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3 Clamshell - conventional clamshell dredges, wire supported, conventional open clam bucket.
4 Enclosed Bucket - wire supported, near watertight or sealed bucket usually incorporating a level cut capability.
5 Articulated Mechanical - backhoe designs, clam-type enclosed buckets, hydraulic closing mechanisms, all supported by articulated fixed-arm.
6 Cutterhead - conventional hydraulic pipeline dredge, with conventional cutterhead.
7 Horizontal Auger - hydraulic pipeline dredge with horizontal auger dredgehead.
8 Plain Suction - hydraulic pipeline dredge using dredgehead design with no cutting action.
9 Pneumatic – air operated submersible pump, pipeline transport, either wire supported or fixed-arm supported.
10 Specialty Dredgeheads - other hydraulic pipeline dredges with specialty dredgeheads or pumping systems 11 Diver Assisted - hand-held hydraulic suction with pipeline transport.
12 Dry Excavation - conventional excavation equipment operating within dewatered containments such as sheet-pile enclosures or cofferdams.
13 OPERATIONAL CHARACTERISTICS - quantitative entries, reflecting capabilities and limitations of dredge types, and are solely a function of the equipment itself.
14 Production Rate - in-situ volume of sediment removed per unit time. Rates shown are for production cuts as opposed to “cleanup passes” and are for active periods of operation under average conditions. Rates for two bucket or pump sizes are shown for comparison. For mechanical dredges, the rates were calculated assuming 80% bucket fill with a bucket cycle time of 2 minutes. For hydraulic dredges, the rates were calculated assuming in-situ sediment 35% solids by weight, 5% solids by weight for slurry, and pump discharge velocity of 10 ft/sec. The rate shown for diver-assisted assumes a maximum pump size of 15 cm and roughly 50% efficiency of diver effort while working. Production rate for dry excavation is would be largely dictated by the time required to isolate and dewater the areas targeted for excavation. A variety of factors may influence the effective operating time per day, week, or season, and should be considered in calculating times required for removal.
15 Percent Solids by Weight - ratio of weight of dry solids to total weight of the dredged material as removed, expressed as a percentage. Percent solids for mechanical dredging is a function of the in-situ percent solids and the effective bucket fill (expressed as a percentage of the bucket capacity filled by in-situ sediment as opposed to free water), and near in-situ percent solids is possible for production cuts. A wide range of percent solids for hydraulic dredges is reported, but 5% solids can be expected for most environmental dredging projects.
6-17 Chapter 6: Dredging and Excavation Highlight 6-7b: Footnotes for Sample Environmental Dredging Operational Characteristics and Selection Factors 16 Vertical Operating Accuracy - the ability to position the dredgehead at a desired depth or elevation for the cut and maintain or repeat that vertical position during the dredging operation. Although positioning instrumentation is accurate to within a few cm, the design of the dredge and the linkages between the dredgehead and the positioning system will affect the accuracy attainable in positioning the dredgehead. A vertical accuracy of cut of approximately 15 cm (one-half foot) is considered attainable for most project conditions. Fixed arm equipment holds some advantage over wire-supported in maintaining vertical operating accuracy. The accuracies achievable for sediment characterization should be considered in setting performance standards for environmental dredging operating accuracy (both vertical and horizontal).
17 Horizontal Operating Accuracy - the ability to position and operate the dredgehead at a desired location or within a desired surface area. Considerations are similar to those for vertical accuracy.
18 Maximum Dredging Depth - physical limitation to reach below a given depth. Wire-supported buckets or pumps can be deployed at substantial depths, so the maximum digging depth generally is limited by stability of the excavation. Reach of fixed arm supported buckets or hydraulic dredges is limited by the length of the arm or ladder. Conventional backhoe equipment is generally limited to about 15 m reach. Smaller hydraulic dredges are usually designed for a maximum dredging depth of about 15 m. Hydraulic dredges usually also have a limiting depth of removal of about 50 ft due to the limitation of atmospheric pressure, but this limitation can often be overcome by addition of a submerged pump on the ladder. The table entries should NOT be considered as hard and fast limits. Larger dredge sizes and designs are available for deeper depths.
19 Minimum Dredging Depth - constraints on draft limitations of some floating dredges or potential loss of pump prime for hydraulic dredges. Such limitations can be managed if the dredge “digs its way into the area.” For smaller dredges, these limitations typically are at approximately the 1m water depth. Pneumatic dredges require a minimum water depth of about 5 m for efficient pump operation.
20 SELECTION FACTORS - qualitative entries, reflecting the potential performance of a given dredge type, and are a function
of both the capability of the equipment type and the site and/or sediment conditions. Entries defined as follows:
(High) - indicating the given dredge type is generally suitable or favorable for a given issue or concern, (Medium) - indicating the given dredge type addresses the issue or concern, but it may not be preferred, and (Low) - indicating the given dredge type may not be a suitable selection for addressing this issue or concern.
21 Limit Sediment Resuspension - potential of a given dredge type in minimizing sediment resuspension. Clamshell (Low) Circular-shaped cutting action, cratered bottom subject to sloughing, open bucket design subject to washout and spillage, scows and workboats working in shallow areas. Enclosed Bucket (High) - Seal around the lips of the bucket and an enclosed top when in the shut position, level cut design minimizes sloughing. Articulated Mechanical (High) - Less resuspension as compared to conventional clamshell dredges. Cutterhead/Horizontal Auger (Medium) - Conventional cutterhead dredges and horizontal augers result in less resuspension as compared to conventional clamshell dredges.
May be fitted with hoods or shrouds to partially control resuspension. Plain Suction/Pneumatic (High) - No mechanical action to dislodge the material. Specialty (High) - Although designs vary, all the so-called specialty dredges have features specifically intended to reduce resuspension. Diver Assisted (High) - Precision of diver assisted hydraulic dredging, the smaller size of the dredgeheads used, and inherently slow speed of operation. Dry Excavation (High) - Completely isolates the excavation process from the water column.
22 Control Contaminant Release - the inherent ability to control sediment resuspension and dissolved and volatile releases for the given equipment type and associated operation. Clamshell (Low) - Can be operated such that the excavation and water column exposure of the bucket is within a silt curtain containment or enclosure; however, high suspended solids within the silt curtain may be released when the curtain is moved. Enclosed Bucket/Articulated Mechanical (Medium) - can be operated such that the excavation and water column exposure of the bucket is within a silt curtain enclosure with relatively small footprint. Enclosed buckets act as a control and greatly reduce resuspension within the enclosures and potential for release. Cutterhead/Plain Suction/Horizontal Auger/Pneumatic/Specialty Dredgeheads (Medium) - Capable of transporting the material directly by pipeline, minimizing exposure to the water column and to volatilization. Can be operated within enclosures, but the footprint of such enclosures would be necessarily larger than that for mechanical dredges. Diver assisted (High) - scale of diver-assisted dredging would seldom require contaminant release controls. Dry Excavation (High) - Dewatering of the dredging area effectively eliminates dissolved releases. Sediment surface exposed to the atmosphere has lower volatile emission rates as compared to the same surface ponded with elevated suspended sediment concentrations.
6-18 Chapter 6: Dredging and Excavation Highlight 6-7b: Footnotes for Sample Environmental Dredging Operational Characteristics and Selection Factors 23 Minimize Residual Sediment - efficiency of the dredge is in removing material without leaving a residual, and potentially meeting a cleanup level. Clamshell (Low) - High potential to leave residual sediment because of the circular-shaped cutting action and the tendency to leave a cratered bottom subject to sloughing. Enclosed Bucket/Articulated Mechanical/Cutterhead/Horizontal Auger/Plain Suction/Pneumatic/Specialty Dredgeheads (Medium) - All dredges with active dredgeheads and/or movement in contact with the bottom sediment will leave some residual sediment. The control offered by the articulated arm provides an advantage for removal of thin residual layers. Diver Assisted (High) - Hand-held action of diver-assisted work has a low potential for generating residual sediment. Dry Excavation (High) - Any fallback of sediment excavated under dry conditions can be readily observed and managed.
24 Transport by Pipeline - compatibility of the dredge with subsequent transport by pipeline. Clamshell/ Enclosed Bucket/Articulated Mechanical (Medium) - All mechanical dredges remove material at near in-situ density, and additional reslurry and rehandling equipment must be employed to allow for pipeline transport. Cutterhead/Plain Suction/Horizontal Auger/Pneumatic/Specialty Dredgeheads/Diver Assisted (High) - All hydraulic and pneumatic dredges are designed for pipeline transport. Dry Excavation (Medium) - Additional reslurry and rehandling equipment must be employed to allow for pipeline transport.
25 Transport y Barge - compatibility of the dredge with subsequent transport by barge. Clamshell/Enclosed Bucket/Articulated Mechanical (High) - Material excavated with mechanical dredges is close to in-situ density and may be directly placed in barges for transport. Cutterhead/Plain Suction/Horizontal Auger/Pneumatic/Specialty Dredgeheads/Diver Assisted (Medium) - Barge transport of hydraulically dredged material is inefficient. Although pneumatic and some specialty dredges are capable of removing soft sediment at high water content, intermittent operation for change-out of barges will significantly reduce efficiency. Dry Excavation (High) - Material excavated in the dry may be placed directly in barges using conveyers or front-end loaders.
26 Positioning Control in Currents/Wind/Tides - ability of the dredge to hold a desired position of the dredgehead horizontally with current, wind, or vertically with fluctuating tides. Clamshell/Enclosed Bucket/Articulated Mechanical (High) - Operate with spuds or jack-up piles and are inherently stable against movement by normal winds and currents. Cutterhead/Plain Suction/Specialty Dredgeheads (High) - Equipped with spuds and use “walking spud” method of operation inherently stable against movement by normal winds and current. Horizontal Auger (Medium) - Free floating and operate using an anchor and cable system, subject to movement with longer anchor sets. Pneumatic (High) - Operate from spudded barges or platforms and are inherently stable against movement by normal winds and currents. Diver Assisted (Medium) - Ability of divers to maintain a desired position will be hampered by currents. Dry Excavation (High) - Not affected by wind and currents.
27 Maneuverability - ability of the dredge to operate effectively in close proximity or around utilities and other infrastructure, narrow channel widths, surface and submerged obstructions, and overhead restrictions. Clamshell/Enclosed Bucket/Articulated Mechanical (High) - Buckets are wire supported or fixed-arm articulated and may be operated close in to infrastructure and within tightly restricted areas. Cutterhead/Plain Suction/Horizontal Auger/Pneumatic/Specialty Dredgeheads (Low) - Swinging action of the walking spud method of operation for hydraulic pipeline dredges and the need for long anchor and cable setup for horizontal auger dredges limits their ability to operate near infrastructure or within tightly restricted areas. Diver Assisted (High) - Can be conducted close to infrastructure and within tightly restricted areas.
Dry Excavation (High) - Containments for dry excavation can be designed for areas near infrastructure and tightly restricted areas may be completely contained.
28 Portability/Access - ability of the dredge to pass under bridges, through narrow channels, or to be transported by truck and easily launched to the site. Clamshell/Enclosed Bucket/Articulated Mechanical/Cutterhead/Plain suction/Horizontal Auger/Pneumatic/Diver Assisted/Dry Excavation (High) - Dredge types considered here are the smaller size and are generally truck transportable. Specialty Dredgeheads (Medium) - Some specialty dredge designs are too large for truck transport.
29 Availability - this factor refers to the potential availability of dredges types to contractors and the potential physical presence of the equipment in the U.S. Clamshell/Enclosed Bucket/Articulated Mechanical/Cutterhead/Plain Suction/Horizontal Auger/Pneumatic/Diver Assisted/Dry Excavation (High) - Most dredge types are readily available.
Specialty Dredgeheads (Medium) - Some specialty dredges are available through only one contractor or may be subject to restrictions under the Jones Act.
6-19 Chapter 6: Dredging and Excavation Highlight 6-7b: Footnotes for Sample Environmental Dredging Operational Characteristics and Selection Factors 30 Debris/Loose Rock/Vegetation - susceptibility of a given dredge type to clogging by debris and subsequent loss of operational efficiency. Clamshell/Enclosed Bucket/Articulated Mechanical (High) - Mechanical dredges can effectively remove sediment containing debris, although leakage may result. Mechanical equipment is the only approach for debris-removal passes. Cutterhead/Plain Suction/Horizontal Auger/ Pneumatic/ Specialty Dredgeheads (Low) - Subject to clogging by debris and are incapable of removing larger pieces of loose rock and larger debris. Loose rock and large debris can also cause inefficient sediment removal. Diver Assisted (Low) - Presence of logs and large debris may present dangerous conditions for diver-assisted dredging. Although divers can remove sediment from around large debris or rocks, this type of operation would be inefficient. Dry Excavation (High) - Dry excavation allows use of conventional excavation equipment. Leakage from buckets caused by debris is not a consideration for dry excavation.
31 Hardpan/Rock Bottom - ability of a dredge type to remove a sediment layer overlying hardpan or rock bottom effciently without leaving excessive residual sediment. Clamshell/Enclosed Bucket/Articulated Mechanical/Cutterhead/Horizontal Auger (Low) - Closing action of buckets and cutting action of dredgeheads result in problems maintaining a desired vertical cutting position and would tend to leave behind excessive residual sediment. Power associated with articulated mechanical has advantage in removing hard materials. Plain Suction/ Pneumatic/ Specialty Dredges (Medium) - Lack an active closing or cutting action and can operate over an uneven hard surface, although removal efficiency may be low.
Diver Assisted (High) - May be the most effective approach for precise cleanup of a hard face, since the divers can feel the surface and adjust the excavation accordingly. Dry Excavation (High) - Allows the visual location of pockets of residual remaining on an uneven hard surface.
32 Flexibility for Varying Conditions - flexibility of a given dredge type in adapting to differing conditions, such as sediment stiffness, variable cut thicknesses, and the overall ability to take thick cuts. Clamshell/Enclosed Bucket (High) - Buckets are capable of taking thin cuts or thicker cuts in proportion to the bucket size, and bucket sizes can be easily switched.