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L` <br />Taj Bahadori <br />July 13, 2015 <br />Page 3 of 6 <br />The minimum specified asperity for 60 -mil DST HDPE geomembrane is 20 mil. Actual asperity <br />height for two HDPE geomembrane roil samples used for interface tests was 30 and 31 mils. <br />Both of these index properties exceed the minimum specified values. The individual test results <br />are included in the attached SGI test report, Appendix A to this memorandum. <br />4.0 Interface Strength Test Results <br />The interface strength test results for all of the five (5) test series are summarized in Table 3 <br />attached, and individual laboratory test data sheets ae presented in SGI test report in Appendix <br />A. Table 3 lists normal stresses, minimum specified post -peak strengths and actual post -peak <br />shear strengths at the specified normal stresses. The table also lists the measured peak <br />strengths to assist in interpretation of the weakest interfaces for the composite liner system <br />(defined by the lowest peak strength), and for comparing the single interface strength test <br />results to multi -layer (sandwich) test results. <br />The following are our key observations and conclusions from the test results summarized in <br />Table 3. <br />4.1 Single Interface Test Series 1 through 3 <br />Test Series 1 — Foundation Laver/GCL Interface <br />® The post -peak strength at normal stresses 6,000 psf and above (6,000, 8,000 <br />and 12,000 psf) do not meet the specified interface strength values and showed <br />GCL internal failure. Looking at peak strength values for this interface, it would <br />appear that the peak strength is much greater than the internal peak strength of <br />soaked GCL at high normal stresses, and therefore, causes the rupture of GCL <br />needle punching resulting in internal GCL failure. The on-site foundation layer <br />soil is silty sand/sandy silt, with a relatively high friction angle and high peak <br />strength. The peel strength of GCL used for this test series was 47.5 lbs. <br />PAM Projects\14-101 ( SJCDPW North County and Foothill)\Cotrespondence\Memo - Foothill Interface Shear Strength Test Results.docx <br />