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Interpretation of SGI Interface Shear Strength Test Results Page 3 Foothill Sanitary Landfill - Area 2A Liner System 8/4/15 • Material Property Specified Property Tested Property GM Asperity Height 20 mils 30 and 31 mils GCL Peel Strength 15 lbs (D4632) 47.5 to 58.1 lbs (D4632) or GCL Peel Strength 2.5 ppi (D6496) 11.9 to 14.5 ppi (D6496) FLS Relative Compaction 90% Mod. Proctor 90% Mod. Proctor (98.5 pcf) FLS Moisture Content OMC to +4% (19 to 23%) OMC + 4% or 23% The interface strength test results for the five (5) test series are summarized in Table 1 below and plotted to assess the minimum interface strength envelope for the sideslope and base liner systems. The detailed SGI test results are presented in Appendix A of the SGI Test Report and only the peak and post -peak strengths are presented in Table 1. The following sections discuss the measured, operational, and required interface strength envelopes for the sideslope and base liner systems separately. 3.0 Sideslope Liner Interface Test Results Figure 2 presents the interface strength test results for the sideslope liner system that are presented in Table 1. Each of the strength envelopes are keyed to the Test Series name in Table 1. For example, Figure 2(a) presents the peak interface strengths for Test Series 1 through 3 because these single interface test series were specified to assess the strengths of the various sideslope liner system components. In addition, the multi -interface test series for the sideslope liner system (Test Series 5 — Sideslope Multiple) with all of the interfaces tested in Test Series 1 through 3 is included for comparison purposes. The single interface tests in Figure 2(a) show the weakest or critical interface for the sideslope system is the GM/GC interface. This critical interface is confirmed by Multi -Interface Test Series 5, which shows shear displacement occurred along the GM/GC interface for all of the normal stresses tested, which matches the GM/GC interface being the critical interface in all of the single interface tests. In addition, the multi -interface tests yielded essentially the same strength envelope as the single interface tests, which is in agreement with similar findings in Stark et al. (2015). This agreement between single and multi -interface tests confirms the critical interface for the sideslope liner system materials tested is the GM/GC interface. If the geosynthetics and soils used during actual construction differ from the materials tested by SGI, the critical interface and lowest peak strength envelope could change which would change the critical post -peak strength envelope. Using the design procedure set forth in Stark and Poeppel (1994) and refined in Stark and Choi (2004), geosynthetic lined slopes should be designed using the post -peak strength envelope for the critical interface. As a result, the post -peak strength envelope for the GM/GC interface in Figure 2(b) should be used for design of the sideslope liner system even though the post -peak envelope for the GCL/FLS interface is lower for normal stresses greater than about 5,000 psf The GCL/FLS post -peak strength envelope should not be used for sideslope design because the GM/GC interface will fail before the GCL/FLS interface fails so a post -peak condition will not develop along the GCL/FLS interface before the GM/GC fails as described by Stark and Choi (2004).