SITE HISTORY - Laserfiche WebLink

Home Browse Search

.� I.. 5.5 Overexcavation and Remediation Following tank and backfill removal, any underlying hydrocarbon-impacted soils (detected — y 0 visually and/or by field screenin methods) will be excavated until they have been removed or practical limits prevent further excavation. These limits are likely to include the maximum practical excavation depth of the backhoe or excavator, the lateral extent of hydrocarbon- impacted soils, etc. When the practical limits of excavation have been reached, confirmatory soil samples will be collected as in Section 5.2. These samples will be analyzed on a 24-hour turnaround. A decision to backfill with clean material (Section 5.5.1), place soil vapor collection piping and/or place a conductor casing in the backfill (Section 5.5.2) will be made based on these sample results. These options are discussed in the following sections. 5.5.1 Backfilling Excavation backfilling will be performed following analysis of confirmatory soil samples — taken during overexcavation (Section 5.5). Backfill will consist of clean gravel, to be overlaid by baserock and asphalt or pavement. Backfill design and installation are discussed in documents prepared by others. 5.5.2 Vapor Collection Piping and Conductor Casing Perforated piping and solid riser may be placed in the excavation during backfilling. This will be done in order to facilitate removal of soil vapor following construction activities, if necessary. The decision to install this piping will be based on the positive detection of hydrocarbons in confirmatory samples described in Section 5.5. Perforated pipe would consist of flexible perforated corrugated polyethylene pipe with slip fittings. The pipe would be placed in a loop near the bottom of the excavation or top of 1-2 feet of gravel. A riser pipe of unperforated PVC pipe with slip fittings would be placed at one end to stub out just below grade. The stub-out would be capped and would be protected by a water resistant, traffic-rated utility box finished slightly above grade. Several feet of gravel would be placed over the loop of perforated collector pipe, followed by a one foot — thick layer of compacted base rock. Backfill above this would continue as in Section 5.5.1. Four 12-inch diameter manholes will be subsequently placed with the pavement over the backfilled area, to allow for possible drilling access without concrete cutting. MA/8-12-91/ENG/9284MER 7