ARCHIVED REPORTS XR0009921 - Laserfiche WebLink

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Corrective Action Plan Page 10 of property affected by discharges of gasoline and other volatile materials It has the principal advantages that affected soil, that acts as a source of groundwater contamination, is rapidly removed from the subsurface and can be thoroughly decontaminated by simple aeration procedures before it is returned to the excavation as clean material For soil remediation by excavation and on-surface treatment to be applicable at a given site, it is necessary that an excavation of the required area and depth can be safely and cost-effectively opened and that all, or substantially all, of the most severely affected soil is accessible to excavation Also, it must be possible to backfill the excavation with the treated soil, or imported material, and compact the soil so that it meets the engineering specifications necessary for the site to be restored to beneficial use For the reasons discussed below, these conditions are not met at the 152 East 11th Street property To remove the soil that is affected by gasoline, an excavation would have to be approximately 15 ft deep over the general area of the subject property In the most severely affected areas the excavation would have to be up to 18 ft deep As soon as such an excavation penetrated below the water table at a depth between 7 ft and S ft beneath the ground surface, contaminated groundwater would flow into it and it would be rapidly flooded Due to their very loose condition, the sands that underlie much of the site at depths between 12 and 25 ft beneath the surface and locally in extensive lenses at shallower depths (See Figures 7 and 8) would immediately collapse into any unsupported excavation made into them Even if supported, such an excavation would be subject to uncontrolled inflow of sand because the sand would rise in the bottom of the pit where heaving, quick- sand conditions would develop This condition can be readily predicted from the geotechnical and hydrogeologic data that has been obtained from the borings and groundwater pumping tests that have been performed at the subject site and is supported V by reports from other sites in the general vicinity where sand flowed into excavations that penetrated beneath the water table When sand flows into an excavation, material is lost from subsurface strata in adjacent areas When the sands are loose, highly permeable and contain a significant fraction of fine material, as is the case beneath the 152 East 11th Street property, this can occur to some degree even if the excavation is supported and the bottom does not heave 1 If any more than minor quantities of loose sand were to flow into a remedial excavation, the sidewalks and pavement of East 11th and "F" Streets could be damaged due to settlement Also, if even small amounts of material were flushed from the strata beneath the un-reinforced, brick structures at 124 and 208 East 11 th Street, there is concern that their foundations could be disturbed and they might suffer serious damaged, become unstable and be rendered unsafe to occupy In addition to the risk of damage to nearby street and structures, excavation beneath the water table poses other significant problems When excavating beneath water, it is difficult