Significant Project Features
- Project conducted by USEPA’s Great Lakes National Program Office at the inner harbor of this active community and required a detailed community relations plan and health and safety controls
- Dredging operations conducted on a 24/7 basis
- Mechanical dredging of approximately 170,000 CY of PCB-contaminated sediments
- Stabilization of sediments using Class C fly ash was required to condition material for offsite transportation and disposal. Perimeter misting system was installed to minimize dust missions
- All water generated from dredging, stabilization, and decon operations was treated onsite through a 200 gpm WWTS
History & Location Details
The Sheboygan River and Harbor Site, Wisconsin includes the lower 14 miles of the river from the Sheboygan Falls Dam downstream to, and including, the inner harbor before entering Lake Michigan. The former Tecumseh Product Co. plant in Sheboygan Falls is located on the upper river and is considered the primary source of PCB contamination in river sediment. Cleanup of the upper portion of the Sheboygan River was completed under separate contracts. The USEPA’s Great Lakes National Program Office (GLNPO) issued a task order to Environmental Quality Management (EQM) under its SRAC contract to execute the harbor dredging work. Sevenson, EQM’s team partner on the SRAC, was subcontracted to perform the dredging and stabilization tasks associated with the project. Sevenson mechanically dredged +/– 170,000 CY of sediment below the 8th Street Bridge to the mouth of the river. This provided a 10 – 16 ft navigational channel. Dredge sediments were loaded into scows and pushed to a dock where sediment was stabilized and transferred to a parcel for loading for offsite T+D. Dredging operations continued 24/7.
Dredging was conducted in two (2) locations. Dredge Area 1 was an upstream section where mechanical dredging was completed to 11 ft below LWD. Dredge Area 1 extended approximately 1,500 ft downstream from the Eight Street Bridge and covered an area of approximately 5 acres. Dredge Area 2 was the downstream section of the harbor where mechanical dredging was completed to 15 ft below LWD. Dredging Area 2 extended approximately 2,150 ft downstream from the downstream end of Area 1 and covered a surface area of approximately 11 acres.
SITE SETUP AND SITE PREPARATION
When the notice to proceed was received mobilization, site setup and site preparation activities were initiated. This included project and support offices. A staging pad for sediment solidification, dewatering, staging and load out was installed. The pad was placed over an existing parking lot and was constructed with 4 inches of asphalt on top of a HDPE liner. Temporary roads were installed for access and loading of trucks as well as delivery of reagents. Work barges were secured in the harbor to receive scows filled with dredged spoils.
Dredging operations were scheduled to take place in a busy harbor. Prior to initiating dredging environmental and marine controls were installed. The bathometric survey was reviewed to determine if debris removal would be required using conventional equipment deployed on a barge with a scow. Debris was loaded and ferried back to the transfer area for off-loading and processing prior to offsite disposal. Dredging was performed utilizing a Komatsu 800 hydraulic excavator equipped with Young 3-yd3 hydraulic clam shell bucket. An IHC XPM System was installed on the excavator to guide and monitor dredging cuts. Dredging was conducted from a work barge platform and placed into position with a tug boat and anchored in specific locations using spuds deployed from the work barge. Dredging progressed from upstream to downstream for both Dredge Areas 1 and 2. Dredging was performed up to the limits of the navigation channel in both Dredge Areas 1 and 2. Dredging began in Area 1. Removal was performed to 11 ft below LWD. Sediment removal thicknesses ranged from
Dredging in Area 2 was performed to 15 ft below LWD. Sediment removal thicknesses ranged from approximately 3 ft to approximately 9 ft.
Three (3) scows were utilized to contain and transfer the dredged sediments to the mixing area. Scows were loaded to allow addition of the fly ash agent with enough free board to allow for mixing. Scows were approximately 42 ft by 167 ft. After the loaded scows were secured to the mixing barge, excess water was pumped from the barge to the settling tank that lead to the water treatment system. Following water removal from the scow, solidification/stabilization operations began.
Dredging was monitored daily to track progress in three ways. The first used an IHC XPM system. The software tracks the dredge bucket in real time and a computer screen displays the bucket depth and footprint during operation. A dredge template is also displayed showing the operator where to stop the dredge bucket. Each time the dredge bucket closes, the footprint of the bucket changes the screen color.
The second was through weekly check systems. Sevenson used a single beam hydrographic survey system to perform weekly volume checks. The survey system on board the survey vessel was an RTK DGS receiver, a fathometer, and a PC computer running Hypack for data collection and post processing.
The third way progress was monitored and involved monthly progress surveys performed by USACE. Data generated by USACE was reviewed jointly.
SOLIDIFICATION/STABILIZATION MIXING OPERATION
The solidification/stabilization mixing process consisted of Class C fly ash. Fly ash was added to the sediment contained in the scows using two long-reach excavators that operated from a work barge. Fly ash was delivered to the site in pneumatic tanker trucks, and the material was blown into a storage pig. Fly ash was deployed from the silos down a shoot to fill excavator buckets. Misting equipment was installed and utilized to control dust emissions when adding or mixing fly ash into the sediment.
DEWATERING AND OFF LOADING SCOWS
Latent water was pumped from the loaded scows and transferred to settling and frac tanks in the waste water treatment plant area. Site water was pumped to a single 18,000-gallon weir tank. Effluent from the weir tank was pumped to three 21,000-gallon frac tanks. These tanks acted as both settling tanks and flow and load equalization tanks. Water was processed through the WWTP which consisted of settling tanks, four (4) bag filters, two (2) sand filters, two (2) granular activated-carbon (GAC) vessels designed to remove volatile and semi-volatile organic compounds to two (2) 1-micron bag filters to capture any carbon fines from the GAC process.
As sediment was off loaded from the scows, it was placed on the staging pad with rubber-tire loaders to promote further dewatering and drying prior to load-out for disposal. Material was spread out over the pad in thin lifts to maximize air contact and exposure to the sun to promote evaporation of residual water. Material was consolidated into taller stockpiles and blended with fly ash to dry any free water remaining.
TRANSPORT AND DISPOSAL
Trucks were staged for loading before entering the site. Load out was done with either rubber-tire loaders or excavators from a stockpile. Truck tires were pressured prior to departing the site. The landfill provided a dedicated access road and dedicated dump area with a convenient circular route or turn-around area. This allowed for fast, safe, expedited flow of truck traffic. A wheel was station was provided.
SITE RESTORATION AND DEMOBILIZATION
Removed all installed structures, items, and materials (e.g., liner materials, temporary pads, roadways, environmental controls, fencing, etc.). Replaced asphalt and any other structures removed during the project. Removed perimeter sediment and erosion controls and performed final restoration of all disturbed areas. Conducted post-installation inspections with appropriate stakeholders. Demobilized site equipment, materials, and facilities.