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W. MICHAEL CARROLL, PE COUNTY OF SAN JOAQUIN DEPARTMENT OF PUBLIC WORKS DECEMBER 22, 2009 . 0 per inch per ASTM D6496 should result in a GCL product that should not fail internally under high normal stress conditions. Specifications for all future modules will require interface shear testing of the materials to be used for module construction. Therefore, actual interface testing completed during each module construction will allow confirmation of the critical interface. The specified linea- interface strength for Module 4 in the 2007 analysis was a friction angle of 10.5 degrees up to a normal stress of 8,000 psf. However, because proposed normal stresses in Module 4 will exceed 8,000 psf, we re-evaluated the interface failure strength. We have assumed a revised, bilinear failure envelope that has a friction angle of 10.5 degrees up to a shear strength of 2,000 psf at 10,790 psf normal stress and a constant shear strength of 2,000 psf for normal stress > 10,790 psf. We believe this failure envelope, shown in Figure B, is conservative as this envelope represents data well below median values. We added Section 6-6', cut through Module 4, to the previous analyses. The location of Section 6-6' is shown in Figures 1 and 2 and the section is presented in Figure 8. Slope stability analysis of Section 6-6' resulted in a static FS of 1.43 and an estimated seismic displacement of 6.2 inches, which are unacceptable per DWR standards. To achieve acceptable results for the conditions represented in Section 6-6', we lowered the final grades over Module 4 to achieve a minimum static FS of 1.5. The top deck elevation over Module 4 was iteratively lowered by 17 feet below the previous grade while maintaining the 5% surface slope toward the west outer slope. The revised grades are reflected in Figure 2. Figure 8 shows the revised Section 6- 6'. The estimated seismic displacement is 3.5 inches. Results are summarized in Table 1 and detailed output is included in Attachment B. We believe our assumptions are conservative and that analysis results indicate the slope is stable under ultimate fill height conditions. 0 Cross Section 1-1' includes a short section of smooth geomembrane/soil interface that is modeled using a friction angle of 17 degrees. The value of 17 degrees does not seem to be discussed in other portions of the report. The value appears too high and unconservative. The side slope liner system in Module 1 is discussed in Section 3.2 of Appendix E of the referenced report; however, the assigned interface shear strength of 17 degrees is not discussed in the text. The operations soil layer on the side slope of Module 1 is underlain by a smooth HDPE geomembrane. Initial modeling of Section I-1' through Module 1 assumed a friction angle of 17 degrees for the side slope section of the liner system based on this interface. We believe a friction angle of 12 degrees to be more appropriate and conservative. The maximum normal stress on the side slope is less than 1500 psf. Interface friction angles tend to increase with reduced normal stress. Our experience indicates that sand/smooth HDPE geomembrane interfaces can have a large displacement interface friction angle of up to 20 degrees for normal stresses up to 5000 psf. Additionally, no adhesion was included in the model. The inclusion of adhesion in the stability analysis, even with a reduced friction angle, would result in increased shear strength for the low normal stresses applied on the side slope liner in Module I and a corresponding higher FS. Koerner and Koerner (2007) show that even a small increase in adhesion can influence the factor of safety significantly. Section I-1' was remodeled with the adjusted strength parameter, not including adhesion. The resulting static FS was 1.50, which is acceptable. Also, during our reanalysis, we realized that Section 1-1' did not include the proposed final access road shown in Figure 2 of Appendix E. A representation of Section 1-I' that reflects the currently proposed final grading was modeled. Furthermore, the smooth geomembrane/soil interface on the side slope was PAGE 6