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WASTE DISCHARGE REQUIREMENTS ORDER NO. R5-2010-0016 -15- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS NORTH COUNTY LANDFILL SAN JOAQUIN COUNTY Existing grading plans for module slopes would be retained (e.g., 3H:1 V interior and exterior, 20 -foot wide benching every 50 vertical feet) and existing precipitation and drainage controls would be extended (and sized, as necessary) to handle the increased flows (and flow velocities) associated with the vertical expansion during a 100 -year storm event. See Findings 52 and 53. The VEP would increase the landfill capacity by approximately 72% from 20.9 million yd to 35.9 million yd3, based on a May 2009 aerial survey. Based on the existing disposal volume (5.3 million yd3) and projected disposal rates, the landfill would reach capacity in the year 2055. Geotechnical Analysis 58. Geotechnical issues evaluated for the vertical expansion plan included (a) the potential for puncture of the geomembrane liner from the overlying LCRS gravel; (b) the ability of LCRS pipes to withstand increased loads (e.g., wall crushing, buckling and deflection); (c) leachate production and head buildup on liner; (d) LCRS geonet and pipe flow capacities; (e) LCRS sump design and capacity; leachate storage and disposal; and slope stability analysis. The results indicated the need for modification of the design of Module 3 and future modules, as follows: a. Existing Modules --Reduce maximum fill height of Module 3 to accommodate vertical loading limits of existing (4 inch, 15.5 SDR) LCRS piping. Other modules within design limits. See Discharge Specification B.1.c. b. Future Modules --Cushion against liner puncture and use stronger LCRS piping to accommodate planned vertical loads. See Construction Specification DA 59. Slope stability analysis identified the following critical cross sections: a. Module 1: N -S section of the northern slope. b. Module 3: E -W section of the western slope. c. Module 4: E -W section of the western slope. d. Module 4 and future modules: 1) N -S section of the southern slope (Modules 6 and 7) where the landfill toe buttress is minimal; and 2) NE -SW section of the NE slope (future Module 11) where the base liner slopes toward the perimeter. e. Interim slopes: N -S section of the central part of the landfill after completion of Module 10, before construction of Module 11. 60. Static slope stability analysis performed on the above critical cross sections (using Slide software developed by Rocscience, Inc.) employed two-dimensional limit equilibrium analysis and the method of slices. Both force and moment equilibrium