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WASTE DISCHARGE REQUIREMENTS ORDER R5-2015-0058-01 -28- <br />SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS <br />FOOTHILL SANITARY LANDFILL, INC. <br />FOOTHILL LANDFILL <br />SAN JOAQUIN COUNTY <br /> <br />Slope Stability <br />121. Updated slope stability analysis (see January 2012 Updated Slope Stability Analysis <br />for Foothill Sanitary Landfill, prepared by Shaw Environmental, Inc.; and 20 January <br />2015 supplemental report Review of the Stability Analysis at the Foothill Landfill, <br />prepared by CB&I) focused on LF-1 and the southern part of LF-2 where Modules 2 <br />and 3 will be developed along the unit perimeter over the next several years, reaching <br />approximate final cover grade. The following representative cross-sections were <br />evaluated in this area to assess both interim and global landfill slope stability: <br />a. Interim: N-S section of southern fill slope over interface between Module 1 and <br />Module 2. <br />b. Interim: E-W section of western fill slope over interface between Module 2 and <br />future Module 3. <br />c. Global: N-S section of southern final cover slope spanning LF-1, M-1, and M-2. <br />122. Slope stability analysis was performed on the above cross sections using the SLIDE© <br />(Version 6.0013) software program developed by Rocscience, Incorporated. The <br />program performs two-dimensional limit equilibrium analysis using the method of <br />slices to compute factors of safety based on various analysis procedures. These <br />procedures are included the Morgenstern-Price method, which considers both force <br />and moment equilibrium. Critical interface failure envelopes were developed for the <br />modules based on the results of laboratory shear testing of liner components up to <br />maximum expected loads and other factors. Computed static safety factors for the two <br />interim fill slopes were 1.3 (southern) and 2.0 (western) and global safety factors <br />computed for the landfill (i.e., southern final cover slope) were to 2.34 (static) and 1.8 <br />(pseudo-static). Critical interfaces included the geocomposite-geomembrane interface, <br />the geomembrane-GCL interface, and internal failure of the GCL. <br />123. A peak horizontal ground acceleration of 0.054 g was used in dynamic slope stability <br />analysis based on an earthquake of moment magnitude 8.1 occurring along the San <br />Andreas fault zone about 85 miles west of the site. Deformation analysis indicated an <br />estimated displacement of approximately 0.3 inches, well below the 12-inch maximum <br />recommended by the USEPA in its1995 publication Seismic Design Guidance for <br />Municipal Solid Waste Landfills. <br />CORRECTIVE ACTION <br />124. Previous WDRs Order No. R5-2003-0020 required that the Discharger implement an <br />Evaluation Monitoring Program (EMP) to investigate the nature and extent of <br />groundwater impacts at the site and, by 1 June 2003, submit an Engineering <br />Feasibility Study (EFS) report proposing corrective action measures, including a plan <br />for closure of LF-1. A partial cover work plan for LF-1 was also required to be <br />submitted by the same date. <br />125. In June 2003, the Discharger completed implementation of the EMP, including <br />installation of temporary borings down gradient of MW-3 to delineate the extent of the <br />release; a deep soil gas probe (SG-1) to sample for VOCs in soil gas near the water <br />table; and VOC sampling. No VOCs were detected in groundwater samples collected <br />from the temporary borings and MW-3. Four VOCs were detected in soil gas at <br />SG-1, but none of them matched those previously detected in groundwater at