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The Hylebos waterway is located on Commencement Bay in Tacoma, Washington and is part of the Commencement Bay Nearshore/Tideflats Superfund Site. The waterway was dredged to its current configuration in the late 1960s and is currently used primarily for commercial and industrial shipping. Since the late 1800s the tide flats area has been used for ship building, oil refining, chemical manufacturing, storage, and other industrial uses. These activities caused the sediment deposited in the waterway to be contaminated with a number of pollutants.
The Head of the Hylebos Waterway Sediment Remediation Project spanned two construction seasons and involved the removal of over 450,000 cubic yards of sediments impacted with a wide range of compounds including PCBs, PAHs, and arsenic. In addition 18,400,000 gallons of sediment-ladened water required treatment prior to release back to the Hylebos Waterway.
Envirocon was initially contracted by the Head of the Hylebos Clean-up Group (HHCG) to construct and operate a sediment trans-load and water management facility to support the removal of contaminated sediment from the Hylebos Waterway. The trans-load facility was constructed to transfer contaminated sediments from barge to rail and consisted of a 12,000 sf containment area that extended onto a dock over the waterway. It was surrounded by ecology blocks and included sumps which collected excess water, directing it to a water management system for treatment. Sediment was removed from the barges using a long reach excavator equipped with a clamshell bucket and the sediment was placed in the containment where front-end wheel loaders would then transfer the sediment from the containment to rail containers for shipment to the landfill. Sediment that could not be directly loaded into rail containers was transported and placed in stockpile locations on site for loading at a later date. Envirocon loaded up to 5,000 tons of contaminated sediment per day onto rail containers over a 16- to 24-hour (two shifts) period.
In addition to the trans-loading of sediments, Envirocon was required to construct and operate a dewatering system that removed and treated excess water from the sediment barges and the trans-loading facility. Water was treated to within the water quality standards set for the project and released back into the waterway. Initially the water treatment consisted of the addition of flocculants followed by routing of the water through a series of weirs where suspended sediments would drop out before returning the water back into the waterway. During the second season a combination of flocculants, mixing tanks, and geotubes were used to remove the suspended sediments. Sediments removed from the water were subsequently loaded onto rail containers and transported to the landfill for disposal.
In the second construction season, Envirocon's scope of work was increased to include the remaining dredging of approximately 65,000 cubic yards of contaminated sediments.
Dredging of the Hylebos Waterway was performed by Envirocon using an excavator that was modified to meet the requirements of the project. Major modifications included:
- An additional cab which would house the electronic equipment for the excavator's on-board positioning system and seating for the project engineer's observer.
- Installation of third member excavator stick and a 3 cubic yard rotating clamshell bucket.
- Installation and testing of an RTK-GPS positioning system that controlled excavation depth to within two-tenths of a foot accuracy in an X, Y, & Z coordinate. The under water positioning of the excavator bucket was through LCD monitors mounted in the cab of the excavator which the operator and observer used to place the clam bucket in a pre-determined excavation pattern.
- Dredging occurred over a 5-month period and was performed using a two-pass process with the bulk of the contaminated sediments being removed during the first pass followed by a second pass which removed the remaining contaminated sediments producing a clean waterway sediment surface.
- At the conclusion of the first season, the HHCG had performed additional sampling over the remaining area to be dredged and determined that some of the areas to be dredged by Envirocon were contaminated with asbestos containing material (ACM). Envirocon was required to dredge, trans-load, and dispose of material in these areas as ACM regulations required. This also required modification of the trans-load operation to meet the requirements for handling ACM. This included additional worker safety training and decontamination facility modification.
- Additional work performed by Envirocon during the Hylebos Sediment Remediation Project included the excavation and backfill of intertidal and under dock sediments. Intertidal excavation and backfill was performed from the shoreline during low-tide periods. Work in the intertidal areas was subject to working around the tidal range which was up to 15 feet over the course of 6 hours, limiting work time within the area. Under dock sediments were removed using an additional excavator with a standard bucket mounted on a barge which worked around the dock structure.
- Special operation procedures were developed for excavation under the dock due to the client's recommendations that oxygen and acetylene bottles may have been lost off the dock during the operation of the plant. Two oxygen bottles were found and disposed of, without incident.
- At the conclusion of the project, over 450,000 cubic yards of contaminated sediments had been removed from the Hylebos Waterway and over 15,000 rail containers had been loaded and disposed off site. The project was completed to the regulatory agency’s standards and the satisfaction of the HHCG.
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