Former Williamsburg Works MGP Site


Location Brooklyn, New York
Contract Value $26,500,000
Period of Performance November 2015 – March 2017
Role Prime Contractor

Significant Project Features

  • In-situ solidification (ISS) and excavation of soils and underground structures from the former MGP holder No. 2
  • Installation of cement-bentonite (CB) slurry (620 LF x 48 ft BGS maximum) steel sheet pile system for the support of deep excavation
  • Excavation and removal of holders and surrounding soil/fill for off-site treatment and disposal or onsite reuse following sampling and analytical testing
  • Dewatering of the deep excavation area
  • Onsite treatment of the water thru a 200 GPM WWTP prior to discharge to the POTW
  • Backfilling all areas to final grades and restoring the site

History & Location Details

The Williamsburg Works MGP site is located in a commercial/industrial area in the Williamsburg section of Brooklyn, New York.  The site is approximately six acres in size and is composed of three parcels of land located on the east bank of the East River between North 11th and North 12th Streets on the west and east sides of Kent Avenue.

A predecessor company to National Grid operated the former Williamsburg Works MGP from approximately the 1850s to the 1930s.  The plant covered several city blocks and produced gas for use in the surrounding community.  The gas production process also produced coal tar which escaped from subsurface structures and pipes.  This tar is still present in subsurface soils under the 50 Kent Avenue property and several adjoining properties.  The Williamsburg MGP was dismantled prior to 1941, and the site was subsequently subdivided, sold, and redeveloped for commercial, industrial, and manufacturing uses.

The IRM is being competed under an Order-on-Consent between the New York State Department of Environmental Conservation (NYSDEC) and National Grid.  The property is owned by the New York City Department of Parks and Recreation.


Project Description

Sevenson was awarded the remediation contract based upon competitive bid.

Layout of Support/Decontamination Zones & Site Preparation

Before commencing excavation activities, Sevenson established the support zones, decontamination zones (CRZ), and exclusion zones, all of which were located within the secured zone. The fenced secured zone enclosed all site remediation activities, and was monitored by 24-hour security to prevent access from unauthorized personnel. Per Sevenson’s Health and Safety Plan (HASP), all signage and safety information was installed in the necessary locations to indicate each work zone. In addition, Sevenson installed and operated a 200-GPM wastewater treatment plant (WWTP), the components of which included one 16,000–21,000 gallon influent weir tank; one feed pump; two bag filter housings; two granular activated carbon vessels; one 16,000–21,000 gallon effluent fractionalization tank; and, one flow meter. The plant was enclosed by a berm and placed on a 20-mil impermeable liner.

For areas that were utilized as the construction road entrance, Sevenson installed two entrances and a 12-foot-wide access road comprised of a 6- to 8-inch layer of dense grade aggregate stone placed over geotextile fabric. Sevenson also designed and constructed separate decontamination (“decon”) facilities for equipment and personnel. The decontamination stations utilized portable steel trays and surrounding splash curtains. All pads were connected to a small pump and portable holding tank into which decontamination water was pumped.

Excavation Support

Sevenson determined that a temporary excavation support system, consisting of a steel sheet pile wall placed within a Cement Bentonite (CB) slurry wall, was required to facilitate excavation activities. To do so, Sevenson constructed the CB slurry wall using the self-hardening slurry trenching construction method. Upon survey and layout of the Support of Excavation perimeter alignment, Sevenson pre-trenched around the perimeter of the alignment to approximately 5 feet BGS. Spoils were cast aside of the trench and placed back into the hole to create a work platform for the CB wall construction. Once pre-trenching was complete, Sevenson self-performed the installation of the CB wall. Self-performing the wall construction process allowed Sevenson to perform CB wall and sheet piling installations concurrently. Sevenson proceeded to excavate the trench with a hydraulic excavator equipped with a custom heavy duty long-reach boom and stick attachments.

Upon commencing CB wall construction, Sevenson utilized a 45-ton hydraulic crane with a vibratory hammer to install AZ-26 700 steel sheet piles within the CB structure. Field crews installed approximately 40–50 LF of sheeting each day. During sheeting installation, Sevenson ensured that the Engineer conducted vibration monitoring to confirm that sheet-driving operations did not disturb nearby structures and the residential community.

Temporary Containment Buildings

Sevenson’s subcontractor designed and erected three temporary containment buildings (TCBs): a 96’ x 98’ (loadout building); 115’ x 118’ (enclosure for BSM ISS work); and, a 98’ x 118’ (213’ x 118’ total over deep excavation) stressed membrane containment structure. The TCB structures were equipped with air handling units designed to capture and remove airborne contamination inside the structures, as well as provide 6 air changes per hour. The small loadout building had one air handling system that consisted of a carbon unit containing approximately 16,000 lbs. of activated carbon. The larger 213’ x 118’ structure, which encompassed the deep excavation zone, had 6 blower units. The TCBs effectively contained odors and absorbed SVOCs and VOCs from being released into the local atmosphere.

Dewatering Procedures and
Water Treatment Facility

During soil removal operations, Sevenson collected and treated contact water obtained from the excavation zones and dewatering process via a dewatering system. Due to the anticipated elevation of the water table, a dewatering system was utilized to keep deep excavation grids in relatively dry conditions. Sevenson’s subcontractor drilled six deep sump wells strategically along the perimeter from grade elevation, which effectively served to dewater two feet below the excavation depth. As contact and dewatering flows were collected, they were pumped to the temporary 200-GPM WWTP, which removed suspended solids, oil and grease, PCBs, VOCs, and SVOCs. Sevenson treated all water as stipulated by the New York City Department of Environmental Protection’s (NYCDEP) discharge criteria.

Excavation Activities

Sevenson performed deep excavation within the TCB. Using three excavators, Sevenson removed a daily average of 450–500 CY of MGP-impacted material. A PC 400 excavator performed the majority of material removal and cast it back to a smaller excavator, which then loaded it in dump trucks for off-site disposal. A PC 300 excavator, equipped with a hoe-ram attachment, crushed and sized all subsurface obstructions. The scope of structural demolition work involved removing the historical gas holder and the MGP building’s asphalt/concrete substructures. The material from these structures was disintegrated and sized utilizing an on-site mobile crushing plant. Sevenson reused portions of crushed material (asphalt, concrete, and brick) as on-site fill within the deep excavation area.

Depending on moisture levels, all removed material was either loaded directly into trucks for off-site disposal at the National Grid-approved waste facility or staged in the 96’ x 98’ TCB for loadout. During the course of remediation operations, Sevenson proactively controlled noxious odors and vapors using both short- and long-term RusFoam® application. Sevenson loaded an average of 32 trucks each day for off-site disposal and received approximately 36 truckloads of approved backfill material. Overall, Sevenson excavated approximately 44,000 tons of MGP-impacted waste to approved off-site landfills, and backfilled about 40,000 tons of approved material during site restoration.

Stockpile Activities

Sevenson segregated and stockpiled materials. While concrete debris from below-grade demolition was stockpiled for subsequent sizing and crushing for on-site reuse and/or loading and offsite disposal, all impacted excavated soil was stockpiled within the TCBs for off-site disposal. MGP-impacted soil stockpiles within the TCB were foamed to prevent the escape of any odors.

Off-Site Transportation
and Disposal

In order to safely and efficiently transport approximately 44,000 tons of MGP-impacted materials, Sevenson managed a transportation subcontractor to relocate all waste to the designated disposal facility. Sevenson’s peak production disposal rate reached an average of 32 truckloads each day, with each load weighing about 22 tons. Swift material removal operations and efficient T&D coordination allowed Sevenson to move approximately 700 tons of material per day during deep excavation operations.

Waste was disposed of at National Grid’s approved waste disposal and recycling facilities. The majority of the waste, mainly MGP-impacted soil and debris, was transported and disposed of at a Low Temperature Thermal Desorption (LTTD) treatment plant, where high-temperature kilns burned away MGP contaminates and purified the exhaust. The cooled soil was ultimately stockpiled, sampled, and tested by an independent certified laboratory to ensure effective treatment and fulfillment of reuse standards. In addition to the disposal of MGP-impacted waste, Sevenson was required to dispose of nonhazardous construction debris, all of which was transported and disposed of at a subtitle D landfill.

Decontamination Procedures

In order to prevent cross-contamination, Sevenson employed multiple decontamination procedures, such as work zone delineation, exercising care during the loading operations, and using “burrito” truck bed liners. The truck bed liners were two 6-millimeter polyethylene sheets and were placed into the truck bed prior to being loaded. After a truck was loaded inside the temporary fabric structure, Sevenson removed residual soil from the truck bed’s side rails before directing the truck to the decontamination pad. At decontamination pad, Sevenson’s crews power washed all excess soil from the truck and tires. Once the tarp was secure and the truck had been cleared of site contaminates, the driver was directed to pull off the decontamination pad. Each decontaminated truck was inspected and issued a numbered manifest by National Grid’s approved representative.

Similar to trucks carrying waste offsite, all equipment that was implemented in the TFS was transferred to the decontamination pad where it was power washed to remove contaminants.  Furthermore, in order to prevent site personnel from inadvertently contaminating clean areas, all personnel who performed work within the TFS’s exclusion area were required to exit the exclusion zone through the crew decontamination pad. All decon water was collected and pumped to the on-site WWTP for proper treatment and discharge.

Backfill and Site Restoration

Sevenson’s scope of work for site restoration included procuring the services of a NYS-licensed land surveyor to obtain as-built shots of the excavation depth. After the Construction Manager / Engineer granted approval to start restoration procedures, Sevenson placed the demarcation barrier at the bottom of the excavation. Once the demarcation barrier was secure, Sevenson placed the approved backfill on the demarcation barrier in 6-inch lifts. As the material was being graded by the bulldozer, a smooth drum roller compacted the material according to contract specifications. Next, Sevenson procured an NYSDOT-approved testing lab to perform compaction testing of each lift. Sevenson’s field crews also managed a union-approved landscaper to hydroseed approximately 73,000 SF. Upon finishing topsoil placement, Sevenson worked with subcontractors to replace sidewalks and fence structures disturbed by work activities.

Rooted in Remedial Construction
Expanded into Environmental Dredging

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