Remediation of MGP Holder Area in Urban Environment



Remediation of MGP Holder Area in Urban Environment

Presenter:  Jon  Sundquist, Ph.D. (AECOM)
Authors:  Jon Sundquist, Ph.D., Reeti Doshi and Allen Zgaljardic, P.E. (AECOM),  Donald Campbell (National Grid)


A former MGP site posed a challenge for remediation as it was located within a highly developed urban environment in a large northeastern city.  The remediation addressed three former holder tanks and their foundations, requiring treatment and/or excavation right up to the property line on three sides (north, east, and south) of the site.  Holder foundation depths dictated excavation to 30 feet bgs.  Groundwater was present at 5 feet bgs.  Many utilities, including shallow and deep sewers, were located just beyond the property line.  AECOM took different design approaches for the east side and the north and south sides.    The utility network was denser on the east side.  Sampling demonstrated lower levels of contamination here, allowing use of in situ solidification for the east side holder.  In situ mixing was implemented via bucket mixing, using a segmented treatment scheme.  Effectiveness was demonstrated through strength and permeability testing, and coring.    The north and south side holders required excavation, which in turn required a support of excavation (SOE) system at the property line.  The simpler and less expensive pile and lagging SOE approach was not selected because it would require a significant drop in water table elevation outside the excavation area and would not provide migration control below the excavation.  Sheet piling was selected, but due to structurally-required depths of 50 to 60 feet and dense soil with boulders, pre-excavation was required.  Slurry was needed to hold open the trench but had to harden to provide the strength needed to support the soil.  Following sheet placement, tiebacks were installed to complete the SOE.    Excavation required dewatering within the SOE walls, primarily with sumps.  Drawdown outside the SOE was greater than modeled, potentially from effects of NAPL on measured permeabilities.  Following backfill, sheets were cut to an elevation below the water table.

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