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Interpretation of SGI Interface Shear Strength Test Results Foothill Sanitary Landfill - Area 2A Liner System Page 11 8/4/15 ® understanding g g the Design Engineer has been asked to obtain a variance to the contract specifications for this difference and they approved the use of the supplied GM and GC materials. These findings are in agreement with AES (2015). • Based on single and multi -interface direct shear tests, the critical interface for the base liner interfaces tested is the GM/GCL interface. These tests also show the base liner system components tested by SGI meet or exceed the project specified post -peak strength envelope. These findings also are in agreement with AES (2015). • If the geosynthetics and soils used during actual construction differ from the materials tested by SGI, the critical interface and lowest post -peak strength envelope could change. • If the design is based on single interface testing, it is recommended that different required strength envelopes be used for different liner system configurations because the critical interface is dependent on the components included and the applied normal stresses (Stark and Choi, 2004). • Stark et al. (2015) and ASTM D7702 discuss the advantages and disadvantages of multi - interface tests. In practice, multi -interface tests are being used for liner system design ® and if there is an anomaly with the multi -interface test results, single interface tests can be conducted to clarify the test results. However in this project, single interface tests were specified and multi -interface tests were used to verify the combination strength envelope from the single interface tests. Regardless, the multi -interface strength envelopes measured herein are in agreement with the single interface test results, which matches the data in Stark et al. (2015). If you have any questions or if I can provide any additional information, please contact me. Sincerely yours, Timothy D. Stark, Ph.D., P.E. v 11