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5.3.4.5 VOID SETTLEMENT AND POTENTIAL LINER DISTRESS As shown in Appendix F, R. W. Beck (1995) also evaluated the magnitude of refuse settlement that could endanger the integrity of the new landfill liner system. Beck assumed that a 60-mil HDPE geomembrane would be installed over existing wastes and native soils, with an initial minimum slope gradient of 5%. The interim final cover system (as discussed in Section 5.2.2.3) is proposed at a 3 to 5 percent minimum gradient, has no geomembrane component and is only intended to serve as an infiltration mitigation measure during placement of the Class III waste above the existing Austin Road refuse. The new Class III waste will be placed with a 5:1 slope ratio on the south facing slope. The 5:1 slope (20 percent gradient) will be overlaid by the proposed intermediate liner system. Beck's calculations indicate that a large void (3 ft. x 3 ft. x 6 ft.) within the existing refuse would need to be located within 10 feet of the new liner to produce ponding conditions on the liner, and would need to be within 3 feet of the liner to endanger the geomembrane. Since the materials that currently exist within the landfill cannot be controlled, a buffer between the liner and potential voids must be installed. The interim final cover is a minimum of four feet thick. Additionally, the new Class III waste placement within 10 feet of the intermediate liner will be monitored to minimize the potential for voids by load checking and waste disposal inspections to verify that large, void promoting decomposable materials are not disposed within 10 feet of the intermediate liner subgrade. A four-foot thick soil zone (interim final cover) (calculated by R.W. Beck to be a sufficient buffer), the monitoring of Class III waste placement within 10 feet of the intermediate liner and 20 percent gradient shall serve to help assure the integrity of the liner and leachate collection systems. 5.3.4.6 IMPLICATIONS TO LINER AND COVER DESIGN Based on the settlement analyses described above, the proposed overall landfill grades are not expected to change significantly after final closure. The anticipated differential settlements are calculated to be small and are not likely to result in significant change in the gradients of the LCRS or ponding of water on the final cover. With appropriate load checks and waste inspections, the integrity of the final cover is not likely to be impacted by the anticipated Forward Landfill JTD 5-20 L\,Allied\2000-793\Reports\..JTD-PCPCMP702:Sec-5.0:5/24/02 Revision 1:07/16%02 BRYAN A.STIRRAT S ASSOCIATES