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• 5.0 SLOPE STABILITY EVALUATION Slope stability analyses were performed on the proposed excavation slopes, refuse moving face slopes, and final landfill slopes under both static and seismic loading conditions, using the STABL computer program (Purdue University). STABL employs the Modified Bishop's Method for circular failure surfaces, and the Janbu Method for irregular failure surfaces. This section summarizes the an- alytical methods, discusses the analysis conditions, and presents the analysis re- sults. 5.1 METHOD OF ANALYSIS 5.1.1 Static Analysis Static stability analysis involves the calculation of a safety factor for assumed failure surfaces through representative slope sections. The static safety factor is defined as the ratio of the forces that act to preserve stability in a slope (resisting forces) with the forces and moments acting to make the slope unstable (driving forces). A safety factor less than about 1.1 (approaching unity) indicates a con- dition of impending slope failure. A static safety factor of 1.5 is the generally accepted minimum value for long-term landfill slope stability. A static safety fac- tor of 1.3 to 1.5 is the generally accepted range of minimum values for short-term slope stability. Critical failure surfaces were automatically determined for selected cross sections by the STABL program. The surfaces were randomly generated circular or irreg- ular geometries, or irregular surfaces forced through weak zones (block sur- faces). The calculated critical failure surface defines a slope mass with the low- est static safety factor. Horizontal forces were applied to the slope elements to determine the safety factor under seismic loadings. The magnitude of the hori- zontal force was varied iteratively to obtain the yield acceleration (Ky), which is the seismically induced horizontal acceleration that reduces the safety factor to unity (1.0). The yield acceleration was used in the seismic slope stability analysis to determine the slope deformations along critical failure surfaces. PJ9 9390217.00W 10 Rev. 0 May 23, 1989