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Remediation Report - 152 East 11th Street, Tracy, California Page 1 I <br /> After examining the pipes in the context of the history of the site and the distribution of <br /> t contamination found in the subsurface From it was conclude that underground tanks, <br /> probably two in number, had been installed under the City of Tracy's property on East <br /> I I th Street at some time (probably in the 1930s or earlier) before the Navarras bought the <br /> 152 East 11 th Street property and in all likelihood those tanks are still present beneath the <br /> street As will be discussed later, this assessment is fully consistent with the hydrogeologic <br /> and geochemical data that has been gathered from the characterization studies that have <br /> been conduced at the site and the geometry of the zone of contaminated soil that was <br /> exposed in the remedial excavation described in this report The projected off-site location <br /> of the leaking underground tanks is shown on Figures 2, 5 and 6 <br /> Excavation of Contaminated Soil <br /> When the clean overburden had been removed from a specific area of the site, the <br /> excavation was deepened in that area to remove the underlying contaminated soil The <br /> Observational Method (Ref 3) for management and control of geotechnical construction <br /> in hazardous ground was employed to ensure safe working conditions and the excavation <br /> was under the continuous observation and control of an experienced, California-licensed <br /> geotechnical engineer <br /> The primary concern during the excavation of the contaminated soil was for the safety of <br /> the excavation and the stability of surrounding property and buildings To reduce the risk <br /> of piping and loss of fine materials from the sands and silty sands in the walls of the <br /> excavation, bottom heave, or development of quick-sand conditions in the floor of the <br /> excavation, the facies of the material excavated from beneath the water table, which was <br /> approximately eight feet below the ground surface at the time the remediation program <br /> was undertaken, were continuously scrutinized and the stability of the walls of the <br /> excavation at those depths was monitored very carefully <br /> As expected based on the hydrostratigraphic sections that had been developed from the <br /> site investigation work performed prior to remediation and that are shown on Figures 3 <br /> and 4, soil containing a significant percentage of silt and fine sands were encountered at <br /> depths between 12 and 14 feet bellow the surface The percentage of clay in there <br /> materials decreased with depth while the silt and sand content increased <br /> When exposed in the walls of the excavation before groundwater flooded the hole opened <br /> by removing an excavator bucket full of soil, the silty and sandy materials could be seen to <br /> be sloughing and it was clear that they would cause instability in the walls and floor of the <br /> excavation if they remained unconfined over large areas, or for more than a short period of <br /> time For this reason, as had been foreseen when the CAP had been prepared, it was <br /> necessary to limit the areal extent and depth of any portion of the excavation beneath the <br /> water table and the time it was allowed to stand open This was done by developing the <br /> total area of the remedial excavation in stages A stage of excavation beneath the water <br /> table was limited to an area within the reach of the excavator Within each limited area, or <br /> cell, contaminated soil was removed from the floor of the excavation to a depth sufficient <br />