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SECTION 3 Slope Stability In accordance with Title 27 regulations, static stability analyses were performed for WMU FU-13. Seismic stability analyses were not performed because WMU FU-13 is an interior module and the interim refuse fill slopes will not coincide with the final landfill grades. Seismic stability analyses are typically only performed for slopes that will become part of the final cover of the landfill. WMU FU-13 will be filled with designated wastes (Class II). Refuse placed in portions of WMU FU-13 will overlie existing municipal solid waste fill (Class III). Interface liners will be placed in these areas to separate the designated and municipal solid wastes. Refuse placed on the northern side WMU FU-13 will be buttressed by waste placed in WMU FU-08 North and will form a contiguous mass. Stability analyses for WMU FU-08 North are documented in the Design and Construction Documents, WMU FU-08, Forward Landfill(Lewis Engineering 2008). The eastern and southern refuse fill slopes of WMU FU-13 are temporary and will be constructed at a 3:1 grade. Refuse will be placed against the 3:1 eastern and southern slopes once future WMUs to the east and south of WMU FU-13 are constructed. The western slopes will be filled against existing refuse slopes. Based on the geometry, construction against adjacent WMUs, and site development plans,the planned refuse fill elevation in WMU FU-13 is approximately 150-feet. The critical stability configurations were selected based on the Construction Drawings and WMU FU- 13 refuse grading plans in Appendix A and B, respectively. Three sections were analyzed as potentially critical for WMU FU-13. The sections included an east-west section, a north-south section, and a northwest-southeast section, as depicted in Appendix B. The critical section was determined to be the northwest-southeast section. The internal stability of the interface and side slope lining system was also considered. A protective operations soil layer will be placed against the slopes as WMU FU-13 filling operations progress(i.e., the operations layer will not be "hung" on the side slope). To minimize slope liner loads, it was assumed that the operations layer would be placed up to a maximum height of 10 feet above the refuse fill level using a wheel loader2. Minimal equipment loads will, therefore, be applied to the slope. 3.1 Analytical Method The slope stability analyses were performed using the computer program XSTABL. The program calculates slope stability using a limit equilibrium analysis based on the method of slices.The method of slices estimates slope stability by assuming a shear surface and calculating the forces that would cause slope movement, and the forces resisting slope movement for the selected shear surface. The ratio of available shear strength (resisting forces) to mobilized shear strength (driving forces) is known as the factor of safety. The computer programs employ a searching routine to determine the critical shear surface with the minimum factor of safety. A factor of safety equal to 1.0 under static loading conditions represents a condition of imminent failure. For temporary slopes, such as those in WMU FU-13, a minimum factor of safety of 1.3 under static loading conditions is considered adequate. 2 The protective operations layer will be placed on the slope using a Wheel Loader. WMU FU-13 REPORT.DOC 3-1