Pledger Creek Remediation

Significant Project Features

• Hydraulic and mechanical dredging in three areas within Pledger Creek to remove 66,725 CY of sediment.
• Placement of a 5 acre sediment cap in three locations within Pledger Creek at the completion of dredging operations.
• Re-graded 21,740 CY of an inactive on-site landfill to prepare the dredged sediment dewatering area.
• Dewatered 66,725 CY of sediment via 70 Geotubes®.
• Designed, installed, and O&M of a 2,000-GPM WWTP; treated and discharged >164M gallons of filtrate.
• Installed 1,050 LF of sheet pile for shoreline stabilization and additional protective measures.
• Recipient of 2023 WEDA Safety Excellence Award.

Project Gallery

History & Location Details

The Pledger Creek site is in Mannington Township, Salem County, New Jersey. The scope of work included remediation in an area of the inactive on-site industrial landfill, dredging contaminated sediments from Northern Pledger Creek (NPC), dredging Central Pledger Creek (CPC), and dredging Southern Pledger Creek (SPC). The creek reaches which required sediment dredging are west, north, and adjacent to the facility. The primary contaminants of concern were PCBs and mercury.

Project Description

Scope of Work

Sevenson’s work at Pledger Creek consisted of re-grading 21,740 CY a landfill for the dewatering area; hydraulic and mechanical dredged 66,725 CY of contaminated sediment from Pledger Creek; dewatered sediment via 70 Geotubes®; and placed 214,900 SF of a sediment cap. Sevenson also designed, installed, operated, and maintained a 2,000-GPM WWTP, which treated and discharged >164M gallons of water.

Site Preparations/Mobilization

Sevenson prepared work plans and mobilized company-owned equipment, including various excavators, dozers, and off-road hauler trucks to re-grade the landfill; media vessels, pumps, frac tanks, pump and bag filters for the WWTP; dredge and HDPE piping for hydraulic dredging; and a specialty excavator barge setup, fabricated in-house in Niagara Falls, NY, – a 24 CY hopper scow – for capping operations. This setup was special because it allowed the use of an excavator on the water in low draft areas (<2 ft.). This was needed for capping capabilities in 2/3 of reaches in the creek. Crews mobilized to the site in September 2021. Erosion control measures were installed around work areas including silt fencing. Crews designed, installed, operates, and maintains a 2,000-GPM  WWTP. Sevenson also conducted approximately 6,000 LF of fusion welding and positioning of a 2-in., 10-in., and 12-in. HDPE pipe from the dredged areas to the sediment dewatering area where it was connected to the Geotubes® on the sediment containment pad for dewatering.

Re-Grading of Landfill

Sevenson utilized a Komatsu D51EX bulldozer (with GPS capabilities) and Komatsu PC300 excavator to conduct debris removal activities and mulching using a tub grinder. Sevenson performed the re-grading of 21,740 CY of material. The bulldozer pushed up material creating the necessary berm around the entire perimeter of the eventual dewatering pad. As the material was being pushed up, the excavator was used to shape and compact the top and side slopes of the berms.  

Subsequently, Sevenson placed 16,000 SF of a Geosynthetic Clay Layer (GCL). This was installed along the western landfill slope and along the banks of the SPC. AquaBarriers were installed within the creek to dewater this slope. At that point, the GCL was deployed and anchored into a 24×18 in. anchor trench. An excavator was used to deploy the GCL with a specialty attachment to assist the land laborers with deployment. On top of the GCL was a 12 in. layer of common fill sand that was placed and graded by a LF excavator. The final layer to this cap was 18 in. of rip rap under laid with filter fabric. Sevenson placed 12,000 SF of filter fabric beneath this rip rap. The rip rap was R-5 and was also placed with the LF excavator. This was a permanent landfill cap that extended from the water’s edge to the new landfill access road.

Then the permanent cap continued from the access road to the top of the landfill dewatering gallery. In this area the permanent cap was composed of 18 in. of low permeability clay, which was compacted to reach a minimum permeability of 10 to the minus 5 cm/sec as defined by lab testing do to moisture in the soil. Sevenson had to strategically place and then rake the clay material (using a specialty excavator attachment) in order to achieve this permeability. The next layer on top of the clay was either 12 in. of Driving Surface Aggregate, under laid with filter fabric, or 6 in. of topsoil. This was along the entire western portion of the landfill.

On the remaining three sides of the landfill, Sevenson prepared the subgrade surface of existing materials, and then imported and placed 6 in. of common fill and 6 in. of vegetative topsoil. This is a temporary grading and vegetative cover until the landfill is permanently capped under a different contract. To prevent erosion on the landfill slopes, Sevenson managed a subcontractor to hydroseed the area to spread seed and erosion control blankets.

Sheet Pile Installation

Sevenson installed temporary sheet pile along the active Salem-County owned railroad line for shoreline stabilization and additional protective measures. Crews installed 1,050 LF of  AZ-19-700 sheet pile, ranging in depths from 25–35 ft. utilizing a CAT 345 excavator and PVE Side Grip Vibratory Hammer. Survey monitoring is being performed during this work to ensure there is no settlement of the railroad. Once all dredging and capping operations are complete, the temporary sheet pile will be extracted using the same equipment crews used to install the sheet piling.

Hydraulic and Mechanical Dredging

Dredging operations began in November of 2021 and finished in July 2022. Sevenson used an 8-in. Moray dredge to hydraulically dredge. Once the sediment was hydraulically dredged, it was pumped into Geotubes® for dewatering. Sevenson hydraulically dredged 10,788 CY from SPC; 17,306 from CPC; and 35,331 from NPC, totaling 63,425 CY.

Sevenson mechanically dredging two areas within Pledger Creek using a PC300 LF excavator on a barge, totaling 3,300 CY of sediment from CPC (2,800) and 500 CY from NPC. Sediments removed mechanically were placed into scows and transported to an unloading area where it is transferred to the staging area for processing and final disposition. Mechanical and hydraulic dredging depths range from 1–5 ft. with a 6 in. vertical overdredge tolerance. In total, crews dredged 66,725 CY of material.

Hydraulic Dredging Challenges

There have been a couple issues with dredging: 1: Sediment within the creeks has been extremely soft and silty, making it difficult for Sevenson to limit over dredging. With responsive quality control and GPS machine control awareness, Sevenson was able to reduce the amount of over dredge. Sevenson increased surveying during dredging operations and is raising the target elevation of the dredge cutterhead allowed for reduced non-pay overdredge volumes. 2: Due to the sediment being extremely soft and silty, material is sloughing back into the dredge prism from areas outside of the dredge limits. Sevenson has increased the amount of dredging outside of the dredge limits to help reduce material from sloughing back. 3: During removal of Phragmites in one of the creeks, a dense clay was found underlying these islands. It was Sevenson’s intention to remove this material with the hydraulic dredge, however the cutterhead quickly became clogged with this clay material. Sevenson then built a composite mat roadway out to the Phragmites and removed the entire footprint of these Phragmites mechanically. The material was transferred to off-road hauler trucks and then dumped into the landfill dewatering gallery.

Material Dewatering

Dewatering activities began in September 2021. Once the sediment is dredged, a dry polymer of Magnafloc 5250 and liquid polymer of Magnafloc 1617 is introduced. The dry polymer is diluted with water in a large on-site tank prior to being sent into the dredged slurry. Once the correct dilution is achieved a remote-control electronic pump sends the polymer to the dredge pipe. The dry polymer hits the dredge sediment prior to the liquid polymer, where it then passes through a static mixer. The dry polymer is a flocculant that helps with the larger fines of sediment to drop out when in the Geotubes®. The liquid polymer is also diluted with water prior to the dredge slurry and is technically a coagulant. This is introduced after the dry polymer and targets the smaller fines of sediment and helps with water quality.

Geotubes® are 65 ft. in circumference and range from 250–330 ft. length. Our Geotubes® are stacked three tubes high and, at this time, it is undetermined what the drying period will be (the bags will remain on-site once Sevenson demobilizes and a separate contract will regrade the material within the bags). To date, Sevenson has utilized 26 Geotubes® to dewater >32,000 CY of sediment. Sevenson utilized 70 Geotubes® (stacked five tubes high due to additional dredging) to dewater >66,725 CY of sediment.

Capping

Capping operations began in early January 2022 and ended in August 2022. A sediment cap consisting of medium sand, fine gravel and stone is being placed in certain parts the creek. The overall area being capped is 214,900 SF. In all three areas, Sevenson will place sand, gravel, and stone along the banks of the creeks to various depths, deepening on dredge cuts. In the open water cap areas, crews will place 6 in. of sand and 6 in. of gravel. Capping operations in the SPC were completed over a 50,900 SF area (1,533 Tons of sand; 1,690 Tons of gravel; 295 Tons of stone were placed); the CPC over a 103,000 SF area (2,940 Tons of sand; 2,185 Tons of gravel; and 900 tons of stone were placed); and the NPC over a 61,000 SF area (2,857 Tons of sand; 1,530 Tons of gravel; and 1,097 Tons of stone were placed). In the SPC, crews used a specialty PC120 excavator barge (see photo) in the shallow creek with a 24 CY hopper scow to move material from land to the barge. Crews utilized a 40×50 ft. capping barge with a PC300 LF excavator atop of it. Sevenson demobilized from the site in October 2022.

WWTP

During site preparations (see above for what equipment was mobilized), Sevenson designed and installed a 2,000-GPM WWTP. Crews are currently operating and maintaining the WWTP, where it has treated and discharged >164M gallons of water back into Pledger Creek via Mannington, Inc.’s Freshwater Wetlands General Permit.

STAKEHOLDER INTERACTION

Sevenson has subcontracted the owner of the rail (SMS Rail) to install a protective layer of fabric and Ballast Stone. This protective stone was needed for Sevenson to traverse heavy equipment along the rail to install/remove sheet pile, and also to haul mechanically removed dredge sediment to the landfill via off-road trucks. SMS Rail will remove this stone at the end of the project. The open line of communication between Sevenson and SMS Rail has been very good during this process.

Quality Control/Health and Safety

Sevenson oversees and implements a stringent QC program at the Pledger Creek Remediation site. Our on-site QC Manager performs daily inspections of all work areas. All turbidity monitoring is done by Langan Engineering. To date, there have been no turbidity exceedances.

Sevenson’s H&S Officer enforces the site-specific H&S Plan. Our H&S Officer holds daily safety tailgate meetings with all site personnel at the start of the day to discuss hazards associated with each day’s work. Work is performed in Level D and Level D Modified PPE under the H&S Officer’s supervision. The H&S Officer performs real-time air monitoring using a Jerome J405, DRX and MiniRae to monitor for dust, lead, methane, and mercury. There have been no exceedances to date. Our H&S Officer works with our Corporate H&S Manager to oversee and implement Sevenson’s COVID-19 safety protocols on-site including following the changing recommendations from the CDC, enforcing the use of face masks (in 2021) and ensure all workstations are clean and sanitized. Sevenson completed >66,000 safe work hours without a lost-time incident.