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Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CA. Page 59 <br /> portions of which that are below the groundwater table being immediately <br /> >� back-filled using engineered fill. <br /> .:_ • Whether placed directly after the ground is opened or later into a shored <br /> excavation, the back-fill would consist of 9-in. to 3-in. sieve-sized crushed <br /> or river run rock containing no fines. SJC has developed and successfully <br /> applied this method at numerous sites where groundwater is affected by <br /> ` fuel hydrocarbons or other chemicals of concern. It permits cost-effective <br /> excavations to be made below the groundwater table so as to move <br /> heavily-affected soil that accumulates in that zone and to provide for <br /> extraction of floating product and groundwater affected by high <br /> concentration of analytes of concern by pumping into vacuum trucks, <br /> while, at the same time, due to the gradation of the rock fill, it does not <br /> pose a risk of liquefaction in the event that the fill is exposed to a seismic <br /> i event. <br /> • Normally, all of the floating product that flows into an excavation or <br /> migrates to it from surrounding areas of the subsurface can be extracted <br /> __1 while the excavation remains open. Similarly, during that time, one or <br /> more volumes of groundwater equal to that of the excavation that are <br /> affected by high concentrations of analytes of concern can be pumped out <br /> `-' and shipped for disposal at a permitted facility. <br /> =;; • Alternately, the rock-fill can be covered by a permeable geotextile <br /> -' membrane, the pit backfilled with standard engineered fill and an 8-in <br /> diameter standpipe can be installed into the rock backfill and used for <br /> periodic removal of floating product that may accumulate over time. <br /> SJC recommends that an engineering design based on the principals noted above, <br /> ! = together with a cost estimate, be prepared for a program for removal of the floating <br /> product on the north side of West Eleventh Street in the area around Well MW-7 so that <br /> it can be considered in detail by SJCEHD and other concerned regulatory agencies. <br /> .7 <br /> SJC <br />