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Taj Bahadori <br />July 13, 2015 <br />Page 5 of 6 <br />Test Series 3 — GM/Geocomposite (GC) Interface <br />• The post -peak strengths for this interface for the side slope liner met the <br />minimum specified values at normal stresses of 6,000, 8,000 and 12,000 psf, but <br />were slightly below the specified values at normal stresses of 1,000 and 4,000 psf. <br />• The specified minimum post -peak value of 404 psf at 1,000 psf normal stress <br />corresponds to a secant friction angle of 22 degrees, which is unrealistic and <br />practically unachievable for the materials specified. The measured post -peak <br />strength was 372 psf, corresponding to a secant angle of 20 degrees which is in <br />line with available data base (typically 18 to 20 degrees). <br />• The post -peak strength at 4,000 psf normal stress was measured at 1,154 psf, <br />slightly below the specified value of 1,174 psf. <br />4.2 Multi -Laver (Sandwich) Interface Tests <br />Test Series 4 — Base Liner <br />• The test results indicate that the specified post -peak strength values were <br />achieved at all 5 normal stress points. <br />• The sliding occurred on the GCL/GM interface. At 12,000 psf normal stress, it <br />eventually resulted in GCL internal failure but the measured post -peak strength <br />was 1,575 psf vs. specified minimum value of 1,555 psf. This result was on GCL <br />sample with the lowest peel strength (47.5 lbs). The retest, at 12,000 psf normal <br />stress, using GCL sample with a peel strength of 53.6 lbs, showed a post -peak <br />strength of 2,391 psf with partial internal failure of GCL. <br />• This sandwich test, therefore, confirmed that the lowest post -peak strength was <br />measured on the GCL/GM interface. The test results from either the sandwich <br />Test Series 4 or from the single interface test, Series 2 for GCL/GM interface, <br />may be used for the base liner in stability calculations. <br />Test Series 5 — Side Slope Liner <br />• Both the peak and post -peak strengths for Test Series 5 are in close agreement <br />with single interface Test Series 3 for GM/GC interface. The failure/sliding <br />occurred on GM/GC interface for all 5 normal stresses. <br />PA14 Projects\14-101 ( SJCDPW North County and Foothill)\Correspondence\Memo - Foothill Interface Shear Strength Test Results.docx <br />