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As discussed in Section 7.1.1, the worst-case flow for the geocomposite drain has been estimated based on the leachate pumping rates between 2005 and 2007. A summary of the pumping rate and the daily calculation is presented in Attachment F. The peak flow is estimated to be approximately 1,000 gpad. Again, the trend in the pumping rates shows decreasing flow with time; therefore, the flow of 1,000 gpad is very likely to be the worst-case flow. The flow capacity calculations are presented in Attachment F. The calculations show a global factor of safety of 35.4 for the geocomposite drain, which is significantly greater than the acceptable value of 2. 7.2 Design of Geocomposite Drain for Future Modules The proposed liner system within the future modules will be similar to that used in Module 4. Therefore, the geocomposite drain is used only on the sideslopes. The anticipated peak flow through this geocomposite drain has been estimated based on the results of HELP3 simulations. The results of the HELP simulation presented in Attachment F show a peak flow of 4,660 gpad. It should be noted that the flow rates predicted from HELP3 modeling are typically very conservative. The required transmissivity of the geocomposite to carry a flow of 4,660 gpad (with all the reduction factors and a global safety factor) is determined using the calculations ispresented in Attachment F. The calculations show that the index transmissivity of the geocomposite drain should be greater than 1.8 x 10-4 m2/sec at a hydraulic gradient of 0.33 and a normal load of 35,000 psf. This requirement should be incorporated into the construction specifications for Modules 5 through 11. U] Geotechnical Analyses for NCRCSL Department of Public Works/Solid Waste Shaw Environmental, Inc. County of San Joaquin, December 12, 2007 7-2