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3.5 LANDFILL SLOPE STABILITY A slope stability analysis is required by 27 CCR, Section 21090 when the closure design includes final slope faces steeper than 3:1 (horizontal to vertical) or a synthetic component in the final cover configuration. The proposed final slopes for the Forward Landfill do not exceed 3:1 and a synthetic component is not proposed, but the slope below the proposed 15-foot wide bench has an inclination of 2:1 (horizontal to vertical). Therefore, a slope stability analysis was conducted pursuant to 27 CCR, Section 21750(f)(5) to review the integrity of final slopes under both static and dynamic conditions. The results of the slope stability analysis are included in Appendix C. To evaluate the static stability of the proposed refuse cover system at various design inclinations and slope heights, the proposed cover configuration was analyzed by typical force equations. For this case, the soil cohesion was neglected. Static refuse cover slope stability (assuming a failure at the cover/MSW interface) for all three slope inclinations analyzed indicates a factor of safety greater than 1.5 To analyze the cover layer itself, the infinite slope method was utilized. This analysis assumes that the failure takes place within the soil cover itself. Two cases were analyzed: unsaturated and fully saturated. Static refuse cover slope stability for the steepest proposed slope inclination of 3.2:1 (horizontal to vertical) indicates an unsaturated factor of safety of 2.31 (typically has to be greater than 1.5) and a saturated factor of safety greater than 1.3 (typically a saturated factor of safety of 1.1 to 1.2 is adequate). The proposed 2:1 (horizontal to vertical) slope below the 15-foot wide benches was analyzed for static and dynamic stability by the use if the computer program SLOPE/W. The results of the static stability analysis indicate a factor of safety of 1.92. The three proposed cover inclinations and heights were analyzed for dynamic displacement. For analysis of the dynamic condition, the cover configuration was iteratively solved for various horizontal ground accelerations for a factor of safety of 1.0. Seismically induced permanent displacement was estimated using procedures described by Bray and Rathje (1998) and Bray et. al. (1998), which account for the failure wedge height, yield acceleration (ky), MHA, shear wave Forward Landfill PFCPCMP 3-9 J:\Allied\Forward\2005.0051 Partial Closure\Partial Closure Plan\FL-sec3.doc:6/15/2005