GZA was selected to complete the delineation and present remedial alternatives to achieve closure at a site impacted by a former Manufactured Gas Plant (MGP). The site, in a densely populated area adjacent to a tidal river, required strong environmental and geotechnical engineering, as well as regulatory expertise.
After delineating the extent of impacted soil, sediment, and groundwater, GZA used probabilistic cost estimating to present several remedial alternatives to our client. The client elected to excavate and thermally treat impacted soil and debris to limit future risk exposure. This alternative presented a number of technical challenges which GZA successfully addressed: removing impacted soil from as deep as 21 feet below grade in sandy soil immediately adjacent to a tidally-influenced river with a mean high tide level a few feet below site grade; installing a sheetpile retaining wall behind a 100-year-old masonry wall at the toe of a 25-foot-high, 45% slope while protecting an apartment building within 10 feet of the top of the slope and excavating 15 feet on the downslope side of the sheeting; installing a new steel bulkhead wall to remove impacted sediment and soil along the bank; temporarily by-passing and replacing subgrade and overhead utilities; and preventing on-site migration of contaminated groundwater from an off-site source during dewatering operations. GZA completed an Ecological Risk Assessment to limit sediment removal within the riverbank and obtained numerous federal and state permits. We also worked closely with the city on restoration plans for roads, sidewalks, water, sanitary and storm lines. An instrumentation program monitored vibrations and slope movements during construction. GZA also developed a groundwater treatment system conceptual design with discharge via a stormwater outfall to the river and completed an asbestos and lead-based paint survey on two structures proposed for demolition. GZA was the Engineer of Record during the 27-month construction project.
GZA met the client’s goal by obtaining an Unrestricted Use Response Action Outcome (RAO) in technically challenging conditions on a waterfront site.