Nuclear Density Probes
Nuclear density probes can be used to measure the density of bottom sediments. Most nuclear density probes work on the principle that a more dense material will absorb a higher percentage of the radiation passing from the source to the detector than will a less dense material.
A typical probe is configured so that the sediment material passes between the source and detector as the probe is lowered. Nuclear density probes can give an accurate graph of sediment density as a function of depth if properly calibrated and used.
A limitation to the use of nuclear density probes, however, is the severe regulations governing their use, including the extensive paperwork involved. Nuclear density probes can be used only by licensed personnel, and the license is difficult to obtain. In addition, nuclear density probes must be stored under special conditions that are expensive to implement and maintain.
Non-Nuclear Density Probes
Non-nuclear density probes operate on various principles, including acoustic and mechanical. One type of acoustic probe, the High Resolution Density Profiler (HRDP), uses ultrasound. An example of a mechanical density probe is the DensiTune Silt Density Probe.
Underwater Sediment Sampling
Samples of the channel sediments to be dredged are often required for adequate characterization of the material and for use in laboratory testing. Sediment sampling and testing are used to determine dredgeability and provide the data necessary for designing the placement and beneficial uses alternatives. The level of effort required for channel sediment sampling is highly project-dependent. If the geophysical survey is required, samples may be required for ground truth interpretations. In the case of routine maintenance work, data from prior samplings and experience with similar material may be available, and the scope of field investigations may be reduced. For unusual maintenance projects or new-work projects, more extensive field investigations are required.
For maintenance work, channel investigations may be based on grab samples of sediment. Since bottom sediments are in an essentially unconsolidated state, grab samples are satisfactory for sediment characterization purposes and are easy and inexpensive to obtain. Grab sampling may indicate relatively homogeneous sediment composition, segregated pockets of coarse- and fine-grained sediment, and/or mixtures. If segregated pockets are present, samples may be taken at a sufficient number of locations in the channel to define spatial variations in the sediment character adequately. In any case, results of grab sampling must allow estimation of the relative proportions of coarse- and fine-grained sediments present. Caution should be exercised in interpreting conditions indicated by grab samples since sediment surface samples do not indicate variation in sediment character with depth
For more detailed information, additional samples may be taken using conventional boring techniques. Samples of sediment taken by conventional boring techniques are normally required only in the case of new-work dredging. Locations for borings should be selected based on information gained from the geophysical survey and/or initial grab sampling. Samples should be taken from within the major zones of spatial variation in sediment type or along the proposed channel centerline at a constant spacing to define stratification within the material to be dredged and to obtain representative samples.
Test Pits and Trenches
Test pits and trenches are usually made with mechanical cutting and removal equipment, such as clamshell (grab), dragline, or backhoe machines. The process of excavating a pit or trench may, in itself, be a form of test dredging. The pit is dug to the sampling or testing depth. Sampling or testing is then done at the surface of the pit using a surface-operated system, by a bottom-supported, remotely operated device or by a driver. The excavated material is usually a representative sample if care is taken in the excavation/sampling process.
Some sediments, such as coarse gravel, cobbles, boulders, shells, and debris, cannot be sampled effectively using the usual boring and sampling methods of geotechnical engineering. A test pit or trench is then the only way of obtaining a representative sample of the sediment. In these instances, in situ strength is usually not a factor, and a disturbed, but representative, sample is very useful for describing the character of the sediments
Placement Site Geotechnical Investigations
Field investigations must also be performed at proposed placement sites to define foundation conditions and to obtain samples for laboratory testing. This is especially important for proposed Confined Disposal Facilities (CDFs). The extent of required field investigations depends on the project size and the foundation conditions at the site. It is particularly important to define foundation conditions (including depth, thickness, extent, and composition of foundation strata), groundwater conditions, and other factors that may influence the construction and operation of the site.
Specific Gravity of Solids
The specific gravity of the solid constituents of a soil is the ratio of the unit weight of the solids to the unit weight of water. While it does not indicate dredging behavior, specific gravity is essential for the calculation of void ratio and porosity. The other properties needed are in situ density and water content. These calculations involve determination of the density and volume of the soil solids as part of the total in situ volume.
Factors Determining Equipment Selection
- Physical characteristics of the material to be dredged.
- Quantities and physical layout of the material to be dredged.
- Dredging depth.
- Location of both the dredging and placement sites and the distance between them.
- The physical environment of and between the dredging and placement areas.
- Contamination level of the sediments.
- Method of placement.
- Production required.
- Type of dredges available
Hydraulic and Mechanical Dredges
Hydraulic and mechanical dredges have enabled the transformation of rivers and harbors throughout the world into navigable waterways, allowing the transport of commerce and people where water passage was historically unavailable. The hydraulic dredge has been a major contributor to this transformation by providing for the movement of large quantities of dredged material in relatively short time periods.
Hydraulic dredges are characterized by the use of a centrifugal pump to dredge sediment and transport it, in a liquid slurry form, to a discharge area. The centrifugal pump was first developed in France in the early 1800s and then adapted to dredging in the 1850s by the USACE. In their present form, hopper and cutterhead pipeline dredges have been in existence since the 1870s and are now common throughout the world.
The major types of hydraulic dredges are hopper dredges and cutterhead pipeline dredges.
- Trailing suction (hydraulic) hopper dredges. Hopper dredges are seagoing vessels that excavate material hydraulically and transport it to a placement site in a hopper built into the hull of the vessel.
- Hydraulic pipeline dredges. Pipeline dredges are normally non-self -propelled dredges that may employ a mechanical cutter to break up the material, which is then excavated hydraulically and transported to the placement site through a pipeline.
The hydraulic pipeline cutterhead dredge is the most commonly used dredging vessel and is generally the most efficient and versatile. Because it is equipped with a rotating cutter apparatus surrounding the intake end of the suction pipe, it can efficiently dig and pump all types of alluvial materials and compacted deposits. This dredge has the capability of pumping dredged material long distances to upland placement areas. Pipeline discharge velocity, under routine working conditions, ranges from 15 to 20 ft/sec (4.5 to 6 m/s).
Mechanical dredges are characterized by the use of some form of bucket to excavate and raise the bottom material. They are not normally assigned to transport the material to the ultimate placement area. In some cases the dredged material can be deposited directly in the water or on the bank immediately adjacent to the dredging area. Normally, however, the mechanical dredge deposits material into a barge that transports it to the placement site. Mechanical dredges may be classified into two subgroups by how their buckets are connected to the dredge: wire rope-connected (clamshell or dragline) and structurally connected (a backhoe).
Dredge Production Rate
Production rate is usually defined as the number of cubic yards of in situ sediments dredged during a given period (commonly expressed in yd3/hr). Cutterhead dredge productivity is dependant primarily on the dredge pumping capacity, depth of cut, advance rate, the height of bank to be cut, cutter size, geometry, horsepower, speed, ladder swing rate, and direction, the width of cut, operator efficiency, dredge efficiency, and sediment characteristics. Parameters that influence pumping capacity include pump horsepower, diameter, and condition, and pipeline configuration (line length and geometry, type of pipeline, vertical lift, and the presence of ladder and/or booster pumps). Booster pumps are used when the pipeline length exceeds the power capability of the dredge pump or a higher production rate is desired. Other site conditions that can have a very significant effect on production include weather, waves, currents, tides, vessel traffic, and the presence of debris and contaminants
Dredge Excavated Material Placement
The excavated material may be placed in areas such as open water sites, on a beach, or in confined placement areas located either in the water or upland. In the case of open–water placement, a floating discharge pipeline may be used. The floating discharge pipeline can consist of sections of pipe mounted on pontoons and held in place by anchors, or it may consist of flexible floating hose (for example, rubber hose encased in buoyant material). Submerged discharge pipeline (either steel or high-density polyethylene [HDPE]) can, under appropriate site -specific conditions, be used to reduce wave- and current-induced forces to enhance pipeline joint connectivity. Additional sections of shore pipeline are required when upland placement is used. In addition, the excavated materials may be placed in hopper barges for subsequent placement in open water or in confined areas that are remote from the dredging area.
Dredging Environmental Concerns
• Increased turbidity
• Contaminant mobilization and transport
• Impacts on endangered species
• Impacts to fish migration and spawning due to turbidity
• Impacts on essential fish habitat
• Loss of habitat or benthos by removal or smothering•Acute and chronic toxicity due to resuspension of contaminants
• Underwater noise impacts on fish
Hydraulic Transport Systems
• Closed System
• Direct connection with hydraulic dredge systems
• No rehandling
• Sediment must be diluted with water for transport
• Reclaiming in situ density (if needed) of fine sediments requires significant time and proper management
• Significant detention may be required to meet water quality standards
MPRSA SECTION 103
Prohibits discharge of dredged material into ocean waters without a permit from the Corps of Engineers
Requires an evaluation of alternatives
Disposal must not “unreasonably degrade or endanger human health, welfare, or amenities, or the marine environment, ecological systems, or economic potentialities”
Corps & EPA published joint testing manuals to evaluate permits
Permits are subject to EPA review and concurrence
CWA SECTION 404
CWA Section 404 prohibits the discharge of dredged or fill material into “waters of the U.S.” without a permit from the Corps of Engineers.
Requires an evaluation of alternatives
No discharge will be allowed that “will degrade the waters of the U.S”…
With the Corps, EPA developed evaluation guidelines and testing manuals to evaluate permits
Permits are subject to EPA review and “veto” if the guidelines are not me
DREDGING PERMIT-TERMS AND CONDITIONS
Type, amount, and source of material to be disposed
Location of placement site
Timing, rate, and methods of placement
Monitoring and reporting
Required operational and engineering controls
Slurry Pipeline Carrying Capacity
The percent solids that can be carried in a pipeline is a function of the slurry velocity, the nature of the solids, and the size of the pipeline.
Turbulence is required to keep the solids in suspension and flowing. Larger lines require a higher velocity to achieve the equivalent turbulence of a smaller line at a lower velocity.
A dredge operator does not require all the soils expertise of the geotech, but must be able to interpret the geotech data. Recognizing the various soils and their effect on the performance of his dredge is essential.
Dredge Barometric Pressure
The only force available to a conventional dredge to push the slurry to the dredge pump is barometric pressure. When barometric pressure is utilized fully, the maximum velocity occurs in the suction line, and maximum capacity is achieved as a function of the area of the suction pipe. Altitude above sea level reduces barometric pressure and air density for fuel combustion and therefore can cause an appreciable reduction in dredge productivity.
Dredge Suction Velocity at Depth
There was a large dredge was working on the harbor of Marseilles, France. The cutter was set at a digging depth of 7 meters. Upon completing a cut, the operator of the dredge lowered the cutter to 10 meters and immediately adjusted his pump speed upward. When asked why he had increased pump speed, he responded, “Everyone knows it takes more horsepower to pick the material up from 10 meters than it does from 7 meters. I just gave it more horsepower.”
And indeed he had, for he increased substantially the water he was pumping. But, he had also reduced his solids payload significantly and was getting nothing in return for his increased fuel consumption except decreased production, increased wear, and additional water problems in the disposal area.
The natural tendency of the operator to increase suction velocity as digging depth increases results in increased costs and lower production.
There is an optimum or correct velocity for every depth. This optimum velocity results in the distribution of the 28 feet of barometric pressure over the several suction losses so as to maximize production rate. An understanding of the suction line and its losses is essential to the efficient operation of the hydraulic dredge. A computer program that calculates the optimum suction velocity is a valuable aid to the operator. Once this velocity is established, other data helpful to the leverman fall into place, e.g., head, specific gravity of slurry, friction loss per 100 feet of line, cubic yard per hour, maximum line length, and HP. Knowing a dredge’s capability can prove highly motivational to its operators.