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TECHNICAL MEMORANDUM Page 10 June 3, 1992 SAC31583.AC.ZZ purposes of the slope stability and settlement analyses, the unit weight of refuse for the proposed landfill extension was assumed to be 50 pounds per cubic foot. For the composite lining, both smooth and textured FML liner were considered in the slope stability analyses. For smooth FML liner, a conservative FML -clay liner interface strength based on residual friction angles on critical interfaces in the Kettleman Hills Landfill Liner System was used (Mitchell et al., 1990). Only limited interface strength properties for textured FML -clay liner is available in the literature. Published friction angles of soil to rough PVC geomembrane ranged from 25 to 27 degrees (Martin et al., 1980). Analyses performed on the Kettleman Hills Landfill Liner System have shown, however, that interface friction angles may be greatly reduced in a saturated condition (Mitchell et al., 1990). Direct shear testing performed for the Keller Canyon Landfill project in Contra Costa County, California, confirmed this trend. For the Keller Canyon project, direct shear testing of FML -clay interface under unsaturated and under saturated conditions confirmed that friction angle may be significantly reduced in saturated conditions (CH2M HILL, 1991). For unsaturated conditions, friction angles ranged from 28.4 to 31.8 degrees with cohesion intercepts of 310 to 809 psf. For saturated conditions, friction angles ranged from 8.5 to 9.5 degrees with cohesion intercepts of 420 to 697 psf. Because of this relatively wide range in properties, both unsaturated and saturated conditions were analyzed for the textured FML liner using direct shear test results from the Keller Canyon Landfill project. UU triaxial compression shear strength and pocket penetrometer test results were used to estimate the shear strength of clay layers. The use of unconsolidated undrained shear strength properties and pocket penetrometer test results for clay layers likely resulted in conservative static safety factors because UU and pocket penetrometer shear strength tests do not take into account consolidation effects of the foundation material under refuse fill loads. Consolidation of the foundation clay material as refuse fill material is placed will result in higher shear strength properties. SPT blow -counts were used to estimate friction angles for sandy layers (sand, silty sand, clayey sand). A summary of material shear strength properties and unit weights used in stability analyses is presented in Table 1. In general, the shear strength properties summarized in Table 1 represent a conservative lower -bound range of the available data.