Former Clifton MGP O.U.2 Remediation


Location Staten Island, NY
Contract Value $23,000,000
Period of Performance January 2012 – August 2014
Role Prime Contractor

Significant Project Features

  • Construction of a temporary waste water treatment plant capable of processing 500 gpm of MGP impacted water
  • Installation of a network of dewatering sumps and connection of WWTP effluent to the discharge point
  • Construction of a subsurface cement-bentonite barrier and buttress wall and management of all spoils
  • Demolition of an existing masonry building and historic subsurface structures
  • Excavation, handling, transportation and offsite disposal of approximately 110,000 tons MGP impacted soil within a temporary fabric structure (TFS)
  • Relocating the temporary fabric structure (TFS) to subsequent excavation areas
  • Site restoration including the placement, grading, and compaction of approved imported backfill, asphalt paving, concrete work and topsoil and seed
  • Installation of NAPL recovery system

History & Location Details

The Clifton former Manufactured Gas Plant (MGP) Site is located in the Rosebank section of Staten Island on two parcels totaling approximately 3 acres. The main plant operations were conducted between approximately 1857 and the early 1950’s at one parcel. A large gas holder and related operations were located at the second parcel. The location is in an urban area and the surrounding community is mostly commercial and residential. In the mid-1950’s, Richmond County Gas Company stopped using the facility as an MGP. Brooklyn Union Gas acquired the site, dismantled the MGP facility, and began using a portion of the site as a service center. In 1994, Brooklyn Union Gas closed the service facility. The parcel has remained vacant with the unused building ever since. In the early 2000’s, a Remedial Investigation (RI) was initiated. The two parcels were identified as distinct Operable Units for further investigation and remediation. OU-1 constituted the former holder area. OU 2 was the former main site.

Plans and specifications were developed for competitive bid for OU-2. Sevenson was awarded the remedial action contract in January 2012.

Project Description

Sevenson was required to complete the following scope of work tasks:


Upon completion of mobilization and site preparation activities, Sevenson initiated the demolition, removal of asbestos-containing material (ACM), and debris removal from the masonry building located onsite. A hazardous materials survey report was completed which included a list of ACM which required removal and disposal.

Prior to initiating demolition and asbestos abatement, all utilities to the building were disconnected. An air monitoring system was set up and managed a by New York State Department of Labor licensed air monitoring subcontractor. The system remained operational for the duration of the decontamination and demolition.

Demolition was accomplished using an excavator equipped with a grapple attachment. The excavator allowed for a controlled demolition of the sidewalls so they fell away from existing utilities. Sevenson continuously applied water during demolition to prevent the spread of dust offsite. A front-end loader and excavator removed metal debris from the concrete and brick rubble. Steel was segregated and sized for scrap. Brick and block debris was transported to an on-site crushing unit for sizing and stockpiled for use as backfill. The concrete slab was tested. When notified it was clean, the slab was demolished using an excavator with a hoe ram attachment.


All water generated from remediation operations required collection and treatment. Sevenson set up, operated and maintained a 500 gpm WWTS. The temporary waste water treatment plant consisted of two (2) 21,000-gallon equalization tanks that received influent from the active excavation zones and the dewatering sumps. Water was treated with sodium hypochorite, as needed, for arsenic, injected with ferric sulfate and polymer, mixed using static mixers, and pumped to a series of 18,000-gallon weir tanks. At the weir tanks, the water was stored to allow a majority of solids to settle out. Once adequate settling time took place, the water was pumped to two (2) multi-media filters vessels, followed by two granular activated carbon vessels. These vessels performed the essential water filtration to remove the primary contaminates of concern. When filtered, the water was pumped through two (2) cartridges each containing 1-micron bag filters. The bag filters captured any remaining suspended solids. Water was then pumped through four ion-exchange columns plumbed in a series to treat the cyanide concerns. Finally, water was pumped to an effluent tank where it was tested and discharged to the sewer system.


Dewatering sumps were installed so that excavation operations could be done in dry conditions. Because of the numerous subsurface obstructions throughout the site, Sevenson pre-trenched locations where sumps were installed. This allowed for relatively easier installation of the sump pumps. Odor suppressant foam was utilized as needed to mitigate odors. The average pipe was installed to a depth of approximately 15 feet. Obstructions included a tar separator, relief holder, tar tank and piping and appurtenances. The objective of the dewatering system was to maintain groundwater levels 2 feet below excavation limits.

The sumps were connected to the influent line at the waste water treatment plant.


The project required the installation of two Barrier and Buttress Cement Bentonite Walls (CBWs) at the perimeter of the site. The barrier wall dimensions were approximately 620 lf x 50 ft x 3 ft. The buttress wall was 790 lf x 13 ft x 6-9 ft. CBWs were constructed using the traditional slurry trenching construction method. This is an established method of installing shallow and deep cutoff walls in difficult conditions when conventional open cut methods are not possible. As the trench was excavated, prepared slurry was added to the trench to support the trench opening and maintain a stable trench. The work platform was prepared by grading the existing site.

Excavation of the slurry walls was accomplished using Caterpillar 385 with a custom heavy duty long-reach stick and boom assembly. This equipment has the capacity to excavate the trench to 55-feet deep. As the trench was excavated, prepared CB slurry was added to the trench to support the trench opening and maintain a stable trench until the CB slurry hardened. Trench spoils were direct loaded into dump trucks for hauling to the Temporary Fabric Structure (TSF) for management and disposal. The slurry wall trench was excavated in a series of approximately 30- to 50- feet long cuts. Each cut of the slurry wall was excavated to full depth before the excavator advanced to begin the next cut. Great care was required to address sewer and water lines which remained in place during installation of the walls.


Sevenson supplied a 96 ft x 148 ft, stressed membrane, temporary fabric structure (TFS) to house all excavation operations. The TFS was designed to facilitate equipment access, excavation and loading. The building structure and frame was anchored to bin blocks placed on the building frame that complied with the Richmond County, NY wind load requirements. After each move, the TFS was re-anchored with the bin blocks.

The TFS was equipped with an air handling system. The air handling system was sized to capture and remove airborne contamination inside the structure. The air handling system consisted of two carbon units each containing approximately 16,000 lbs. of activated carbon, and flow rates of 10,000 cfm per unit, capable of maintaining negative pressure within the building. The blower motors were 100 hp each. The temporary fabric building was installed with vents and louvers to provide makeup air for the system. A sound insulated structure was constructed around the air handler blower unit to mitigate noise.

The initial erection of the TFS was done at the tar separator excavation area. At the conclusion of the excavation and backfill operations at each TFS location, Sevenson relocated the TFS unit to the next area. Based on the excavation footprint required inside the TFS, Sevenson relocated the structure 18 times. The locations took into account overlap to allow for safe excavation.


Approximately 110,000 tons of MGP soils required excavation, characterization and disposal. Sevenson excavated soil using multiple backhoes. One excavator was used for bulk excavation and the second was utilized to load dump trailers for offsite disposal. Sevenson applied odor suppressing foam to stockpiled soils and utilized polyethylene liner as cover to prevent the migration of odors. Due to the large number of concrete substructures encountered during excavation operations, Sevenson equipped the excavator with a Hoe Ram to break up debris. The debris was sized to meet the requirements of the disposal facilities and was loaded out with the bulk soils. The perimeter excavations that were not supported by the cement-bentonite buttress wall were supported by the use of a trench box.

A tarping station was set up outside the TFS. Once the tarp was installed, the truck was directed to enter the TFS and loaded with MGP impacted soil for offsite disposal. Once the truck was fully loaded, the driver was directed to exit the TFS and enter the truck decontamination pad. At the decontamination pad, Sevenson performed a high-pressure water decontamination procedure. Sevenson also secured the flaps of the 3-mil truck liner.

When Sevenson completed the soils excavation beneath the footprint of the TFS, an as-built survey was performed. Sevenson then installed a fabric demarcation barrier. The demarcation barrier was placed at the bottom of the excavation and extended the entire footprint of the excavation. Once the demarcation barrier was installed, Sevenson initiated backfilling operations using approved fill material. Backfill was placed in 8-inch lifts and compacted using a hoe pack. Once backfill was completed within the TFS, Sevenson relocated the TFS and initiated excavation operations as mentioned above.


Site restoration was initiated upon completion of excavation and backfill operations. Restoration was completed in compliance with project plans and specifications. It required Sevenson to restore the site to original pre-remediation conditions.

Activities included placement of topsoil and seed, replacement asphalt to streets and parking areas to meet State and NYCDOT requirements; replacement of concrete curbs and sidewalks; and replacement of lighting and fencing.

Rooted in Remedial Construction
Expanded into Environmental Dredging

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