Sediment Cap Design, Modeling, and Construction at a Former MGP Site
Tom Boom, Vice President and Senior Environmental Engineer, Barr Engineering
Co-Authors: Andrew Santini, Consumers Energy, Dale Kolstad, Barr Engineering and Steve Shaw, Sevenson Environmental Services
Sediment remediation at a former manufactured gas plant (MGP) site in Flint, MI was completed during the 2017 construction season that included dredging sediment and placing a multi-layer sediment cap within the Flint River. The objectives of the cap were to: 1) create a barrier between remaining MGP-impacted sediments below the cap and the river, 2) provide stable riverbanks and riverbed, and 3) incorporate bedform diversity elements for improved aquatic habitat. Meeting these objectives required evaluating multiple options for establishing a cap along 1,700 feet of the riverbed. Blended Barrier (a patented mixture of AquaBlok® and gravel) was selected for in-water cap applications and geomembrane for above water capping. Bench scale testing was performed on the material under simulated field conditions. The testing data was used to demonstrate capability to meet barrier objectives, provide inputs for geotechnical modeling, used in designing supplemental cap layers, and construction requirements. A filter layer evaluation, hydrodynamic evaluation, and global slope stability and veneer cover stability modeling were completed iteratively within the cap design process. Pragmatic construction specifications were established that included a wide range of core elements: on-site Blended Barrier preparation (mixing) and management, appropriate barge spudding, equipment setbacks, and allowable tolerances and timeframes for placement of cap materials. Successful implementation of the selected remedy relied upon the placement of the cap materials within the construction QC requirements. During construction, conforming with the specifications required adaptive management, modifications as conditions changed, heightened coordination, and expedited review of survey data. The project outcome is a physically stable and functional multi-layer cap installed to meet the design intent. Results that will be discussed include requirements that were modified based on construction limitations, field observations, construction QC data collected and used during implementation, and other constructability and design lessons learned.
Tom Boom is a civil engineer and vice president at Barr Engineering who has a wide variety of experience managing large, complex MGP sites. He has been the project manager for multiple MGP sediment remedies.