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Sediment isolation using sheet piling commonly involves driving interlocking metal plates (i.e., sheet piles) into the subsurface, and thereby either blocking off designated areas or splitting a stream down the center. Highlight 6-3 shows an example of where this technology has been used. If a stream is split down its center, then one side of the stream may be excavated in the dry, after pumping out the trapped water. When the excavation of the first side of the stream is completed, water may be diverted back to the excavated side and sediment on the other side may be excavated. Sheet piling may not be feasible where bedrock or hard strata are present at or near the bottom surface. Where sheet piling is used to isolate a dredging or excavation action, project managers should consider potential hydraulic impacts of the diverted flow. Such diversion in most cases will increase natural flow velocity, which may scour sediment outside the diversion wall. If the sediment is also contaminated, as is likely to be the case, the increased dispersion of the sediment should be considered in design choices. Temporarily rerouting a water body with dams is sometimes done for small streams or ponds (Highlight 6-4). This includes the use of temporary dams to divert the water flow allowing excavation of now “dry” contaminated sediment.
The ability and cost to provide hydraulic isolation of the contaminated area during remediation is a major factor in selecting the appropriate removal technology.
6-7 Chapter 6: Dredging and Excavation Once isolated, standing water within the excavation area will need to be removed. Although surface water flows are eliminated, ground water may infiltrate the confined area. The ground water can be collected in sumps or dewatering wells. After collection, the ground water should be characterized, managed, treated (if necessary), and discharged to an appropriate receiving water body. Management of water within the confined area is another important logistical and cost factor that can influence the decision of wet versus dry removal techniques.
Highlight 6-3: Example of Excavation Following Isolation Using Sheet Piling Source: Pine River/Velsicol, EPA Region 5 Isolation and dewatering of the area is normally followed by excavation using conventional earthmoving equipment such as a backhoe or dragline. Where sediment is soft, support of the excavation equipment in the dewatered area can be problematic because underlying materials may not have the strength to support equipment weight. This also may reduce excavation depth precision. Both factors should be accounted for in design. When the excavation activities are complete, temporary dam(s) or sheet piling(s) are removed, and the water body is restored to its original hydraulic condition.
Another less common type of excavation project involves permanent relocation of a water body (also shown in Highlight 6-4). This, for example, was accomplished at the Triana/Tennessee River Superfund Site in Alabama and is being implemented at the Moss-American Superfund site in Wisconsin.
The initial phases of such a project may be similar to excavation projects that temporarily reroute a water body. However, in a permanent stream relocation project, a replacement stream normally is constructed and then the original water body is excavated or capped and converted into an upland area. To the extent the original water body is covered over, direct exposure to residual contamination is generally eliminated.
6-8 Chapter 6: Dredging and Excavation Highlight 6-4: Examples of Permanent or Temporary Rerouting of a Water Body A: Permanent River Relocation – Triana/Tennessee River Site The Triana/Tennessee River site consists of an 11-mile stretch of two tributaries, the Huntsville Spring Branch (HSB) and Indian Creek, which both empty into the Tennessee River. Remedial actions involved rerouting of the channel in Huntsville Spring Branch (HSB mile 5.4 to 4.0), the filling and burial in place of the total DDT (dichloro diphenyl trichloroethane and its metabolites) in the old channel, the construction of diversion structures at the upper and lower end of the stream to prevent stream reversion to the former stream channel, and the diversion of storm water runoff to prevent flow across the filled channel. Remedial actions for HSB mile 4.0 to 2.4 consisted of constructing four diversion structures; excavating a new channel between HSB mile 3.4 and 2.4; filling three areas;
constructing a diversion ditch around the fill areas; and excavating portions of the sediment from the channel.
These remedial actions effectively isolated in place 93% of the total DDT in the Huntsville Spring Branch-Indian Creek system of the Tennessee River. These remedial actions began on April 1, 1986, and were completed on October 16, 1987. Through March 1, 2001, the remedial actions have been inspected yearly by a federal and state Review Panel. The remedial action has not required any repair of the structures to maintain their integrity, and monitoring has shown that total DDT concentrations in fish and water continue to decline.
B: Temporary ReRouting of a River – Bryant Mill Pond Project at the Allied Paper, Inc./Portage Creek/Kalamazoo River Site In EPA Region 5, an EPA-conducted removal and onsite containment action removed polychlorinated biphenyls (PCBs)-contaminated sediment from the Bryant Mill Pond area of Portage Creek. During the removal action, that was conducted from June 1998 - May 1999, Portage Creek was temporarily diverted from its normal streambed so that 150,000 yds3 of the creek bed and floodplain soils could be excavated using conventional excavation equipment.
PCB concentrations remaining after the removal action were below 1 ppm.
Source: U.S. EPA Region 5
Excavation may also include excavation of sediment in areas that experience occasional dry conditions, such as intermittent streams and wetlands. These types of projects generally are logistically similar to upland construction projects and frequently use conventional earthmoving equipment.
6.5 DREDGING TECHNOLOGIES
For purposes of this guidance the term “dredging” means the removal of sediment from an underwater environment, typically using floating excavators called dredges. Dredging involves mechanically grabbing, raking, cutting, or hydraulically scouring the bottom of a waterway to dislodge the sediment. Once dislodged, the sediment may be removed from a waterway either mechanically with buckets or hydraulically by pumping. Therefore, dredges may be categorized as either mechanical or hydraulic depending on the basic means of removing the dredged material. Some dredges employ
pneumatic (compressed air) systems to pump the sediment out of the waterway (U.S. EPA 1994d);
however, these have not gained general acceptance on environmental dredging projects.
6.5.1 Mechanical Dredging The fundamental difference between mechanical and hydraulic dredging equipment is how the sediment is removed. Mechanical dredges offer the advantage of removing the sediment at nearly the same solids content and, therefore, volume as the in-situ material. Little additional water is entrained with the sediment as it is removed. Thus, the volumes of contaminated material and process water to be disposed, managed, and/or treated are minimized. However, the water that is present in the bucket above the sediment must either be collected, managed, and treated, or be permitted to leak out, which generally leads to higher contaminant losses during dredging.
The mechanical dredges most commonly used in the U.S. for environmental dredging are the
following (Palermo et al. 2004):
• Enclosed bucket: Wire supported, near watertight or sealed bucket as compared to conventional open clam bucket (recent designs also incorporate a level cut capability as compared to a circular-shaped cut for conventional buckets, for example, the Cable Arm and Boskalis Horizontal Closing Environmental Grab); and • Articulated mechanical: Backhoe designs, clam-type enclosed buckets, hydraulic closing mechanisms, all supported by articulated fixed-arm (e.g., Ham Visor Grab, Bean Horizontal Profiling Grab (HPG), Toa High Density Transport, and the Dry Dredge).
The mechanical dredge types listed above reflect equipment used for environmental dredging and generally are readily available in the U.S. The enclosed bucket dredges were designed to address a number of issues often raised relative to remedial dredging including contaminant removal efficiency and minimizing sediment resuspension. However, newly redesigned dredging equipment may not be costeffective or preferred at every site. For example, in some environments, an enclosed bucket may be most useful for soft sediment but may not close efficiently on debris. A conventional clamshell dredge may have greater leverage and be able to close on or cut debris in some cases; however, material mounded over the top may be resuspended. An articulated mechanical dredge may have advantage in stiffer sediment since the fixed-arm arrangement can push the bucket into the sediment to the desired cut-level, and not rely on the weight of the bucket for penetration. Highlight 6-5 shows two examples of mechanical dredges.
6.5.2 Hydraulic Dredging
Hydraulic dredges remove and transport sediment in the form of a slurry through the inclusion or addition of high volumes of water at some point in the removal process (Zappi and Hayes 1991). The total volume of material processed may be greatly increased and the solids content of the slurry may be considerably less than that of the in-situ sediment although solids content varies between dredges (U.S.
EPA 1994d). The excess water is usually discharged as effluent at the treatment or disposal site and often 6-10 Chapter 6: Dredging and Excavation
Note: A = Cable Arm Corp. dredge (Source: Cable Arm, Corp.) B = Bean Company Horizontal Profiling Grab (HPG) dredge, New Bedford Harbor Site (Source: Barbara Bergen, U.S. EPA) needs treatment prior to discharge. Hydraulic dredges may be equipped with rotating blades, augers, or high-pressure water jets to loosen the sediment (U.S. EPA 1995b). The hydraulic dredges most
commonly used in the U.S. for environmental dredging are the following (Palermo et al. 2004):
• Cutterhead: Conventional hydraulic pipeline dredge, with conventional cutterhead;
• Plain suction: Hydraulic pipeline dredge using dredgehead design with no cutting action, plain suction (e.g., cutterhead dredge with no cutter basket mounted, Matchbox dredgehead, articulated Slope Cleaner, Scoop-Dredge BRABO, etc.);
• Pneumatic: Air operated submersible pump, pipeline transport, either wire supported or fixed-arm supported (e.g., Japanese Oozer, Italian Pneuma, Dutch “d,” Japanese Refresher, etc.);
• Specialty dredgeheads: Other hydraulic pipeline dredges with specialty dredgeheads or pumping systems (e.g., Boskalis Environmental Disc Cutter, Slope Cleaner, Clean Sweep, Water Refresher, Clean Up, Swan 21 Systems, etc.); and • Diver assisted: Hand-held hydraulic suction with pipeline transport.
Some of the hydraulic dredges included above have been specifically developed to reduce resuspension during the removal process. As with modified mechanical dredges, project managers should be aware that there may be tradeoffs in terms of production rate and ability to handle debris with many of these modifications. Highlight 6-6 presents examples of hydraulic dredges.
6.5.3 Dredge Equipment Selection
The selection of appropriate dredging equipment is generally essential for an effective environmental dredging operation. The operational characteristics of the three types of mechanical and six types of hydraulic dredges presented in the guidance sections above are listed in Highlights 6-7a and 6-7b. This information was reviewed by an expert panel and attendees at a special session on environment dredging at the Meeting of the Western Dredging Association (WEDA XXI) and the 33rd Annual Texas A&M Dredging Seminar in Houston, Texas. The operational characteristics and identified selection factors presented in Highlights 6-7a and 6-7b have been drawn from information compiled for this guidance as well as earlier published reviews of dredge characteristics. Quantitative operational characteristics (both capabilities and limitations) are summarized for conditions likely to be encountered for many environmental dredging projects. The numbers are not representative of all dredge designs and sizes available, but represent those most commonly used for environmental dredging. Qualitative selection factors for each dredge type are presented based on the best professional judgment of the panel and/or their interpretation of readily available data. Site-specific results and supporting references are available in Operational Characteristics and Equipment Selection Factors for Environmental Dredging (Palermo et al. 2004).
The information in Highlights 6-7a and 6-7b is intended to help project managers make initial screening assessments of general dredge capabilities and identify equipment types for further evaluation at the feasibility study stage or for pilot field testing. Note that whenever an equipment type receives a rating of “high,” it means that a particular dredge type should perform better for that selection factor. It is not intended as a guide for final equipment selection for remedy implementation. There are many sitespecific circumstances that dictate which equipment type is most appropriate for any given situation, and each type can be applied in different ways to adapt to site conditions. Project managers should use their own experience and judgment in using this information, and may find it useful to consider other sources of information for purposes of comparison. In addition, because new equipment is being continuously developed and tested, project managers will need to consult with experts who are familiar with the latest in equipment technologies. Experience has shown that an effective environmental dredging operation also depends on the use of highly skilled dredge operators familiar with the goals of environmental remediation, in addition to close monitoring and management of the dredging operation.
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Note: For additional information on development and technical basis for the entries in this table refer to: Palermo, M., N. Francingues, and D. Averett. 2004.
Operational Characteristics and Equipment Selection Factors for Environmental Dredging. Journal of Dredging Engineering, Western Dredging Association.
Chapter 6: Dredging and Excavation Highlight 6-7b: Footnotes for Sample Environmental Dredging Operational Characteristics and Selection Factors 1 This table provides some of the currently available general information that can help project managers initially assess dredge capabilities, and screen and select equipment types for evaluation at the feasibility study stage or for pilot field testing. This table is NOT intended as a guide for final equipment selection for remedy implementation, and regions may find it useful to consider other sources of information for purposes of comparison. There are many site-specific, sediment-specific, and project-specific circumstances that will indicate which equipment is most appropriate for any given situation, and each equipment type can be applied in different ways to adapt to site and sediment conditions. In addition, because new equipment is being continuously developed, project managers should consult with experts who are familiar with the latest technologies.
2 Equipment types shown here are considered the most commonly used for environmental dredging in the U.S. Other dredge types are available. Equipment used for environmental dredging is usually smaller in size than that commonly used for navigation dredging. Information presented here is tailored for mechanical bucket sizes from 3 to 10 cubic yards (about 2 to 8 m3), and hydraulic/pneumatic pump sizes from 6 to 12 inches (about 15 to 30 cm). Larger sizes are available for many equipment types.