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Mr. Michael Carroll -5- 30 November 2011 6. End-of-roll overlapped seams should be constructed with a minimum overlap of 24 inches (600 mm) for Bentomat ST. Seams at the ends of the panels should be constructed such that they are shingled in the direction of the grade to prevent the potential for runoff flow to enter the overlap zone. 7. To avoid seam separation, the GCL should not be put in excessive tension by the weight or expansion of textured geomembrane on steep slopes. The project Engineer should be consulted about the potential for GCL tension to develop. Regional Board staff is concerned that without the design engineer specifying the panel layout for the proposed cover it is impossible for staff to determine if the design engineer took into consideration the constraints listed above that are necessary to meet published performance specifications for the GCL. For example, the 9% slope does not show intermediate anchor trenches perpendicular to the downward slope to secure GCL running parallel to the 300 foot slope. The alternative is to run 12 foot minimum wide GCL panels perpendicular to the 9% slope which is suspect due to its close proximity to the limitation (9% slope versus 10% slope) identified in item 4. above. c. Bentomat ST Guidelines 3.3.113. "Subgrade surfaces consisting of granular soils or gravels may not be acceptable due to their large void fraction and puncture potential. In applications where the GCL is the only barrier, subgrade soils should have a particle- size distribution at least 80 percent finer than the #60 sieve (0.25 mm)." Concerns: The proposed design for the CHSL cover extension is a single barrier GCL over a 1 foot thick subgrade above the refuse. The design engineer does not specify the particle-size distribution for the subgrade that meets or exceeds the Bentomat ST guideline specifications. The smaller the particle size in the subgrade the lower the permeability of the subgrade which provides additional groundwater protection. d. Bentomat ST Guidelines 3.3.F. "At the top of sloped areas of the job site, an anchor trench for the GCL shall be excavated or an equivalent runout shall be utilized in accordance with the project plans and specifications and as approved by the CQA Inspector." Concerns: The current design proposes laying the GCL over the top of the existing barrier soil. The design does not provide adequate anchoring of the GCL. The current design does not include intermediate anchor trenches along the 9% slope if the design engineer envisions running the GCL parallel to the downward 9% slope. e. Bentomat ST Guidelines 3.9.A. "Cover soils shall be free of angular stones or other foreign matter that could damage the GCL. Cover soils should be approved the project Engineer with respect to particle size, uniformity and chemical compatibility. Cover soils with high concentrations of calcium (e.g., limestone, dolomite) are not acceptable." Concerns: Sodium based GCLs should not be placed in an environment where ion exchange can occur with the sodium ions. Such exchange reduces the swelling characteristics of the GCL allowing greater opportunity for leakage. The Discharger must perform testing on the overlying earthen fill to ensure that it does not contain unacceptable levels of calcium.