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6.0 SUMMARY OF ANALYSIS AND RECOMMENDATIONS The Austin Road Landfill expansion was evaluated for mechanical compression settlement and decomposition settlement of the expansion final cover, the vertical expansion liner, and the horizontal expansion liner. The results from the analysis are summarized as follows: ■ Assuming a total settlement rate of 25% over the 30-year post-closure period and a consistent refuse settlement, the final minimum closure grade will decrease from the 5% design grade to 3.75%. ■ Assuming a differential settlement range of 22% to 28%, the final closure grade would vary from 1% to 6% for the 5% design grade areas. A greater range of differential settlement will result in some localized ponding. ■ A large void (3' x 3' x 6') in the existing landfill refuse would have to be within 10 feet of the final cover to produce ponding conditions for a 5% slope. The void would have to be within approximately 3 feet of the final cover to endanger the cover integrity. ■ A large void ('0' x 3' x 6') would have to be within 10 feet of the vertical expansion liner to produce ponding conditions for a 5% slope. The void would have to be within approximately 3 feet of the vertical expansion liner to endanger the geomembrane liner. The liner system can be reinforced for tensile strength with the inclusion of materials such as geogrids. ■ Compressive settlement of the existing landfill refuse under the vertical expansion liner is expected to range from approximately 2 to 4 feet. The 5% design grade after settlement will vary from 4.9% to 5.6% after settlement. ■ Assuming a 10-foot clay layer and a 30-foot silty clay layer subgrade for the horizontal expansion area, up to 1.7 feet of settlement for the horizontal expansion liner subgrade could be expected. The post settlement grades for the leachate collection system with 1% design grade would vary from 0.7% to 0.9%. The following summarizes the recommendations from this report: ■ The issue of cover and liner integrity relative to differential settlement should be reviewed and reanalyzed at the time of final design based on the final design materials and detailed design. ■ Additional geotechnical investigation work be conducted prior to the final design of the vertical expansion to define the compressive characteristics of the existing landfill refuse. It is also recommended that the existing landfill post-closure settlement monitoring program be planned to provide background data for the final design of the vertical expansion. WW-1811-CAI-AA/3110 Austin Road Landfill Expansion August 2,1995 MLI033.DOC Landfill Settlement Page 8