Significant Project Features
- Self-performed mechanical dredging, characterization, stabilization, and T&D of 1,544 CY of river sediment impacted by MGP contaminants
- Designed, constructed, operated, and maintained an on-site 100-GPM temporary WWTP that treated >18,000 gallons of impacted water
- Installed a 3,462-SY, multilayer sediment cap to contain MGP-related residuals from impacting local riverine habitats and the general public
- Implemented structural improvements to an existing combined sewer overflow outfall that effectively reduced soil erosion issues
- Installed two new DNAPL collection wells to eliminate DNAPL accumulation
- Implemented dust, odor and vapor mitigation controls and an air monitoring program in a densely populated urban area; completed > 13,000 workhours without a lost-time incident
History & Location Details
The St. Mary’s River flows into Fort Wayne from the south. The Site’s former MGP operations began in the early 1850s and continued into the 1940s. Before commencing sediment removal operations, Sevenson established a staging area at the Old Fort property, which consisted of a dewatering pad, a temporary WWTP pad, a dredged sediment staging pad, and an access point to the river. Sevenson also constructed a temporary access road to the dewatering pad. Field crews installed a turbidity curtain downstream of the proposed work area. Crews also built a temporary tracking control pad on which all heavy equipment was decontaminated. Also integral to site preparation was the design, construction, operation, and maintenance of a temporary on-site 100-GPM WWTP, which consisted of four frac tanks, bag/sand filters, and carbon vessels. All treated water was discharged to the St. Mary’s River.
Sevenson executed work at two areas of the Fort Wayne Former MGP Site. These areas include:
St. Mary’s River (AOI-5)
Sevenson mobilized a fleet of equipment including a 70-ton crane, three hopper barges, a push boat, and PC400/PC360 long-front excavators (equipped with an environmental clam buckets and GPS), and off-road haulers. Site preparation included installing erosion and sediment controls, such as silt fencing, check dams, drainage ditches/barriers, and sediment filter socks; and utility demarcation. Clearing and grubbing operations required using a PC220 excavator (equipped with a grapple) to remove and place river debris into hopper barges. Hopper barges transported removed debris to a boat ramp area, where it was offloaded and placed into a truck for off-site disposal. Field crews also dismantled and disposed of an existing wooden dock.
Mechanical Dredging, Dewatering, & Stabilization
Using a 70-ton crane, Sevenson constructed a work barge in the river by placing six 40-ft. x 10-ft. x 5-ft. Flexi Float modular barges together, which were supported by spuds and winches. Sevenson mechanically dredged 1,544 CY of contaminated sediment, to depths of 0 ft. to 2 ft. BGS, from river waters ranging from 5 ft. to 13 ft. in depth. Dredging was performed from atop of the floating barge. Excavators were equipped with a 1-CY bucket and guided by GPS software that eliminated potential risks of over- and under-dredging. Sevenson began dredging at the west bank and proceeded to the middle of the river, and progressed downstream until reaching the dredge limits. One site-specific challenge involved maintaining turbidity curtains. This river was prone to flash floods and storm events with more than .75-in. of rain caused conditions that would breach the curtain. To ensure that activities adhered to water quality criteria, field crews installed a second curtain that was supported by additional anchors. Another challenge involved encountering an unidentified underwater utility cable. Sevenson contacted the utility owner and confirmed that it was no longer in operation.
Dredged sediment was loaded into hopper barges and transported to the boat ramp, where a PC300 excavator offloaded and transferred material to off-road haulers. The off-road haulers delivered dredged sediments to the dewatering pad. At the dewatering pad, sediments were stockpiled and gravity drained. Following dewatering, Sevenson used a PC300 to mix Portland cement to further stabilize sediments. Following mixing, Sevenson collected samples of the processed material for paint filter tests. If the treated sediment passed the paint filter test, the sediment was loaded for off-site T&D. If the test failed, Sevenson added additional conditioning agent. Surface water on the dewatering pad was collected in sumps and pumped to the influent tank of the on-site WWTP.
Following mechanical dredging operations, Sevenson installed a multilayer reactive cap. The purpose of the cap was to encapsulate hazardous MGP residuals in the sediment and riverbank to prevent the downstream migration of contaminants, as well as to protect the general public and riverine organisms (fish, reptiles, and benthic organisms). Cap installation began with placing a six-in. layer of Organoclay and Aquagate material. Cap material was loaded into scows from the boat ramp on the Old Fort property, then delivered to the proposed cap location and placed in sections parallel to the riverbank. Materials were placed by utilizing the PC400 long-front excavator equipped with an environmental clam bucket. The clam bucket carried the material to an elevation just above the water surface, where it was deposited.
The next cap layer was a reactive core mat, which was placed perpendicular to the west riverbank. The mats were deployed from the small work barge by using an excavator equipped with a spreader bar off the boom. Field crews then placed mixed native stone to secure the mats in place. Sevenson verified the accuracy of mat installation with subcontracted underwater divers. Sevenson proceeded to install a six-in. layer of stone using hopper barges, as well as a one-ft. layer of riprap. A layer of articulating block was also installed at an outfall to provide additional bank protection from hydraulic forces.
CSO Outfall Improvements at Former MGP Site (AOI-1)
The Former MGP Site (AOI-1) is located on the west side of the St. Mary’s River work area. A combined sewer overflow (CSO) outfall adjacent to the former MGP had severely eroded banks due to frequent stormwater discharge to the river. The CSO outfall improvements included extending the existing 48-in. outfall culvert further into the river. The new outfall is now supported by sheet piling and six ft. of structural fill. Sevenson also installed two new DNAPL collection wells.
Sheet Pile Installation
Before sheet pile installation, Sevenson placed a 40-ft. x 60-ft. wooden mat platform on an existing asphalt parking lot adjacent to the CSO structure. Crews mobilized a 150-ton crawler crane, equipped with a 160-ft. main boom, as well as a work barge with a PC400, to excavate a four-ft. bench along the H-pile alignment to provide access to the piling crew. Sheeting materials consisted of HP 14×89 soldier piles and PZC-26 sheet piles.
Solider piles were installed 7 ft. apart using a PTC 60HV vibratory hammer, to embed them to a minimum of 14 ft. into the glacial till substrate layer. Following H-pile installation, the PZC-26 sheets were driven to a depth of 2 ft. into the glacial till. The elevations at which the sheet piles were installed were determined based on how effective they would be in creating a barrier to potential DNAPL seepage. Sevenson cut one-in.-diameter perforations into the sheet piles at the water table level in order to facilitate the passage of groundwater without allowing NAPL to permeate the sheet piles. The restored slope, which was constructed using structural/general fill and mixed native stone, provided erosion resistance. The installed culvert was then backfilled with structural fill, placed in six-in. lifts, and compacted to 95%. A six-in. articulated block was also installed at the mouth of the culvert to protect the sediment cap and provide additional bank protection.
Sevenson executed site restoration to address disturbed terrains at both areas of the site. Due to the site being an erosion-prone area, Sevenson was required to installed effective erosion controls to protect the riverbanks and the installed cap. Although the currents of the St. Mary’s River are typically slow, flows increase considerably following storm events. Field crews installed four-in. riprap on the channel bottom and six-in.-diameter stone on the side slopes. Angular, instead of rounded, riprap was also placed due to its greater capacity for erosion resistance. Following riprap placement, Sevenson installed stone on disturbed riverbank areas, and topsoil and tree/shrub plantings to impacted upland areas.