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Interpretation of SGI Interface Shear Strength Test Results <br />Foothill Sanitary Landfill - Area 2A Liner System <br />Base Post -Peak Required Strengths <br />Brost W Stress S t <br />\01 <br />—.... Win£ 5 A - ea44N <br />Page 9 <br />8/4/15 <br />Figure 3: Base Liner System: (a) measured peak strengths, (b) measured post -peak <br />strengths, (c) combination post -peak strength envelope, and (d) comparison <br />of combination post -peak strength envelope and design required post -peak <br />strengths <br />The use of the post -peak strength envelope for the critical interface, i.e., GM/GCL, is again <br />confirmed by Multi -Interface Test Series 4, which yielded a slightly lower post -peak strength <br />® envelope than the single interface tests on the GM/GCL interface. In other words, the GCL/FLS <br />interface did not fail before the GM/GCL interface so the lower post -peak strength envelope <br />measured for the GCL/FLS interface should not be used for base liner design as described by <br />Stark and Choi (2004). <br />n <br />U <br />At a normal stress of 1,000 psf, shear displacement occurred on the GM/gravel interface instead <br />of the GM/GCL interface although the resulting strength is in agreement with other normal <br />stresses. This change in interface at low normal stresses is due to the gravel not embedding as <br />deeply into the HDPE geomembrane as occurs at higher normal stresses. Nevertheless, the <br />resulting post -peak strength (535 psf) at 1,000 psf exceeds the project required strength (404 <br />psf). Being able to observe this interface change and interaction is a benefit of multi -interface <br />testing. <br />Figure 3(c) shows the combination post -peak strength envelope developed using the procedure <br />set forth in Stark and Choi (2004), which essentially coincides with the post -peak strength <br />envelope for the GM/GCL interface because sliding occurred at this interface for all of the <br />normal stresses tested. Figure 3(d) adds the project specified strength envelope to the envelopes <br />in Figure 3(c) and shows the post -peak strength envelope of the GM/GCL interface meets or <br />exceeds the project specified strength envelope especially if a GCL peel strength greater than <br />47.5 lbs (ASTM D4632) is used. <br />9 <br />