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TECHNICAL MEMORANDUM <br /> Page 8 <br /> June 3, 1992 <br /> ® SAC31583.AC.ZZ <br /> 3 <br /> typically ranged from approximately 4,000 psf to over 10,000 psf. Based on these CPT logs, <br /> a lower bound shear strength value of 2,000 psf was used in Insitu Tech's stability evaluation <br /> for WMU E (Insitu-Tech, 1987). <br /> Groundwater <br /> Groundwater levels could not be observed at the time of drilling because the borings were <br /> drilled using drilling mud. In the Comprehensive Landfill Description report (CH2M HILL, <br /> 1992), the shallow groundwater in the vicinity of the site was reportedly encountered at 70 to <br /> 80 feet below ground surface. Groundwater measured during EMCON's subsurface <br /> exploration in 1972 indicated groundwater at depths of between 55 and 60 feet below the <br /> ground surface. In one of EMCON's boring, in the southwestern corner of the site, aone- <br /> foot thick lens of perched water was encountered at a depth of 21 feet. Based on a review <br /> of available groundwater data, there is, however, little potential for significant perched water <br /> across the site. A review of the Comprehensive Landfill Description report (CH2M HILL, <br /> 1992) also indicated that the groundwater levels in the area have declined over the past 30 <br /> years because of groundwater pumping for irrigation and domestic use and because of the <br /> current drought. This reported decline in the groundwater level in the area partly explains <br /> the higher groundwater levels measured by EMCON in 1972. <br /> Based on these groundwater table depths an .available history of groundwater levels in the <br /> area, a groundwater table depth of 75 feet was assumed for purposes of the stability and <br /> settlement analyses. <br /> Results of Slope Stability Analyses <br /> Method of Analyses <br /> Slope stability analyses weFe performed using PCSTABL5M (J. Verduin, 1987), a computer <br /> program for analyzing general two-dimensional slope cross sections by limit equilibrium <br /> methods.. This computer program is a modified version of STABL (Siegel, 1975). This <br /> program was used to determine safety factors against slope failure for static (gravity) loading <br /> and to calculate yield accelerations for pseuostatic (earthquake) loading. Safety factors and <br /> yield accelerations were calculated using the modified Janbu procedure. <br /> Two cross sections were analyzed to estimate static safety factors. The location of these two <br />