Contaminated Sediments Program
Table of Contents
Sediment Assessment and Remediation Report
Guidance for In-Situ Subaqueous Capping of
Michael R. Palermo and Steve Maynord
U.S. Army Engineer Waterways Experiment Station
U.S. Army Engineer Division, Great Lakes and Ohio River
Danny D. Reible
Louisiana State University
Baton Rouge, Louisiana
Equipment and Placement TechniquesThis chapter describes considerations in selecting equipment and placement techniques for in-situ cap placement. Considerations for both granular capping materials such as sediments and soils and geosynthetic fabrics and armoring materials are provided.
A variety of equipment types and placement techniques have been used for capping projects. Conceptual illustrations of equipment which can be considered for capping are shown in Figure 8.
For granular cap components, the major consideration in selection of equipment and placement of the cap is the need for controlled, accurate placement and the resulting density and rate of application of capping material. In general, the cap material should be placed so that it accumulates in a layer covering the contaminated material. The use of equipment or placement rates which might result in the capping material displacing or mixing with the contaminated material should be avoided. Sand caps have been successfully placed over fine-grained contaminated material with minimal mixing of the cap with the contaminated sediment (Mansky, 1984a, 1984b; Bokuniewicz, 1989; Bruin, Hattem and Wijnen, 1985, Zeman and Patterson 1996 a and 1996b). Since the surface area to be capped may be several hundred feet or more in diameter, placement of a cap of required thickness over such an area may require placement techniques to spread the material to some degree to achieve coverage.
Site considerations that can influence equipment selection include water depths and wave/current conditions. Other site conditions such as bottom topography, other vessel traffic, thermal/salinity stratification of the water columns (for deep water sites), etc. may also have an influence. Pipeline and barge placement of dredged material for ISC projects is appropriate in more open areas such as harbors or wide rivers. In constricted areas, narrow channels, or shallow nearshore areas, conventional land-based construction equipment may also be considered.
|Figure 8. Conceptual illustrations of equipment which can be considered for capping.|
Potential resuspension of in-situ contaminated material by impact of capping material should be considered in selecting equipment and placement technique for the cap. There is no standardized method presently available to calculate the potential resuspension of sediment and associated contaminant release due to such resuspension. Monitoring conducted at capping sites has generally focused on cap thickness and coverage rather than sediment resuspension.
At an ISC demonstration in Hamilton Harbor, Environment Canada monitored the water column and tracked a small plume of suspended material. Analysis of the material in suspension indicated that it was predominantly fines that had been washed off the sand capping material during placement and not resuspended contaminated sediments (Zeman and Patterson 1996a and 1996b).
Equipment and Placement Techniques for Granular Cap Materials
Granular cap material can be handled and placed in a number of ways. Materials that have been mechanically dredged and soils excavated from an upland site or quarry have relatively little free water, and can be handled mechanically in a "dry" state until released into the water over the ISC site. These mechanical methods rely on the gravity settling of cap materials in the water column, and may be depth-limited in their application. Granular cap materials can also be entrained in a water slurry, and carried to the cap site where they are discharged into the water column at the surface or at depth. These hydraulic methods offer the potential for a more precise placement, although the energy required for slurry transport may require dissipation to prevent resuspension of contaminated sediments.
Direct Mechanical Placement
Land-based cap placement at Sheboygan River.
If the area to be capped is nearshore and appropriate access is available, direct mechanical placement of capping material with land-based equipment can be considered. The reach of the equipment is the major limitation. The capping material would likely be trucked to the site with this method, so access for the trucks and traffic should be considered. Land-based methods might include backhoes, clamshells, end-dumping from trucks, spreading with dozers (during low water periods) etc. A cap with layers of gravel and geotextile was placed using land-based equipment (Figure 9) at a site on the Sheboygan River (Eleder 1992). At the GM Superfund site in Massena, New York, sand and gravel cap materials were placed in the St. Lawrence River with a backhoe bucket from a work barge (Kenna, pers com).
Surface Discharge Using Conventional Dredging Equipment
Field experiences with dredged material capping operations in Long Island Sound and the New York Bight have shown that contaminated sediment mounds have been successfully capped with both mechanically-dredged material released from barges and with material released from hopper dredges (O'Conner and O'Conner 1983, Morton 1987). The surface release of mechanically-dredged material from barges results in a faster descent, tighter mound, and less water column dispersion as compared to surface discharge of hydraulically-dredged material from a pipeline, while surface release of hydraulically-dredged material from a hopper dredge has characteristics somewhat between barge and pipeline discharges.
Surface discharge of material from barges or hopper dredges would not normally be considered for in-situ capping unless special provisions were made for gradual release of the material and spreading the material over a larger area. Point discharges from hopper dredges or barges would normally not be applicable for in-situ capping of soft fine-grained contaminated sediments.
Spreading by Barge Movement
A layer of capping material can be spread or gradually built up using bottom-dump barges if provisions are made for controlled opening or movement of the barges. This can be accomplished by slowly opening a conventional split-hull barge over a 30 to 60 minute interval, depending on the size of the barge. Such techniques have been successfully used for controlled placement of predominantly coarse-grained, sandy capping materials at the Denny Way and other sites in Puget Sound (Sumeri 1989 and 1995). The gradual opening of the split-hull or multi compartmented barges allows the material to be released slowly from the barge in a sprinkling manner.
|Figure 10. Spreading techniques for capping by barge movement at Denny Way, Puget Sound.|
Spreading of thin layers of cap material over large areas can also be accomplished by gradually opening a conventional split-hull barge while underway by tow. This technique has been successfully used for capping operations at Eagle Harbor, WA (Nelson, Vanerberden and Schuldt 1994, Sumeri 1995). Use of barges for spreading cap materials may not be suitable in shallow water depths, because of the water depths needed for barge draft, door openings and consideration of the propeller wash from tug boats.
Hydraulic Washing of Coarse Sand
Hydraulic washing of coarse sand,
Eable Harbor, Puget Sound
|Figure 12. Spreader place for hydraulic pipeline
Spreader box or sand box for hydraulic pipeline discharge,
Simpson Kraft Tacoma, Puget Sound
Pipeline with Baffle Plate or Sand Box
Where granular cap material is excavated by a hydraulic dredge or transported in a slurry form through a pipeline, spreading placement capping operations can be easily accomplished with surface discharge by an energy dissipating device such as a baffle plate or sand box attached to the end of the pipeline. Hydraulic placement is well-suited to placement of thin layers over large surface areas.
A baffle plate (Figure 12), sometimes called an impingement or momentum plate, serves two functions. First, as the pipeline discharge strikes the plate, the discharge is sprayed in a radial fashion and the dscharge is allowed to fall vertically into the water column. The decrease in velocity reduces the potential of the discharge to erode material already in place. Second, the angle of the plate can be adjusted so that the momentum of the discharge exerts a force which can be used to swing the end of the floating pipeline in an arc. Such plates are commonly used in river dredging operations where material is deposited in thin layers in areas adjacent to the dredged channel (Elliot 1932). Such equipment can be used in capping operations to spreadvery thin layers of material over a large area, thereby gradually building up the required capping thickness.
A device called a "sand box"(Figure 13) serves a similar function. This device acts as a diffuser box with baffles and side boards to dissipate the energy of the discharge. The bottom and sides of the box are constructed as an open grid or with a pattern of holes so that the discharge is released through the entire box. The sand box was used to successfully apply a sand cap at the Simpson Kraft Tacoma site in Puget Sound (Sumeri 1989).
A submerged diffuser (Figure 14) can be used to provide additional control for submerged pipeline discharge (Neal, et al. 1978; and Palermo 1994). The diffuser consists of conical and radial sections joined to form the diffuser assembly, which is mounted to the end of the discharge pipeline. A small discharge barge is required to position the diffuser and pipeline vertically in the water column. By positioning the diffuser several feet above the bottom, the discharge is isolated from the upper water column. The diffuser design allows material to be radially discharged parallel to the bottom and with a reduced velocity. Movement of the discharge barge can serve to spread the discharge to cap larger areas. The diffuser can also be used with any hydraulic pipeline operation including hydraulic pipeline dredges, pump-out from hopper dredges, and reslurried pump-out from barges.
|Figure 14.. Submerged diffuser syste, including the diffuser and discharge barge.|
Sand Spreader Barge
Specialized equipment for hydraulic spreading of sand for capping has been used by the Japanese (Kikegawa 1983, Sanderson and McKnight 1986). This equipment employs the basic features of a hydraulic dredge with submerged discharge (Figure 15). Material is brought to the spreader by barge, where water is added to slurry the sand. The spreader then pumps the slurried sand through a submerged pipeline. A winch and anchoring system is used to swing the spreader from side to side and forward, thereby capping a large area.
|Figure 15. Hydraulic barge unloader and sand spreader barge (from Kikegawa 1983.|
Gravity-fed Downpipe (Tremie)
Tremie equipment can be used for submerged discharge of either mechanically or hydraulically handled granular cap material. The equipment consists of a large-diameter conduit extending vertically from the surface through the water column to some point near or above the bottom. The conduit provides the desired isolation of the discharge from the upper water column and improved placement accuracy. However, because the conduit is a large-diameter straight vertical section, there is little reduction in momentum or impact energy over conventional surface discharge. The weight and rigid nature of the conduit requires a sound structural design and consideration of the forces due to currents and waves.
Conveyor unloading barge with tremie (from Togashi 1983).
The Japanese have used tremie technology in the design of specialized conveyor barges for capping operations (Togashi 1983, Sanderson and McKnight 1986). This equipment consists of a tremie conduit attached to a barge equipped with a conveyor (Figure 16). The material is initially placed in the barge mechanically. The conveyor then mechanically feeds the material to the tremie conduit. A telescoping feature of the tremie allows placement at depths of up to approximately 40 feet. Anchor and winch systems are used to swing the barge from side to side and forward so that larger areas can be capped, similar to the sand spreader barge.
Tremie system employed at Hamilton Harbor.
A variation on the tremie system was used at the ISC demonstration in Hamilton Harbor (Zeman and Patterson 1996a and 1996b). Sand, piled on a flat-deck barge, was placed into a hopper using a small front-end loader. Inside the hopper, the sand was slurried and routed into a number of 6-inch diameter, PVC plastic tubes (Figure 17). The tubes extended 30-feet down, where the sand exited about 5-10 feet above the sediment. An anchor and winch system was used to position the barge.
Hopper Dredge Pump-down
Some hopper dredges have pump-out capability by which material from the hoppers is discharged like a conventional hydraulic pipeline dredge. In addition, some have further modifications that allow pumps to be reversed so that material is pumped down through the dredge's extended dragarms. Because of the expansion at the draghead, the result is similar to using a diffuser section. Pump-out depth is limited, however, to the maximum dredging depth, typically about 60-70 ft.
Equipment and Placement Techniques for Armoring Layers
Placement of armor layers on in-situ caps can apply techniques commonly used for purposes of streambank and shoreline erosion protection. The Sheboygan River ISC was constructed using stone (1-2 inch cobbles) for erosion protection. Armor stone was also used at GM Massena site. Although there is very little experience with armor stone at ISC applications, guidance from streambank and shoreline erosion protection (USACE 1990, 1994) may be applicable to some ISC sites.
|Figure 18. Stone placement at Sheboygan River.
Methods that have been used for placing armor stone include placing by hand; machine placing, such as from some form of bucket; and dumping from trucks and spreading by bulldozer. Placement of cobbles at the Sheboygan River ISC was by bucket from a land-based crane with support from workers wading in the shallow river (see Figure 18). Gravel-sized armor stone was placed onto the cap at Massena using a backhoe which was emptied a few feet above the cap. Where gravel, cobbles or small stone must be placed in deeper water, it may be possible to push them over the side of a flat deck barge or down a modified tremie. Potential effects with such methods that should be considered include the disruption or penetration of other cap components by the armor stone impact and the differential settling of graded stone. In order to reduce the force of impact it may be necessary to handle the stone by bucket and release it closer to the cap surface or pass the material down some type of slide towed behind the barge.
As noted in the previous chapter, because of the uncertainties associated with underwater placement of stone, the design thickness of the erosion component should be increased by 50 percent.
Placement of Geosynthetic Fabrics and Membranes
Experience with placement of geosynthetic fabrics in subaqueous conditions is limited. At the Chicago Area Confined Disposal Facility (CDF), a plastic liner was pulled from a workbarge in sections which were heat welded together on the barge surface (Savage 1986). Cranes have been used to place geotubes prior to filling, directly lifting folded fabric tubes from working barges. Longer lengths of tube have been deployed from large reels mounted on barges. A membrane measuring 110 feet by 240 feet was placed as a temporary subaqueous cap at Manistique River by crane from a workbarge and anchored using concrete blocks (Hahnenberg, pers com). This operation required some manipulation of the cover by divers. A geotextile cap was deployed using a reel at Eitrheim Bay in Norway (Instanes 1994). Geosynthetic fabric was also used at Sheboygan, comprising two layers of the armoring.
Geosynthetics have been fabricated with anchors around the perimeter and other locations to simplify aquatic deployment. In most cases, the placement of geosynthetic fabrics at an ISC will require the coordinated actions of several crews and vessels. The material will have to be anchored quickly, especially where currents, waves or tidal conditions are subject to rapid changes.
The ability to keep barges and work vessels in position may require considerable effort at sites subject to currents, waves and tidal movements. Where granular cap material is placed by surface discharge, barge spreading, or hydraulic washing, vessels can be positioned by tug boats or other support vessels. Spuds, long steel posts attached to some barges that are lowered into sediments to maintain position, may not be appropriate for use during cap placement, as the spuds might penetrate and damage the cap. Cables attached to large "deadman" anchors deployed outside the cap footprint have been used to position work barges for ISC construction at Hamilton Harbor (Zeman and Patterson 1996a and 1996b).
Once the equipment and placement techniques for the various cap components are selected, the needs for land-based surveys or navigation and positioning equipment and controls can be addressed. The survey or navigation controls must be adequate to insure that the cap can be placed (whether by land-based equipment, bargeload, hopperload or by pipeline) at the desired location in a consistently accurate manner. Global positioning equipment (GPS) using the differential mode (DGPS) was used at the Hamilton Harbor capping demonstration (Zeman and Patterson 1996b).