LAND completes environmental remediation at former MGP site in Tonawanda, New york, Feature Article

This article was written by GEI, Dan Kopcow P.E., PMP and Paul JansenGEI, Dan Kopcow P.E., PMP and Paul Jansen. The full article can be found here

National Fuel Gas Distribution Corporation (NFG) has successfully completed an environmental remediation of their former Manufactured Gas Plant (MGP) site in Tonawanda, NY. One of the main elements of success was implementing in-situ solidification (ISS) as a key component in a multi-technology remedial approach. The approach also involved dredging and one-pass recovery trenches to obtain regulatory approval and resolve constructability issues and redevelopment concerns from the third party site owner. There were many complexities that included Record of Decision (ROD) amendments, managing multiple stakeholders, and technical issues in the remediation execution. The ISS design and implementation addressed many of these issues and resulted in an improved site ready for anticipated redevelopment that preserved the industrial character of the adjacent stretch of the historic Erie Canal.

Site Background

The site, located on a commercial/industrial lot in a residential area, was impacted with MGP wastes including impacted soil, sediment, groundwater, and non-aqueous phase liquid (NAPL) on-site and in the sediments of the adjacent highly-trafficked creek. NFG is not the current property owner of the site, which added complexity to site access and restoration. The site includes several old buildings still in use, many of which had major structural issues and undocumented foundation construction. The buildings were occupied with active small businesses and maintaining access during construction required careful sequencing, scheduling, and close coordination with the building owner.

NYSDEC conducted site characterization and remedial investigations to delineate the nature and extent of the impacts. NYSDEC then issued a ROD. It required:

• Demolition of buildings.
• Soil excavation for the entire property.
• Displacement of industrial and commercial tenants.
• A complex series of groundwater interceptor trenches.
• A challenging excavation footprint that would have required substantial shoring design and installation.
• A high level of dewatering because the groundwater table was just 5-feet below ground surface.
• Initial Record of Decision Elements

NFG signed a Consent Order in July 2008 and contracted with GEI on interim remedial work and supplemental investigations of the site. GEI worked with NFG, the property owner, and tenants to convey to the NYSDEC that their proposed remedy was unnecessarily disruptive to the site businesses. GEI demonstrated that a revised approach would meet the collective needs of the regulators, NFG, and site owners (protective of the environment, less disruptive to site tenants, remediation within the same schedule, reduced cost, less risk). The revised multi-technology approach supported substantive negotiations and discussions that helped align stakeholder goals and move the project forward.

NFG was successful at obtaining an Amended ROD in April 2013 on the basis that NYSDEC viewed ISS as a “new technology” for remediation of MGP impacts.

Amended Record of Decision Elements

ISS Multi-Tool Approach

Once the ROD was amended (resulting in significant savings to NFG in remediation costs), GEI discussed various project delivery methods with NFG. Ultimately, NFG asked GEI to serve as the prime entity for a design-build project delivery model. The value to NFG was having a single point of contact to manage the project, creative design/build solutions from competitively-bidding remediation contractors, and cooperatively aligned interests to complete the design and reduce the price and risk.

Using ISS as the remediation centerpiece, a 75% design package was offered to pre-qualified contractors for competitive bids. Bid evaluations were based, in part, on the remediation contractor’s experience successfully implementing ISS. Upon award, the design was completed with input from the Remediation Contractor to drive down the overall price through cost efficiencies while maintaining quality and reducing or sharing risk in an equitable manner. As a result of this collaborative design process, GEI saved on NFG’s remedial costs while maintaining the same field schedule, same level of quality, regulatory approval, and reduced risk. Careful stakeholder management practices were implemented with the existing tenants to maintain continuous business operations during remediation.

This multi-tool approach was successful, in large part, because of the revised remediation approach focusing on ISS as the centerpiece in a multi-technology approach. The ISS design took advantage of multiple ways of implementing an ISS project, as follows.

ISS Bucket Mixing

For the majority of the site, ISS via bucket mixing was selected as an alternative to auger mixing for solidification closer to the buildings.

ISS Bucket Mixing

As mentioned earlier, these buildings were in poor condition with shallow masonry foundations. GEIs geotechnical and structural groups determined that bucket mixing would be the safest way to ensure proper remediation while protecting the buildings. Geotechnical and vibration monitoring was also implemented to ensure there were no adverse impacts from the bucket mixing during construction.

Overview of the Structural Monitoring Array. Each Green Square Represents a Monitoring Point.

The Remediation Contractor performed pre-ISS excavations to 6 feet in depth in the general bucket mixing area with a shallower 4-feet depth pre-excavation within 15 feet of the buildings. The average depth of bucket mixing was 23 feet below ground surface. Overall, there were 37 bucket mixing grid cells completed which ranged from 112 cubic yards (CY) to 500 CY each. The volume of ISS was approximately 15,000 CY.

The cement to wet soil ratio was 5% (i.e. 5 lbs. of cement to 100 lbs. of soil at in-situ moisture content) and a cement to water ratio of 1:1 (i.e. 5 lbs. of water to 5 lbs. of cement). During implementation the water to cement ratio was increased to 1.5:1 to better mix and fluidize the deep clay layers.

Performance criteria were established in the design to define success. This included an unconfined compressive strength (UCS) minimum of 50 pounds per square inch (psi) and maximum of 500 psi and a permeability of 1×10-6 centimeters/second (cm/sec).

ISS Auger Mixing

Holder Removal: A former holder was located adjacent to a building. To safely address the holder and preserve the building, ISS auger mixed columns were constructed in a secant pile compression ring application around the subsurface gas holder walls. Creating this ring wall of overlapping ISS columns allowed the removal of the gas holder contents as well as the gas holder structure itself while protecting the adjacent building.

ISS Auger Mixing

The cement/water design mix was increased to 10% at a 1.5 water:1 cement ratio since the ring wall was serving as structural support for the gas holder and adjacent building. The performance criteria included a UCS minimum of 200 psi and a permeability maximum of 1×10-6 cm/sec.

The remediation contractor installed 38 ISS columns with a 10% or more overlap around the gas holder structure. The ISS columns were augered at least 1 foot into the competent clay beneath the gas holder, which corresponded to a depth of approximately 24 feet below ground surface.

Once the gas holder contents and structure were removed, the bottom of the gas holder area was solidified utilizing ISS bucket mixing.

Management of Dredged Sediment

Approximately 2,000 CY of impacted sediments were dredged Tonawanda Creek downgradient of the site. The original plan was to dewater the dredged sediments on site and transport and dispose of them off site at an appropriate disposal facility. However, NYSDEC approved of placing the dredged sediments in the former gas holder once the ISS ring wall was installed and the gas holder contents and structure were removed. Once the sediments were placed within the ring wall, they were stabilized with ISS bucket mixing to the same performance criteria as the general ISS bucket mixing material. This “green” solution (no trucking and no disposal fees) also produced significant cost savings for NFG.

Tonawanda Creek Sediment Removal inside Silt Curtain

The final component of the revised ROD called for the installation of NAPL recovery trenches at various locations along the site. These trenches would be installed in close proximity to sensitive infrastructure (canal) and third party owned abutting structures. The target depth for installation of the trenches was the top of the clay confining layer strata that was on average approximately 24 feet below the existing grade.

Given the challenging environment and target depth, the one-pass trench installation technique was selected as the preferred installation method because it was the most efficient and constructible. A specialty subcontractor excavated each trench, and placed the collection pipe and site-specific backfill in one continuous process. This alleviated concerns about potential damage to nearby structures by reducing the area for open excavation by a conventional track-hoe and supported project completion on-time and on budget.

One-Pass Trenching

Redevelopment

Preservation of the existing structures was a key component for the property owner’s buy-in. Replacement of the buildings would be expensive. The multi-technology approach allowed ISS implementation very close to the buildings, and encircled them with a low permeability matrix to prevent migration of impacts that may remain underneath the buildings. The NAPL collection trench installed along the creek will capture residual impacts downgradient of the buildings and prevent migration into the creek. The ISS mass is also a significant ground improvement. The original soils were unacceptable for new construction, and would have required installation of expensive deep foundation systems (e.g. piles). New structures can now be built with a less expensive slab on grade approach. As such, new structures can be built faster, which will in turn allow tenants to resume normal business operations sooner.

Value Added by ISS Multi-Technology Approach
Utilizing the ISS-centric multi-technology approach provided the following value to NFG:

• All the field work was performed safely with no incidents as ISS is a safer approach than shoring, dewatering and deep excavation while reducing truck traffic significantly.
• Allowed the work to proceed via the Amended ROD at significantly less than the original projected remediation cost while allowing NFG to retain control of the project.
• Preserved buildings while minimizing disruption to the existing tenants.
• Lowered the short-term risk of structural failures, emissions, and odors impacting the community.
• Long-term risk and schedule remained the same.
• Treating dredged sediments in the former gas holder area within ISS ring wall reduced transportation and disposal costs.

Closing

Remediation dollars can be significantly stretched via out-of-the-box thinking on the reuse of impacted material. Given the high cost of disposal, and sometime unavailability of thermal desorption facilities, the reuse of impacted soil on-site reduces cost. It also serves as a potential selling point to future site owners (e.g. ground improvement).

Please take a look at the video below for a visual summary of some of the remedial activities.