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FORWARD LANDFILL NON-WATER CORRECTIVE ACTION PLAN • and unit weight were based on average values from the testing performed on samples obtained from previous studies (GLA, 2008a and 2008b). Parameters for the MSW are based on published information(Singh and Murphy, 1990). Shear strength of geosynthetic interfaces were obtained from previous interface shear testing of actual materials used onsite (Lewis Engineering;personal communication, 2011) and generally accepted interface shear strength parameters (Koerner, et.al., 2005). A summary of the geotechnical material parameters used in the slope stability analyses is presented in the following table. Geotechnical Material Parameter Strength Summary Unit Weight Friction Angle Cohesion/ Material (pcf) (degrees) Adhesion ( sf) Native Soils: Victor Formation(average of properties of clayey to sandy silt and silty 115 30 650 sand for non-lined landfill areas;modeled as bedrock for lined landfill areas) Refuse Fill(MSW) 80 30 200 Non-woven Geotextile/Textured HDPE Interface(base liner),peak strength 10* 21 210 Non-woven Geotextile/Smooth HDPE Interface(slope liner),residual strength 10* 7 0 Textured HDPE/Low-Permeability Soil Liner,(peak strength) 10* 18 1 210 GCL Internal Strength;(slope liner),residual strength 10* 5 400 Notes: "/"Implies interface. *Not significant in the analysis. By review of the above properties, in lined landfill floor areas, the NW geotextile to textured HDPE interface (friction angle=21 degrees; cohesion=210 psf) is a stronger interface than the HDPE to low-permeability soil liner interface(friction angle= 18 degrees; cohesion=210 psf). As appropriate, the stability analysis herein used the weaker(HDPE/low permeability soil) interface in the calculations for the floor areas. For the lined slope areas, the NW geotextile/smooth HDPE (residual strength) was used to model this interface strength. In the south central portion of the southern landfill area, the Ash Pit has a GCL covering the ash at a depth below the existing landfill slope. Two critical interfaces were modeled for this GCL covering, the textured HDPE on the GCL/NW geotextile interface, as well as the internal strength of the GCL, because it was not readily apparent which interface was more critical. Standard practice within the solid waste industry is to use peak shear strength parameters for base liners and large deformation(a.k.a., residual) shear strength parameters for slope liners (Stark and Choi, 2004; Koerner and Bowman, 2003; Gilbert, 2001; Stark and Poeppel, 1994). The basis for this approach is that refuse settlement along the slope liner would cause displacement and mobilize residual strength conditions, whereas with the base liner, no such corresponding pre-shearing occurs. This procedure was followed for this analysis. The analysis MS2011-012MWFA REPORT.DOCX 12 Geo-Logic Associates