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0 GeoSyntec Consultants <br />FS = (c/(yZcos2p) + tan � [1-y,,, (Z - d,) / yZ] - ks tan P tan (ks + tan (3) <br />where: <br />FS - factor of safety <br />c and � - soil or soil/geosynthetic interface strength parameters along the failure plane; <br />P - slope angle; <br />d,,, - depth of water below ground surface; <br />y - unit weight of materials; <br />yW - unit weight of water; and <br />ks - the seismic coefficient. <br />The assumed material properties used in the final cover stability analyses and the <br />sources of information are listed in Table 3-2. The shear strength parameters are based <br />on a normal stress range of 100 to 300 pounds per square foot (psf) (5 to 15 kPa) at <br />large shear displacement. Prior to construction, material specific interface direct shear <br />tests (ASTM D 5321) will be performed to confirm the assumed shear strengths used in <br />the final cover slope stability analysis. <br />The static stability was evaluated in terms of acceptable factor of safety against <br />slope failure for both long-term, dry conditions and short-term, fully saturated <br />conditions. The factor of safety is defined as the ratio of available shear strength (i.e., <br />resisting forces) to mobilized shear strength (i.e., driving forces). The generally <br />accepted static factor of safety for the long-term loading condition is 1.5 [Duncan, <br />1992]. For specific conditions, such as during or soon after a heavy rain storm, the <br />generally accepted static factor of safety is 1.2. The seismic stability was evaluated in <br />terms of acceptable levels of seismic deformation. For this analysis, the acceptable <br />level of permanent seismic displacement within the final cover system was assumed to <br />be 12 -in. (300 -mm) or less [Seed and Bonaparte, 1992]. <br />J:\ALLIED\FORWARD\CLOSUREPLAN\REPORT 3-10 99.11.30 <br />0 <br />