ARCHIVED REPORTS XR0011751 - Laserfiche WebLink

Home Browse Search

E.r. Work Plan fpr Refined Plume Definition and Management of Floating Product-7500 W 111h St., Tracy, CA. Page 54 along the length of the trench so that its surface is just above the water table. Plate 3 shows v crushed rock being placed in a remediation trench at a site where a large volume of LNAPL was present on the groundwater. Under some circumstances, such as when flowing sand is present in the bottom of a trench, as may be the case at the 7500 West Eleventh Street site, the walls of the trench tend to �., become unstable a short time after excavation is opened or before it can be excavated to sufficient length. If such a situation occurs at the 7500 West Eleventh Street site, SJC will open the trench using a series of overlapping excavation cells of limited length. Each excavated cell, in turn, is left open only for the minimum time necessary for spoil to be removed from it and a sample recovered from the bottom before the rock is placed into it. The rock fill stabilizes the trench as it is introduced into it. The maximum allowable size of ..• an excavation cell is set by the project engineer in the field and the cells are successively overlapped to achieve complete excavation over the entire length of the trench. Whether it is placed as a single operation, or by successive backfilling of excavated cells, when the crushed rock rises above the water table to a height just sufficient to make a hard standing, it is leveled and compacted using a vibratory roller. Clean, no-fines rock is readily •— compacted to a high relative density by vibration and the resulting compacted mass has a very high permeability. Unlike other materials commonly used when excavations containing groundwater have to be backfilled, such as hydraulically placed sand, such rock is not i susceptible to liquefaction during a seismic event. To ensure that the trench will continue to function as a cut-off trench for LNAPL migrating down the groundwater gradient, and to provide for future pumping of floating product or contaminated groundwater, should such be necessary, the surface of the crushed rock will be raised above the elevation of the water table extant at the time the trench is backfilled, so that it will, at all times, be approximately one foot higher than the highest groundwater elevation that has been recorded in Monitoring Well MW-7. 9.4.1.6 Disposal of Soil Excavated from Cut-off Trench As noted in Section 9.4.1.2, when it is being excavated, the soil from the cut-off trench will be segregated into two stockpiles; one containing soil affected by fuel hydrocarbons and the other containing soils excavated from shallow depth that are judged in the field using visual and olfactory indicators to be free of contamination. The hydrocarbon-affected soil will be loaded onto trucks and transported as a special waste to the Class H Forward Landfill in Manteca, California. Because SJC has previously developed a waste profile for affected soil from the 7500 West Eleventh Street site, extensive waste characterization analyses, beyond the results obtained from samples recovered from the floor of the trench as it is being excavated, will not be required for its disposal at that facility. .. R To check that the stockpile of soil designated as clean at the time of its excavation is, in fact, free of analytes of concern, it will be tested before its final disposal site is determined. The r...r sic