Hard Hat Services (HHS) is an organization with a strong experience base in sediment restoration. The experience base covers not only the investigation and assessment of sediments, but includes designing and managing sediment restoration actions.
HSS offers services in:
- Sediment Sampling
- Sediment Testing (physical tests)
- Preparation of Dredging Permits
- Assessment of Sediment Fate and Transport
- Setting Reasonable, Mass-Based Remedial Action Objectives
- Assessing Remedial Options
- Engineered Enhancements for Natural Recovery
- Siting and Designing Confined Disposal Facilities
- Designing Dredge Water Treatment Facilities
- Designing Armoring and In-Situ Caps
- Selecting Specialized Dredge Equipment
- Evaluating Sediment Treatment Technologies
- Managing Remedial Dredging Projects
- Implementing Turn-key Sediment Remediation
1. Sediment Sampling
Sediment sampling is complicated by the soft or loose nature of the typical sediment requiring restoration. Sampling procedures must account for disturbance effects in sampling, separation of the sample constituents, and loss of the fluid mud section above the solid sediment surface. Often, the methods of recovering a sample must be modified in the field to obtain a representative sample of the sediment.
Most sampling today occurs with a vibrating head sampler that disturbs the very loose sediment sample. Because of the vibration disturbance, the conclusions drawn from the sampling are sometimes in error where the location of stratigraphic layers in the sediment is important.
HHS personnel have designed and/or implement the following types of sediment sampling to obtain representative sediment samples:
- Piston tube sampling in thin walled tubes
- Air driven linear hammer sampling with thin wall tubes
- Use of foam sealant to retain tube samples
- Use of sphincter technology for sample retention
- Vibrating core sampling (where disturbance is not a problem) • Vacuum suction sampling of zone settling area
- Grab sampling with clamshell type sampler
- Sediment probing using settling plate and probe
- Sediment probing using cone penetrometer
- Sediment probing using jetted probe
- In-situ vane shear testing
- Turbidity monitoring
- Bathymetric surveys
- Water column sampling
2. Sediment Testing - Physical Tests
Sediment testing refers to the physical sediment tests needed to assess sediment handling and/or dewatering characteristics. Testing is required to determine the in-situ density and loosened density of the sediment after excavation. The ratio of loosened to in-situ density (bulking ratio) is used to determine the volume required to hold the excavated sediment. This is probably one of the simplest concepts, but is misunderstood by a large number of service providers in the sediment restoration business. Sediment physical testing also includes properties such as water content, specific gravity, and organic content.
When sediment is suspended in water during excavation by hydraulic dredging methods (the most common way to remove fluid mud), the sediment settles and deposits through three distinct stages. The first stage is the suspension of the sediment particles in the water with the particles settling in accordance with the principles of Stoke’s Law. The second stage is zone settling, where the particles get close enough together to slow the movement of water out from between the soil particles (this is the fluid mud stage often found on the top of natural occurring sediment deposits). The third stage is compression settling, where the soil particles in the sediment are in contact with each other and are transmitting load from particle to particle. Testing to measure the rate at which these phases occur is important in the design of facilities to remove water from sediment. The testing must be conducted in large diameter settling columns to prevent the edge of the settling vessel from influencing the test.
Dewatered sediment must often be capped or excavated and moved to another location. The time rate of drainage of the sediment is an important factor in determining when further activities may occur with the sediment. Testing of the rate of drainage may be done using large volume settling columns, but the time to obtain test results is often prohibitive. Other state-of-the-art technologies, such as seepage consolidation are used by HHS to reduce the time required to obtain the time rate of drainage.
Physical testing also involves the determination of chemical additives or reagents to improve the removal of fine sediments from water. The tests are performed in jars with the additives mixed under similar conditions. It is important that someone who understands the actual field practices introduce the additives to the water-sediment mixture so that the laboratory tests reflect the actual practice in the field. Testing for solidifying agents is done in a similar manner, with paper cups substituted for the jars.
For all sediment testing programs, it is important that the testing personnel have experience with full-scale sediment handling and restoration. Experienced personnel will recognize the problems that occur during testing and will be able to adjust the testing program to obtain useful results. Tests performed by personnel that have no sediment restoration or commercial dredging experience often result in unusable information.
HHS personnel are experienced in the application of dredging to sediment restoration and have performed or managed the following testing:
- Water content
- Grain size and hydrometer
- Loss on ignition
- Specific gravity
- Settling tests (USACOE EM 1110-2-5027)
- Self-weight consolidation testing (Seepage Consolidation)
- Dissolved organic content
- Hydraulic parameter testing
3. Preparation of Dredging Permits
Filling or excavating activities in a navigable water body require a permit under Section 404(b)(1) of the Federal Clean Water Act. The 404 permit is granted by the Army Corps of Engineers with the exception of a few states where the State has the 404 permit authority. A permit is also required under Section 401 of the Clean Water Act for any activities that may impact water quality. The 401 permit is usually granted by the State. The supporting documentation for these permits is usually a composite of the information collected during sampling, testing, and other design activities.
- Prepare support for Section 404(b)(1) compliance determinations
- Prepare support documents for Certification for Dredging, Dredge Material Disposal and Filling in Waters under Section 401 of the Clean Water Act
- Prepare specific support documents for state requirements under Section 401
4. Assessment of Sediment Fate and Transport
Sediment fate and transport assessments using mathematical models is becoming a more common way to assess the impact of sediment restoration. The modeling of harbors and bays and the tidal effects on sediment transport is possible with today’s computer systems. However, the ability to determine if the computer model is correct depends on the experience of the person evaluating the results of the model. Even under the best of conditions the model will depend on some assumed parameters. The choice of these parameters may have a significant impact on the answer obtained. A sound understanding of the assumptions and that the answers are not absolute is critical in interpreting the results of mathematical projections.
HHS is familiar with the actual restoration processes and the factors that affect the movement of sediment. HHS is able to look at analytical projections and separate reality from mathematical magic.
HHS’s expertise includes:
- Sediment transport analysis
- Experience with screening level analysis
- Models of stream flow
- Contaminant mass transfer between sediment-water-air
- Understanding of the variability in modeling parameters
5. Setting Reasonable, Mass-Based Remedial Action Objectives
The real goal for sediment restoration should always be mass removal from contact with the environment. The most difficult problem with any sediment restoration commitment is agreement on the proper action level for restoration. The problem is that determinations are usually based on an established maximum value and the assumption that the entire restored area is at that value following restoration. The regulatory enforced clean-up levels for this approach are usually unattainable.
The deposition of a chemical of concern in sediment occurs by several processes. The processes are the same ones that deposit minerals of commercial value within a geologic formation. The feasibility of mining mineral deposits is often measured by projecting the mass of mineral present based on limited sampling of the geologic media containing the mineral. The techniques used in mineral exploration are directly applicable to calculating the mass of a chemical of concern in a given sediment volume. The number of sediment samples required may be determined using mining exploration principals to avoid over sampling.
HHS has significant experience in setting practical clean-up levels and approaches to projects that produce an end point for sediment restoration.
Some of those approaches are:
- Mass versus absolute limit concept
- Statistical versus absolute minimum concept
- Consideration of natural recovery effect
- Combined removal and containment
- Use of Kriging to establish fixed quantities (USEPA’s Geo-EAS & SURFER)
- Use of Monte Carlo method for parametric variations
6. Assessing Remedial Options
HHS personnel have experience with sediment restoration at numerous sites. The sediment restoration has included everything from natural recovery to removal of sediment. Successful projects usually include some aspects of several technologies for sediment restoration. HHS is familiar with the following broad classes of technologies for sediment restoration:
- Natural Recovery - The natural covering of old sediments by new sediments results in lowered exposure to the contaminants in the old sediments. This occurs in sediment depositional areas where erosive forces are small.
- In-Situ Capping/Armoring - Where erosive forces predominate the isolating effect of natural recovery may be obtained by placing clean sediment and armoring the sediment to keep it in place.
- Consumptive Remedies - Many contaminants dissipate due to biological or chemical reactions. Sometimes these reactions may be accelerated by engineered intervention.
- Removal - Removal includes dredging using hydraulic, mechanical or pneumatic means or some combination of these methods. Removal may also mean the excavation of sediments from a dewatered portion of a water body
- Stream Bypasses - Stream bypasses may be used for either a temporary or permanent relocation of a stream. For temporary use the bypass usually involves pumping the stream while work is ongoing in the dewatered stream. Permanent bypasses are used where sediment is contained in-place.
7. Engineered Enhancements for Natural Recovery
In many water bodies like ponds, lakes, harbors and backwater areas, natural recovery will result in the restoration of contaminated sediment through capping with clean sediment through natural deposition processes. These processes may be accelerated or improved by engineered additions to the water body.
HHS personnel are experienced in the design of engineered sediment control systems including:
- Baffles for flow control
- Headwater structures
- Silt curtains
- Steel sheet pile dividers
- Earth berms
- Rip-rap mounds
8. Siting and Designing Confined Disposal Facilities
Dredge materials are usually placed in an earthen berm containment referred to as a confined disposal facility (CDF) located near the dredging area. The earth berm area may be located in the water or on an upland area above the normal high water level. The CDF is often designed as the permanent location for the sediments and is closed out and capped after filling. The CDF may also be a temporary location for settling and dewatering the sediments prior to treating the sediment or prior to moving the sediment to another final disposal location.
HHS is experienced in the geotechnical and environmental factors for selecting a CDF location. HHS also has extensive experience in the public policy participation process for finding acceptable locations:
- Assessing site suitability parameters
- Rating sites
- Design of site liners
- Design of dewatering systems
- Design of horizontal barriers
- Design of protection for in-water site
9. Designing Dredge Water Treatment Facilities
The most critical part of the environmental dredging process may be the treatment of the return water. Large volumes of fluid mud sediment are usually moved by hydraulic methods. The sediment normally makes up 10 to 15% by weight of the pumped slurry. The remainder of the slurry is carriage water used to transport the sediment to the CDF (settling basin). The carriage water must be removed and treated to some specific level before return to the body of water.
Carriage water treatment is predominantly a solids removal process. The bulk of the sediment in the carriage water is removed in the primary settling basin. The residual is usually removed by secondary polishing in a second settling basin and by final polishing with pressure sand filters. At sites where physical settling basins are not possible, mechanical dewatering methods may be used to remove solids. Mechanical methods are more expensive and inherently riskier to operate than the settling basin approach.
Dredge water treatment facility discharge standards must be realistic to make sediment removal possible. No impact carriage water treatment standards are often requested, but unrealistic goals. A sound understanding of mass removal goals in concert with reasonable carriage water treatment standards is necessary for the removal of sediment by hydraulic dredging methods.
HHS is experienced with the following carriage water treatment technologies:
- Coagulant and flocculent additives
- Filtration methods
- Sedimentation techniques
- Recessed chamber presses
- Belt filter presses
- Vibrating screen decks
- Mineral jigs
- Sand screws and spiral classifiers
- Fenton’s reaction
- Ultraviolet oxidation
- Oil-water separation
10. Designing Armoring and In-Situ Caps
For sediments located in areas of active erosion, capping with armor protection is a good way to reduce exposure and to prevent transport and redeposit of contaminated sediment. HHS is experienced with not only the methods for designing capping and armoring systems, but also the analysis of hydraulic changes that will result from the capping.
- Selection of isolation cap material
- Selection of proper thickness of isolation cap
- Selection of armoring material and thickness
- Analysis of hydraulic impact from armoring
11. Selecting Specialized Dredge Equipment
HHS personnel have worked in the production and the environmental dredging businesses. HHS is able to help in selecting or designing specialized equipment and in the application of the equipment to environmental projects.
12. Evaluating Sediment Treatment Technologies
HHS is experienced in application of treatment technologies for contaminated sediments. The simplest technologies often offer the most restoration for the dollar expended. Dewatering of the sediment reduces the weight for landfilling and for further treatment. Other technologies to reduce the concentration of chemicals of concern may be appropriate where the concentrations of the chemicals are high. However, most sediment restoration involves large volumes of sediment with low concentrations of the chemical of concern.
HHS is experienced with:
- Thermal desorption
- Sediment washing
- Sediment separation
13. Managing Remedial Dredging Projects
Environmental dredging requires constant attention to sediment control, water treatment, and removal objectives. All of these issues must be constantly monitored and controlled as conditions change in the operation. These management demands differ from production dredging where yards moved per hour is the predominant objective.
HHS knows the types of equipment available for sediment restoration projects. HHS understands the difference between sediment restoration work in small industrial lagoons and work in open bodies of water. HHS prepares cost estimates for sediment restoration based on estimates of the cost of input factors (labor, equipment, and expenses) to complete the work. Unit prices are not reliable in the estimating of environmental dredging as in more conventional earthwork or buildings.