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Interpretation of SGI Interface Shear Strength Test Results Page 10 Foothill Sanitary Landfill - Area 2A Liner System 8/4/15 Another consideration in deciding that the GCL/GM interface is the critical interface and that the post -peak strength of this interface should be used for design purposes is the field compaction conditions versus the laboratory compaction conditions for the FLS. SGI compacted the FLS material to a relative compaction of 90% based on Modified Proctor compaction (ASTM D 1557) and a moisture content of 23%, which corresponds to the optimum moisture content (OMC) +4% or the wettest condition allowed by project compaction requirements. The field compaction requirements for the FLS in the specifications are a relative compaction of greater than or equal to 90% based on Modified Proctor compaction (ASTM D1557) and a moisture content of OMC to +4% above OMC. Eighty-six (86) field compaction control tests provided by AES show the following field compaction levels for the base liner system: • Relative Compaction: 90 to 100% with an average of 95% • Compaction Moisture Content: OMC — 0.3% to +4.3% with an average of OMC + 1.8% As a result, it is anticipated the GCL/FLS interface will be as strong or stronger in the field than in the SGI testing assuming a high peel strength (>47.5 lbs) is used for the GCL because the stronger GCL/FLS will force internal GCL failure instead failure along the GCL/FLS interface. A stronger FLS will result in a stronger GCL/FLS interface, which reinforces the conclusion above that the GM/GCL interface will be the critical interface for the base liner system especially if a GCL peel strength greater than 47.5 lbs (ASTM D4623) is used. In summary, the critical interface for the base liner system materials tested by SGI is the GM/GCL interface. The GM/GCL post -peak strengths for the base liner system meet or exceed the project specified post -peak strength envelope. 5.0 Conclusions and Recommendations This letter report presents my interpretation of the results of single and multiple interface shear strength tests performed on soil/geosynthetic, and geosynthetic/geosynthetic interfaces for the sideslope and base liner system components proposed for the Foothill Landfill, Area 2A liner system. This testing and my interpretation resulted in the following conclusions and recommendations: Based on single and multi -interface direct shear tests, the critical interface for the sideslope liner system interfaces tested is the GM/GC interface. These tests also show the sideslope liner system components tested by SGI meet or exceed the project specified post -peak strength envelope for the sideslope liner system except at normal stresses of 1,000 and 4,000 psf where the measured interface strengths are slightly lower than the required values (3 65 & 372 v. 404 psf and 1,154 & 1,135 v. 1,174 psf). This small difference between the measured and required post -peak strengths at normal stresses less than 4,000 psf should not impact the factor of safety significantly. It is my 10