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0 GeoSyntec Consultants <br />Monolithic covers have also been used in closure designs at the Coyote Canyon, <br />Milikan, and Phelan landfills. <br />Forward will evaluate the appropriateness of a monolithic final cover at the landfill <br />site. If appropriate, Forward will propose a monolithic final cover design to be <br />incorporated into the closure construction and an amended CQA Plan. <br />3.4.2.3 Landfill Slope Stability <br />A stability and settlement analysis was performed by CH2M HILL [1994] for the <br />entire Forward Landfill as part of the Preliminary Closure and Post -Closure <br />Maintenance Plan (see Appendix D). The landfill stability analysis was performed for a <br />proposed final grading plan with 3H:1V slopes to a final elevation of 210 -ft. Multiple <br />shear surfaces were evaluated through the subgrade and along the future geomembrane <br />lined Class II disposal Units. Subsurface, waste, and base liner strength properties were <br />based on a geotechnical investigation and typical values from the literature [CH2M <br />HILL, 1994]. <br />Static loading factors of safety were found to be greater than 1.5, and permanent <br />seismic displacements were estimated to be negligible (less than 1 -inch) during the <br />maximum credible earthquake (MCE). The critical shear surface was found along the <br />interface between the geomembrane and compacted clay liner for future lined WMUs to <br />be constructed within the expansion area of the landfill [CH2M HILL, 1994]. <br />3.4.2.4 Final Cover Slope Stability <br />Per §21090, final cover designs having any slopes steeper than 3H:1 V, shall be <br />evaluated with a slope stability analysis. As the WMU B design includes existing <br />2H:1 V slopes, a final cover stability analysis has been performed under static and <br />seismic loading conditions. Since the cover length to thickness ratio is very large, an <br />infinite slope analysis approach was followed. The general equation for calculating <br />factor of safety (FS) for an infinite slope of soil thickness (z) above a weak (failure) <br />plane, as proposed by Matasovic [1991], is given by the following equation (similar <br />equations with special cases are also presented in Lambe and Whitman [1969] and <br />Sharma and Lewis [1994]): <br />J:\ALLIED\FORWARD\CLOSUREPLAN\REPORT 3-9 99.11.30 <br />0 <br />