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W. MICHAEL CARROLL, PE COUNTY OF SAN JOAQUIN DEPARTMENT OF PUBLIC WORKS DECEMBER 22, 2009 permitted height of the landfill is about 125 feet. The proposed raise is to 320 feet msl, or a height of about 250 feet. Maximum stresses at the base of the proposed landfill are estimated to be about 20,000 psf at full height. Section 9.2.1 of Appendix E notes that the maximum waste thickness will be about 255 feet, corresponding to a maximum normal stress of about 20,000 psf. Section 9.2.1 also contains a recommendation that the interface shear strength of the future liner systems for Modules 5 through 11 should be tested to a maximum normal stress of 20,000 psf. The maximum thickness of waste in Modules 1 and 3 are about 140 feet and 190 feet, respectively. Bottom and Side Slone Liner In line with previous landfill reviews over the last couple of years, DWR recommends that only reinforced geosynthetic clay liner (GCL) be used as part of the bottom and side slope liners underlying refuse. We agree with DWR's recommendation for only using reinforced GCLs. Our standard practice is to specify use of reinforced GCLs for liner system construction. Section 9.2.1 of Appendix E includes a recommendation that a reinforced GCL consisting of a needle -punched GCL with non -woven geotextile on both sides be used for the liner construction. Waste Strength 0 The strength assumed for the waste (friction angle — 33 degrees, cohesion — 100 psi) is slightly outside the range that DWR has recommended in the past. This is unconservative. It is true that the assumed waste shear strength (friction angle — 33 degrees, cohesion — 100 psf) is slightly unconservative compared to the bilinear model recommended by Kavazanjian et al. (1995), which we understand is favored by DWR. To be consistent with DWR recommendations, the failure envelope described by Kavazanjian et al. (1995) as presented by Duncan and Wright (2005) (cohesion = 500 psf for normal stress < 770 psf; friction angle = 33 degrees, cohesion = 0 psf for normal stress > 770 psf) has been used in the revised modeling. An updated Table l showing static factors of safety and estimated seismic displacements is attached. Liner Strength The GCL/HDPE liner strengths vary for the different modules. Except for Module 4, the existing modules seem to have a friction angle of 10 degrees and use smooth HDPE or single -side textured [geomembrane]. Module 4 and the proposed modules seem to have a friction angle of 12 degrees and use double -side textured HDPE (geomembrane]. The critical liner interface strengths vary according to the liner system materials used or proposed in each module. Because the liner system materials have varied in some modules, the critical interface strengths also vary. The liner system components for each module were presented in Appendix E of the 2007 report. Those descriptions are reproduced here for ease of reference. PAGE 2