Significant Project Features
- Excavation, transportation, and disposal of more than 12,000 CY of MGP-contaminated sediment
- Successfully utilized custom-fabricated hopper scows and DOS scows to move and dewater sediment prior to T&D
- Transported material in DOS scows using tugboats, via the Hackensack River, to the designated disposal facility
- Sevenson completed over 9,276 safe work hours without a lost-time incident
History & Location Details
The Former Hackensack Gas Works Site is located in the southern portion of Hackensack, New Jersey, along the western bank of the Hackensack River. The site was used as a former manufactured gas plant (MGP) from 1867 to 1908 and is comprised of several Areas of Concerns (AOCs); however, scope of work for this project was limited to Area E. The area consists of impacted sediments and surface water within the Hackensack River and adjacent Brosses Creek.
Site preparation included mobilization of equipment; installation of erosion control measures; steel sheet pile installation; and set up of river access locations (including a floating pier). Sevenson also located and verified all existing utilities/structures; installed a turbidity curtain and turbidity monitoring controls; and conducted topographic/bathymetric surveys.
Before dredging, Sevenson located several utilities, including a municipal sewer line, gas lines, electrical lines, and fiberoptic lines running under the Hackensack River within the project limits of disturbance. Utilities were located using several methods including geophysical survey with soft dig, submarine utility survey, and submarine electromagnetic tip survey. After utilities were located, they are marked in Hackensack River using marker buoys. Following utility location activities, Sevenson removed large debris within the dredge area (in the top several feet) using a PC800 long-front excavator with an environmental clamshell bucket. Debris near the shoreline was visually identified, while submerged debris was identified through visual and hydrographic means (including the pre-construction hydrographic survey). All pre-dredging debris was placed into hopper scows and transported to the designated disposal facility. Any additional debris encountered during production was comingled in the DOS scow and transported to the disposal facility.
For dredging operations, Sevenson utilized a PC800 long-front excavator with a 5.5 CY CableArm environmental clamshell bucket mounted on a 40-ft. x 80-ft. spud barge. Dredged material was loaded directly into a Department of Sanitation (DOS) scow (with a total capacity of 800 CY). Once at full capacity, the DOS scow was towed to the designated off-site disposal facility.
In the DOS scows, dredged sediment was dewatered to the extent required by the designated disposal facility. Decant water was conveyed, using portable pumps and hoses, from the hopper scow to a separate decant DOS scow prior to transportation and disposal.
Dredging was performed in approximately 40-ft. lanes, which allowed for approximately 10-ft of excavator overlap reach into adjacent lanes, across four main areas of the site. Due to the shallow existing sediment elevations, dredging began in deeper dredging areas and progressed to shallower areas with cuts no deeper than 2.5 ft. at a time. Crews removed approximately 1,200 CY per day. In total, Sevenson dredged >12K of contaminated sediment.
Sevenson minimized over-dredging by utilizing Trimble’s Teledyne PDS positioning and tracking software. This software allowed the dredge operator to simultaneously view the location of the bucket attachment and the position of the excavator in real-time. Positioning was calculated using an integrated RTK GPS/GNSS receiver with two antennae and a 450 MHz radio, as well as a series of inclinometer and rotation sensors on the boom and stick. Two additional inclinometers were placed perpendicular to each other at the machine center of rotation. The measurements taken by these sensors allowed the software to determine the position of the cutting face of the bucket and its orientation in relation to bathymetry and target depth surfaces, as well as previous cuts. Sevenson checked the bucket position daily using RTK survey methods, or known elevations, at the top of marked sheet piles. RTK verification was performed by placing the dredge bucket at a measured point and then comparing the RTK measurement with Teledyne positioning output.
TRANSPORTATION AND DISPOSAL
DOS scows containing sediment and decant water were transported to their respective disposal facilities using a local tug service subcontractor. Tugboats with approximately 1,500 HP were used to push DOS scows to and from the disposal facility on the Hackensack River. Upon arrival at the disposal facility, each loaded scow was secured against the dock using a series of winches and cables/ropes. During T&D operations, Sevenson ensured transport did not interfere with marine traffic in occupied areas of the Hackensack River by implementing our Navigation and Waterway Control Plan and closely coordinating with adjacent property owners to avoid disruptions.
Following T&D, Sevenson proceeded with backfill of the river bottom. Areas closest to shore were backfilled first, with deeper areas of the river backfilled last to avoid grounding of Sevenson vessels at low tide. In specified areas, Sevenson placed a layer of sand and Organoclay© that was transported to the area using a barge, and mixed in 100-CY hopper scows using an excavator bucket. Backfill material was placed to the required thickness and verified using lead line measurements.
Sevenson also conducted restoration of the upland area of the site (the upland area consists of all areas along the shoreline adjacent to dredging). This area was restored to pre-construction conditions, as shown on the pre-construction topographic survey. Sevenson performed all of the work from the water and did not disturb upland soil, plants or trees. Existing rip-rap areas were restored with subsoil to 12 in. of existing grade and a 12-in.-thick layer of 6-in. D50 stone.