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W. MICHAEL CARROLL, PE COUNTY OF SAN JOAQUIN DEPARTMENT OF PUBLIC WORKS DECEMBER 22, 2009 modeled as having a friction angle of 12 degrees. The waste failure envelope defined by Kavazanjian et al. (1995) as presented by Duncan and Wright (2005) was used. The resulting static FS for the revised cross section analyzed is 1.50 and estimated permanent seismic displacement is < 0.1 inch. Detailed output is included in Attachment B. Testing should reflect the range of stresses to be encountered. Based on the proposed height of the landfill, maximum stresses will be on the order of 20,000 psf. Because failure envelopes of liner interfaces are actually curvilinear and flatten out at high stresses, it is important to consider strength tests at the maximum stresses. Current testing seems to be limited to 10,000 psf or less. While the testing of liner materials used in Modules 1 through 4 was limited to a maximum normal stress of 10,000 psf, friction angles used in modeling were conservative compared to testing results, as discussed above. Note that Modules 1 through 4 were designed and constructed prior to the vertical expansion concept. Therefore, normal stresses used during previous interface shear testing considered the maximum design height and resulting maximum normal stresses on the liner at the time. The maximum waste height in Module 1 is about 140 feet resulting in a maximum normal stress of about 11,000 psf. We believe the reported testing and associated results are adequate for Module 1. The maximum waste height in Module 3 is about 190 feet resulting in a maximum normal stress of about 17,000 psf, but the normal stress on much of the liner is between 10,000 and 14,000 psi Test results of the geonet-smooth geomembrane interface present in Module 3 performed during the construction phase indicated a large displacement friction angle > 12 degrees for a maximum normal stress of 10,000 psf. However, to be conservative a value of 10 degrees was used in the modeling. We believe the use of a conservative 10 degree friction angle compensates for the potential flattening out of the failure envelope at higher normal stresses. Also, the calculated factor of safety for Section 2-2' through Module 3 is > 2, indicating that use of an even more conservative liner interface shear strength would likely still result in an acceptable static FS. As discussed previously, testing for Module 4 was conducted for normal stresses up to 8,000 psf. We constructed the bilinear liner failure envelope shown in Figure B based on the liner interface strength specification of 10.5 degrees. Section 6-6', shown in Figure 8, was cut through Module 4. The location of the section is shown in Figures 1 and 2. The slope stability analysis resulted in a static FS of 1.43 and an estimated seismic displacement of 6.2 inches. We believe our assumptions are conservative and the slope is stable. As discussed above, tests of textured HDPE geomembrane-GCL interface strength at normal stresses up to 20,000 psf were performed for slope stability analysis of the future modules. Detailed results are presented in Attachment A. A multi -linear failure envelope, defined in Figure A, was developed using the large displacement shear strengths from testing and applied to the slope stability analysis of future modules (5 through 11). Results are summarized in Table 1. Detailed output is included in Attachment B. The static factors of safety are > 1.5 and the estimated permanent seismic displacements are < 1 inch. Additionally, other available GCL-textured geomembrane interface tests obtained from CETCO and GSE, included in Attachment C, show that a large displacement friction angle of 12 degrees is achievable for normal stress up to 21,600 psf. All included tests allowed for internal failure of the GCL PAGE 7