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5.4 SLOPE STABILITY ANALYSES 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 with the assumption that a 1 -foot -thick clay liner will be used at the site, using the STABL computer program (Purdue University). STABL em- ploys the Modified Bishop's Method for circular failure surfaces, and the Janbu Method for irregular failure surfaces. This section summarizes the analytical methods, discusses the analyses conditions, and presents the analyses results. 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 ac- cepted minimum value for long-term landfill slope stability. A static safety factor 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). Table 2 (see Appendix B) summarizes the results of the static stability analyses. Critical failure surfaces under individual scenarios are shown on Figures 6,_7, and 8 (of Appendix B), respectively. In general, the static safety factors are 1.6 to 6.3 and are greater than the minimum acceptable safety factor of 1.5. Because the refuse strength parameters are generally weaker than the foundation soils, criti- cal failure surfaces are in the refuse as shown on Figures 6 and 8 (see Ap- pendix B). The Newmark Method of analysis (1965) was used to evaluate the stability of slopes under seismic loading conditions. This method evaluates the slope stability in terms of the permanent slope displacements that result from the seismic loading. The procedure is based on the assumption that a slope will PJ9 9390218A.00W 41 Rev. 0 July 20, 1989