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9 3.2.1.3 Shear Strength Testin <br />Based on the above discussion and Shaw's past experience with similar liner systems, the <br />smooth HDPE geomembrane-geonet and the geonet-geotextile interfaces are considered <br />to have the lowest shear strengths within Modules 1 and 3. To determine the shear <br />strengths of these critical interfaces, direct shear tests (American Society of Testing and <br />Material [ASTM], 1995, Method, D532]) were performed on materials actually used in <br />the construction of these modules. The test samples were provided by the County. It is <br />our understanding that some of the samples were obtained directly from the field and <br />some of the samples were leftovers from construction. <br />The results of the direct shear testing are included in Attachment A. These test results <br />suggest that the large -displacement (or residual) friction angles for both the critical <br />interfaces noted above are greater than 12 degrees. Based on Shaw's past experience, the <br />measured interface friction angles are higher than the typical values for these interfaces <br />(i.e., 8 to 10 degrees). Therefore, to be conservative, a residual friction angle of 10 <br />degrees has been used in the slope stability analyses for Modules 1 and 3. <br />Geotechnical Analyses for NCRCSL Department of Public Works/Solid Waste <br />Shaw Environmental, Inc. County of San Joaquin, December 12, 2007 <br />3-3 <br />