• Construction Management & Oversight
• Former MGP Site
• Remedial Implementation – ISS Bucket Mixing down to 35ft bgs
• Remedial Design
• ISS Bench Scale Mix Testing
• QA/QC Sample Testing
• Vibration Monitoring Adjacent to Sensitive Structures
• Perimeter Air Monitoring
• Building Demolition/ Asbestos Abatement
• Environmental Footprint Analysis
In 2014, GZA assisted a confidential client with remedial action development, construction management, and oversight for remediation of coal tar-impacted soil at a former manufactured gas plant (MGP) site in Massachusetts. The work was performed in accordance with the Massachusetts Contingency Plan (MCP). The remediation implementation contract was approximately $1.8 million.
Challenge
The initial remedial design called for excavation and off-site disposal of tar-impacted soil to depths exceeding 20 feet below ground surface, extending below groundwater into highly permeable soils. Pump tests recommended by GZA demonstrated that this approach would require significant dewatering and water treatment. The project required an alternative source control method capable of treating deeper impacted soils, along with controls and monitoring to address odors, air quality, vibration, and construction health and safety requirements.
Solution
GZA performed bench-scale mix testing to evaluate in-situ stabilization/solidification (ISS) as an alternative to excavation and disposal. Following selection of ISS, GZA prepared contract drawings, specifications, and bid documents for treatment of tar-impacted soils to depths of up to 35 feet below ground surface and assisted National Grid with bid evaluation. The contract drawings and specifications were prepared in May 2014, and remedial implementation began in September 2014.
GZA provided full-time construction management and remediation oversight for demolition and asbestos abatement of an on-site garage; excavation and disposal of approximately 450 cubic yards of MGP-impacted material from an underground gas holder; and ISS treatment of approximately 6,900 cubic yards of tar-impacted soil at depths ranging from 15 to 35 feet. The gas holder was emptied, cleaned, and backfilled with uncoated crushed concrete reused from the former garage foundation. ISS was performed using a bucket mixing technique believed to be among the deepest applications of this method. Odor-suppressant foam, vibration monitoring adjacent to perimeter buildings and structures, and continuous perimeter air monitoring using Air Logics’ systems supported implementation.
GZA’s responsibilities included monitoring compliance with contract drawings and specifications, reviewing monthly contractor payment applications, maintaining the submittal registry and perimeter air monitoring system, collecting and testing quality control samples of ISS-treated material, observing contractor compliance with project health and safety requirements, and participating in weekly construction coordination meetings. GZA also used EPA’s Spreadsheets for Environmental Footprint Analysis (SEFA) to quantify carbon dioxide-equivalent savings associated with ISS compared with an excavation, removal, and thermal desorption alternative.
Benefit
GZA’s evaluation supported selection of a source control approach that avoided the significant dewatering and water treatment effort associated with the initial excavation design. ISS treatment was completed in December 2015 in accordance with the project specifications. GZA’s integrated design support, construction management, and quality control services supported implementation of the selected remedy, while the environmental footprint analysis quantified its carbon dioxide-equivalent savings relative to the evaluated alternative.