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f <br /> 'i 5.5 Overexcavation and Remediation <br /> Following tank and backfill removal, any underlying hydrocarbon-impacted soils (detected <br /> f0-or <br /> visually and/or by field screening methods will be excavated until they have been removed <br /> practical limits prevent further excavation. These limits are likely to include the maximum <br /> practical excavation depth of the backhoe or excavator, the lateral extent of hydrocarbon- <br /> impacted soils, etc. When the practical limits of excavation have been reached, confirmatory <br /> soil samples will be collected as in Section 5.2. These samples will be analyzed on a 24-hour <br /> turnaround. A decision to backfill with clean material (Section 5.5.1), place soil vapor <br /> collection piping and/or place a conductor casing in the backfill (Section 5.5.2) will be made <br /> based on these sample results. These options are discussed in the following sections. <br /> 5.5.1 Backfilling <br /> Excavation backfilling will be performed following analysis of confirmatory soil samples <br /> taken during overexcavation (Section 5.5). Backfill will consist of clean gravel, to be <br /> overlaid by baserock and asphalt or pavement. Backfill design and installation are discussed <br /> in documents prepared by others. <br /> 5.5.2 Vapor Collection Piping and Conductor Casing <br /> 'may Perforated piping and solid riser may be placed in the excavation during backfilling. This <br /> will be done in order to facilitate removal of soil vapor following construction activities, if <br /> necessary. The decision to install this piping will be based on the positive detection of <br /> hydrocarbons in confirmatory samples described in Section 5.5. <br /> Perforated pipe would consist of flexible perforated corrugated polyethylene pipe with slip <br /> fittings. The pipe would be placed in a loop near the bottom of the excavation or top of 1-2 <br /> feet of gravel. A riser pipe of unperforated PVC pipe with slip fittings would be placed at <br /> one end to stub out just below grade. The stub-out would be capped and would be protected <br /> by a water resistant, traffic-rated utility box finished slightly above grade. Several feet of <br /> gravel would be placed over the loop of perforated collector pipe, followed by a one foot <br /> thick layer of compacted base rock. Backfill above this would continue as in Section 5.5.1. <br /> Four 12-inch diameter manholes will be subsequently placed with the pavement over the <br /> backfilled area, to allow for possible drilling access without concrete cutting. <br /> MA/8-12-91/ENG/9284MER 7 <br />