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File No. 20-1008-48 KLEINFELDER January 3 , 1992 Native Underlying Soil For the purposes of this stability analysis, we have assumed the native, undisturbed underlying soil to have the same strength and density parameters as the overlying closure cap. We have assumed the underlying soils to have an in-place moist density of approximately 120 pcf, a cohesion value of 1000 psf, and a friction angle of 30 degrees. We note that blow count information indicates that portions of the underlying native soil are slightly cemented. This information was considered in the selection of strength properties. STABILITY ANALYSIS Using the values described above, we performed a slope stability analysis on the steepest profile provided to our firm which indicated a 2 : 1 side slope for the landfill. Our slope stability analyses were performed using the computer program 11TSTAB, 11 which uses the Bishop's modified method of analysis. The 11TSTAB11 program analyzes circular slip surfaces and has the capability to search for critical circles. Variations of slope geometry and properties can also be made easily with this program. The effect of ground shaking on the computed factors of safety was made by applying a site acceleration of .20g. As discussed in the geology section of this report, the Harney Lane landfill site in general is considered to be in a low seismic area. our design value of i, 20g correlates well with accelerations by Seed and Idriss 1985, Campbell 1981 and Joyner and Bore, 1981. Based on our assumption, the computed minimum factor of safety for the 2 : 1 slope configuration was 1.5. This meets the requirement of Section 17777 , A. B, and C. We note that in the program 11TSTAB, 11 horizontal tangents are defined to which the critical circle must be tangent. Once a limiting tangent is specified, the program is instructed to search for the center of a critical circle defined as that circle which develops the lowest factor of safety. In our analysis, five tangents were specified for the two slope configurations evaluated. One slope configuration, corresponding to the primary landfill, does not incorporate a soil berm at the toe of the slope. The minimum factor of safety against slope failure by circular slip calculated for the slope was found to be 1.5. Another slope configuration which utilized a 15 foot wide soil berm at the base of the slope, corresponding to the secondary portion of the landfill, was found to have a minimum factor of safety of 2.3 , as analyzed. We note that due to the configurations of the soil berm for the secondary site and by the method which the program TSTAB analyzed the geometry of the Copyright 1992 KleinfeLder, Inc. Page 8 of 9