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