Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report (2018) / Chapter Skim
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4.1 4. INSTALLATION GUIDANCE AND APPRAISAL OF RESEARCH RESULTS 4.1 Overview This chapter presents a summary of information, observations, and recommended guidance for the installation of filters under water.
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4.2 4.3.1 Construction Constraints: Filter Selection Based on Site Access and Overhead Clearance Construction constraints take into account the different needs and challenges required for placing a filter underwater and/or in flowing water. Construction constraints for filter placement are primarily concerned with site access issues in general, and, particularly when working in and around bridges, the overhead clearance between the water surface during construction and the low chord of the bridge deck section.
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4.3 Figure 4.1. Selection based on access and clearance.
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4.4 Figure 4.2. Selection based on velocity and depth.
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4.5 4.3.3 Additional Considerations Additional issues to consider when selecting a filter to be placed under water include: Granular Filters: • Availability of filter material of the required size and gradation • Haul distance All Filter Types: • Site access • Equipment requirements, including barge if necessary • Environmental and water quality issues and permitting requirements • Habitat and/or migration issues for threatened and endangered species • Traffic control during construction activities Lastly, materials and installation costs will be a factor in the selection of a filter when more than one filter type and/or construction technique is viable at a particular site: 1. Initial construction materials and delivery costs 2.
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4.6 4.4.2 Installation Underwater installation of granular filters should only be performed by clamshell bucket or tremie, with the filter material being released on or very near the bed. The tremie method of placement can be accomplished using either rigid pipe from the surface, or a flexible hose through which the filter material is pumped in a water slurry to divers at the end of the hose.
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4.7 Table 4.1. Recommended Tests and Allowable Values for Aggregate Quality (modified from FHWA HEC-23, Lagasse et al.
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4.8 Table 4.2. Standard Coarse Aggregate Gradations From ASTM C33.
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4.9 Aggregate quality: Aggregate for granular filters should be hard, dense, durable, and should conform to the allowable values for the standard tests in Table 4.1. Filter layer thickness: For practicality of placement, the nominal filter layer thickness when placed under water should be no less than 6 inches.
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4.10 Quality Control: Quality control procedures for the underwater placement of granular filters include the following checklist items: 1. Checking contractor's submittals for material quality testing and size gradation in conformance with project specifications.
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4.11 In considering environmental impacts and permitting of a countermeasure which includes a granular filter placed in flowing water, issues that could be of concern would likely be limited to potential short term increases in turbidity and changes in the characteristics of the stream substrate. Here, a concern for the physical habitat would be infilling of interstices in the existing substrate (e.g., impacts on spawning habitat)
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4.12 4.5.3 Inspection and Maintenance Immediately after placing the geotextile filter, and before the armor layer is installed on top of it, the filter should be inspected for sufficient areal coverage and overlaps. Underwater inspection is best accomplished by divers.
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4.13 Table 4.3. Recommended Tests and Allowable Values for Geotextile Properties.
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4.14 4.5.6 Quality Assurance / Quality Control In construction, the quality assurance program focuses on the procedures to ensure that the design meets quality standards. This includes design calculations, construction plans and detail drawings, and specifications for required material properties.
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4.15 4.5.7 Environmental and Permitting Considerations As noted in Section 4.4.7, for aquatic organisms, habitat includes the stream, its boundaries (bed and banks) , and existing vegetation.
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4.16 4.6 Applications In the following sections, two projects involving underwater placement of geotextile filters are used to illustrate and supplement the discussion of the current state of practice in the U.S.
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4.17 Although the DOT intended that the scour holes would be filled with clean gravel, an environmental resource agency prohibited this even though the intent was to use a washed gravel. As a result the A-Jacks® modules were placed on the streambed and in some cases where the scour holes were extremely deep, the A-Jacks® modules were placed in tiers.
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4.18 Idaho DOT (IDT) required that the Contractor: • Fit modules as close as possible to each other and to the pier structure.
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4.19 Figure 4.6. Applying the geotextile filter to an A-Jacks® module.
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4.20 Figure 4.8. Installation at the bridge started downstream and proceeded upstream.
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4.21 Figure 4.10. Diver inspecting in-place A-Jacks® module at Ferry Butte Bridge.
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4.22 NCDOT, with collaboration and cooperation of other agencies and private contractors, stabilized the scour condition allowing the bridge to be reopened on Sunday, December 15, 2014. Restoring traffic to the Outer Banks was crucial in advance of the holiday influx of tourists headed south to the 60 miles of barrier islands, seven historic villages, and gateway to Ocracoke Island.
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4.23 December 5th: NC Governor issues Emergency Declaration which will facilitate negotiations with other agencies and contractors. USACE has a contract maintenance dredging project of the navigation channel through the inlet bar.
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4.24 December 12th: Divers confirm consolidation of dredge placed sand at Bent 166. First shipment of A-Jacks® elements (Figure 4.12)
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4.25 Assembly area Stockpile of assembled A-Jacks Figure 4.13. A-Jacks® assembly area and stockpile at Bonner Bridge (Henderson 2014)
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4.26 Figure 4.15. Contractor apparatus for A-Jacks® module placement (Henderson 2014)
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4.27 Figure 4.17. Placement of geotextile containers around Bent 166 at Bonner Bridge (Henderson 2014)
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4.28 The final numbers for work performed were: 78 - 3'x3'x3' geotextile bags 158 - 4'x4'x4' geotextile bags 980 - A-Jacks® elements Cost = $1.79 million (excluding dredge activity estimated at +/- $1 million) Summary Observations.
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4.29 4.7.2 Observations From the Survey of Practitioners • A total of 138 survey forms were distributed to practitioners across a wide variety of local, state, and federal agencies in the United States. Of the 42 responses received, 31 were from state government personnel (primarily DOTs)
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4.30 Two flow rates were established for the divers to work in; the first was a relatively modest approach velocity of 0.6 feet per second, while the second was more severe with an approach velocity of 3.5 feet per second. At all times the divers were tethered and in communication with each other as well as with line handlers topside.
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4.31 4.8.2 The Market The market or audience for the results of this research are hydraulic engineers and maintenance and inspection personnel in local, state, and federal agencies with responsibility for countermeasures for scour and erosion control at bridges and highway facilities, including: • State Highway Agencies / Departments of Transportation • FHWA • City/County Bridge Engineers • U.S. Army Corps of Engineers • U.S.
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4.32 AASHTO. The American Association of State Highway and Transportation Officials is the developer and sanctioning agency for standards, methods, and specifications.
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4.33 courses. A supplemental critique sheet could be used during Course 135048 or 132013 to provide feedback on the applicability of the guidance and suggestions for improvement.
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4.34 Learning Outcomes At the end of Part I, Participants will be able to: • Describe the purpose, need, and functions of geotextile and granular filters • Identify sources of design guidance for geotextile and granular filters • Discuss experience with and local agency guidance for installing filters underwater • Describe and list several techniques currently used to install filters underwater for typical armoring countermeasures (e.g., riprap, ACBs, etc.) • List and discuss the lessons learned from a recent U.S.
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