ARCHIVED REPORTS_2010 - Laserfiche WebLink

Home Browse Search

7.1 Check of Existing Geonet/Geocomposite Drainage Layers 7.1.1 Geonet.Drain in Module 1 and 3 As noted in Section 3.2.1, a geonet drain with a geotextile filter directly above it was used as the leachate collection system within both Modules 1 and 3. The geonet was used both on the base and on the sideslopes. The actual transmissivity of this geonet has been determined by Shaw based on laboratory testing using geotextile, geonet, and geomembrane samples provided by the County. The transmissivity test results are included in Attachment E. The tests measured a transmissivity of 7.128 x 10-5 square meters/second (m2/sec) for the geonet. The adequacy of this geonet to convey the worst-case flow has been checked based on the historical leachate pumping rates between 2005 and 2007. A summary of the leachate flow in gallons per acre per day (gpad) is presented in Attachment F. This table shows that the leachate flow is gradually decreasing with time as more and more waste is placed within these modules. This is consistent with the fact that as the volume of waste placed within any module increases, the absorptive capacity increases, which results in decreased flow. Therefore, additional waste placement in these modules is not likely to increase the flow beyond the already recorded maximum flow. The maximum flow is estimated to be 100 gpad based on the summary table presented in Attachment F. The flow capacity calculations are also presented in Attachment F. The calculations show a global factor of safety of 4 for the geonet with respect to the maximum flow of 100 gpad. The computed factor of safety is greater than the acceptable value of 2. 7.1.2 Geocomposite Drain in Module 4 A gravel drainage layer was used as the leachate collection system within the base of Module 4. However, geocomposite drain was used on the sideslope. This section discusses the calculations performed to determine the adequacy of this geocomposite to convey the worst-case leachate flow. The transmissivity tests performed as part of the construction quality assurance (CQA) for Module 4 show a minimum index transmissivity of 6.94 x 10-5 M2 /see for the geocomposite drain. This value has been used to assess the flow capacity of the existing geocomposite in Module 4. Geotechnical Analyses for NCRCSL Department of Public Works/Solid Waste Shaw Environmental, Inc. County of San Joaquin, December 12, 2007 7-1