Environmental Dredging: New Techniques from the USA

Dredging is a convenient and often essential means of removing contaminated sediments from waterways, but concern exists about the potential environmental effects of localized resuspension of sediment and release of contaminants during the dredging process. New 'environmentally friendly' techniques are being developed using new and modified equipment to ensure that polluted material does not enter the water column during the dredging process.

One of the more major environmental dredging projects undertaken in the US in recent years is the Marathon Battery Super Fund Project on the east bank of the Hudson River. For the purpose of remediation, the Super Fund site was divided into three key areas: the former Marathon Battery manufacturing facility and surrounding area; the Hudson River in the Cold Spring Pier Area (CPSA); and a number of coves and waterways such as the East Foundry Cove (EFC), West Foundry Cove (WFC), East Foundry Cove Marsh (EFCM), and Constitution Marsh.

All of these areas had become seriously contaminated over the years through the discharge through stormwater lines and sewage lines of water heavily polluted with effluent from a nickel-cadmium battery factory. Sediments removed from these areas would need to be excavated and dewatered, chemicals in the spoil would be fixed, and treated material would then be transported for offsite disposal and site restoration.

After the proposed remediation plan had been defined, a request for tenders for the project was issued in May 1992. A contract for the work was subsequently awarded to Sevenson Environmental Services (with Aqua Dredge Inc. acting as sub-contractor for the dredging) in May 1993. After the contract was awarded and the contractor put forward his proposed remediation methods, a number of Value Engineering Change Proposals (VECPs) were subsequently discussed with the US Army Corps of Engineers, the EPA, and consultants Malcolm Pirnie Inc.

Sevenson Environmental Services proposed a mechanical dewatering process rather than using containment or settling basins. Under this proposal, dredged slurry would be separated into solid and liquid materials. During the VECP process, concern was expressed about the ability of the screens proposed by the contractor to deal with the spoil, and to take account of these concerns Sevenson Environment Services set up a new system consisting of centrifuges, screens and pumps. Problems experienced with this design led the contractor to design a revised scheme based on mechanical dewatering and a modified settling basin treatment as originally proposed in the contract document.

Further discussions between the contractor and the US Army Corps of Engineers/EPA team took place with regard to the nature of the containment structure for the settling basins. The contract document had specified that containment dikes and basins be constructed out of earth and select fill materials. Sevenson Environmental Services proposed a water-filled, tube-like structure. After much discussion in the VECP process, it was decided that the contractor should use earth dikes as well as the water-filled structure.

Also under the VECP process, the contractor proposed a patented, proprietary Maectite treatment for the chemical fixation of pollutants. In order to meet the EPA's concerns, the contractor undertook a series of studies which determined that the Maectite process was indeed capable of treating a range of chemicals. Aqua Dredge Inc., the company responsible for undertaking the dredging work, used a MUD CAT Model MC-915 hydraulic dredge with a horizontal auger for most of the work, at one point mobilizing a second hydraulic dredge to work on the CSPA of the Hudson River. In some areas, such as Cold Spring Pier, hydraulic dredging was rendered ineffective by large quantities of rock and debris, and a local dredging subcontractor was called in to use a crane with clamshell bucket, operating from the deck of a spud barge.

All of the goals of the Marathon Battery have now been successfully completed through a wide-ranging consultative process between the contractor, government agencies, and contract designers. As with many more complex environmental dredging projects, the complexity of the problems on site required contract documents that were definitive yet flexible enough to respond to changing conditions, while the contractor was required to introduce new ideas through an interactive engineering review process.

Excerpted from Port Engineering Management