TenCate Geosynthetics Netherlands bv

Geotube Dewatering Technology A Key Component In Hylebos Waterway Cleanup - Case Study


Courtesy of TenCate Geosynthetics Netherlands bv

The Hylebos Waterway was named after a 19th century priest who served parishes in Tacoma and Steliacoom, WA. For years, however, the name was associated with one of the largest Superfund sites in the nation.

The Hylebos Waterway contains a toxic layer of sediment contaminated with PCBs, PAHs, arsenic, hexachlorobenzene, hexachloro-butadiene, and other organics and metals. The contamination comes from a variety of industries that have been operating in the area for more than a century, including chemical manufacturing plants, scrap metal recycling, log transfer facilities, and shipbuilding.

But the waterway is now becoming recognized as one of the most successful large-scale dredging cleanup operations ever attempted, thanks in part to Geotube dewatering technology.

TenCate develops and produces materials that function to increase performance, reduce cost, and deliver measurable results by working with our customers to provide advanced solutions. For this situation, Geotube dewatering technology would provide a cost-effective way to help contain hazardous materials, isolate them, and allow effluent to be
released back into local waterways safely.

In late 2005 and early 2006, marine dredging began at the headwaters of the Hylebos Waterway, part of the Commence Bay/ Nearshore Tideflats Superfund Site in Puget Sound. A total of 150,000 cy (115,000 cm) was dredged during the operation.  While the majority of the dredged material was dewatered using settling ponds and mechanical separation systems, the challenge remained to effectively capture the 2.5% solids in the overflow from the sediment barge used in the dredging operation.

The environmental engineering firm of Dalton, Olmstead and Fuglevand (DOF) in Kirkland, Washington, value engineered the use of Geotube dewatering technology to dewater 75,000 cy (57,000 cm) of turbid water in the sediment barge. The barge sediments represented just 6% of total />7s/fc/material; however, failure to dewater the fine marine sediments contained in the barge would have resulted in a three to four inch (7.6—10.2 cm) thick layer of residual sediments covering the entire dredged waterway. Without the Geotube dewatering technology solution, the project's clean up objectives would not have been met.

The majority of the dredging was conducted by Envirocon, Inc., of Portland, Oregon, using an articulated fixed arm dredge with a 3.0 cy (2.3 cm) closed hydraulic clamshell bucket. A second crane barge belonging to Quigg Bros., Inc., of Aberdeen, Washington, was outfitted with a 2.5 cy (1.9 cm) conventional rehandling clamshell bucket and a 4.0 cy (3.0 cm) cable arm closed bucket. Dredged material was then placed in a barge with 4 ft (1.2 m) high water tight walls and transported to a nearby dock for offloading.

To test the dewatering capabilities of Geotube dewatering technology with the Hylebos Waterway marine sediments, DOF engineers decided to deploy pilot units.

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