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C, <br />ANALYSES3 MATERIAL PARAMETERS FOR SLOPE STABILITY <br />3.1 Subsurface Conditions <br />The summary of subsurface conditions provided in this section is based on foundation <br />soil description in the existing Slope Stability Evaluation, North County Sanitary <br />Landfill, San Joaquin County, California (EMCON Associates, 1988). <br />The foundation soils to a depth of 100 feet from the ground surface consist of medium <br />dense silty sand (SM) and clayey sands (SC) with occasional medium stiff sandy silt <br />(ML) and sandy clay (CL) interbeds. The standard penetration resistance (N) values <br />varied from 19 to 68 with an average of 40. Below 100 feet from the ground surface, the <br />soils generally consist of medium dense to dense, poorly graded sand (SP), and gravelly <br />sand (SP -GP), with average standard penetration blow counts of about 50 for 2 inches of <br />penetration. <br />Groundwater level data from the existing monitoring wells indicate a groundwater level <br />isof 125 to 150 feet below ground surface. <br />3.2 Shear Strength of the Liner System <br />The subsurface conditions described above suggest that the landfill subgrade has higher <br />shear strength relative to the shear strength of the landfill liner system. Therefore, failure <br />surfaces through the liner system are more critical than failure surfaces through the <br />landfill subgrade. In other words, the slope stability of the landfill is controlled by the <br />internal/interface shear strength of the liner system. The shear strength of the liner <br />systems within both existing and future modules is discussed below. <br />3.2.1 Liner Systems/interfaces within Modules 1 and 3 <br />3.2.1.1 Module 1 <br />The existing base liner system within Module 1 consists of the following components <br />(starting from the bottom): <br />• 6 -inch compacted soil Layer <br />• 60 -mil smooth HDPE geomembrane <br />• Geonet <br />• 10 ounce per square -yard (oz/yd2) non -woven geotextile <br />Geotechnical Analyses for NCRCSL Department of Public Works/Solid Waste <br />Shaw Environmental, Inc. County of San Joaquin, December 12, 2007 <br />3-1 <br />