likely that surface effects of any voids existing at the time of construction will be filled during the foundation grading for the cover system. It is anticipated that the final cover of the existing landfill will have a total of four feet of soil material, this could be sufficient to provide adequate support for the geomembrane. A common method of evaluating vertical expansions is to assume that a void develops directly beneath the liner leaving the liner unsupported. Based on the discussion above,this may be an overly conservative approach. However, an analysis of this condition will be conducted here as a means to illustrate liner system reinforcement. A typical method used for protecting the liner system from excessive strain is to use a geogrid for tensile reinforcement under the liner layers. The method of analysis is one proposed by Berg and Collin(s) for design of landfill liners over yielding foundations. For the purposes of demonstration a high strength biaxial HDPE, geogrid material is assumed, and a maximum allowable strain of approximately 3% was assumed for the geomembrane. Based on the analysis,a single high strength geogrid can limit the strain of the liner to 3% for voids directly under the liner of approximately 5 feet in diameter,with a factor of safety greater than 4 (see attached Calculation No. 4). The issue of liner integrity should be reviewed and analyzed at the time of liner system design based on the proposed geosynthetic materials to be used. 4.3 LANDFILL IMPROVEMENT METHODS There are methods that can be used to decrease the settlement of landfills. Generally, three techniques are used: preloading, dynamic compaction, and grouting. Preloading involves placing a deep layer of soil over the landfill to promote consolidation. Dynamic compaction is involves dropping a large weight from a crane to compact the soil. Grouting involves injecting the waste with a mortar-type mixture. All of these methods could be excessively expensive if WW-1811-CAl-AA/3110 Austin Road Landfill Expansion August 2, 5_95 MLI033.DOC Landfill Settlement Page 6