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expansion. Similar calculations for existing leachate collection pipes in Module 3 show that maximum waste above the most critical pipe should be limited to 165 feet; the proposed final grading reflects this height limitation. The calculations for Modules 5 through 11 show that an HDPE with an SDR of 13.5 should be used to achieve adequate factors of safety under the proposed landfill heights. 9.1.3 Flow Capacity of Geonets/Geocomposite Drains Geonet drains were used on both the base and sideslopes of existing Modules 1 and 3. A geocomposite drain is used only on the sideslopes of Module 4. A geocomposite drain will also be used on the sideslopes of all future modules. Based on the computed peak flow from the historical pumping rates and the measured transmissivity of the geonet, the calculations for the existing geonet drains in Modules 1 and 3 show adequate flow capacity. Similarly, existing geocomposite drain on the sideslopes of Module 3 shows adequate flow capacity. The design calculations for the geocomposite drain on the sideslopes of the future modules, based on the peak flow estimated from the HELP simulation, shows that geocomposite drain should have a minimum index transmissivity of 1.8 x 10-4 m2/sec at a hydraulic gradient of 0.33 and a normal load of 35,000 psf. 9.1.4 Puncture Protection of Geomembranes Since a geonet drain is used as the leachate collection system in Modules 1 and 3, the puncture protection of the geomembrane within these modules is not a concern. Rounded/subrounded gravel was used as the leachate drainage layer in Module 4. The puncture protection calculations based on the actual grain -size of this gravel shows that the existing geomembrane can withstand the additional loading without a geotextile cushion. Similar calculations show that if rounded/subrounded gravel is used as leachate drainage layer in the future modules, a geotextile cushion would not be required. However, if crushed gravel is used, a 16-oz/yd2 non -woven geotextile cushion should be provided for puncture protection of the underlying geomembrane. 9.2.1 Liner System Shear Strength Requirements As discussed in Section 4.2.4, a minimum residual shear strength defined by a friction angle of 12 degrees was assumed for the liner system within all the future modules. The end of filling of Module 9 represents the most critical interim slope stability condition. Geotechnical Analyses for NCRCSL Department of Public Works/Solid Waste Shaw Environmental, hic. County of San Joaquin, December 12, 2007 9-2