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5.1 5. CONCLUSIONS AND SUGGESTED RESEARCH 5.1 Conclusions and Applicability of Results 5.1.1 Conclusions 1, This research has resulted in a synthesis of the state of the practice for installing filter systems underwater for bridge scour and other erosion control countermeasures. Materials, equipment, and placement techniques developed in Europe, the U.S., and other countries are described in detail in Chapter 2 of this report. 2. Although it was not a primary objective of this study, it was necessary to clearly establish the functions of, and need for, granular and/or geotextile filters as an essential component of any armoring system. These are established in Chapter 2, together with accepted filter design procedures presented in Chapter 2 and Appendix B. 3. A survey of practitioners across the U.S. revealed that some underwater installation practices in other countries are still new and largely untried in the U.S. The use of self- sinking geotextile composite fabrics (e.g., a sandmat) is one such practice. Another example is the use of a flexible tremie hose to deliver a slurry of coarse granular filter material to divers for placement underwater. When conditions allow divers to safely access the work area, they can, under moderate flow conditions, place granular material loosely on the bank or bed. 4. A proof-of-concept demonstration of underwater filter placement by divers was conducted as part of this research project. The divers were able to install a variety of filter materials around a prototype bridge pier in flowing water. The trials were conducted in the large outdoor River Engineering Flume at Colorado State University under different flow rates and velocities. The various trials are described in detail in Chapter 3 of this report. 5, For both granular and geotextile filters, recommended design procedures, specifications, material testing requirements, installation alternatives, and QA/QC checklist items are provided in Chapter 4. In addition, filter selection guidance (in the form of flow charts) is provided based on site-specific conditions which include flow depth, flow velocity, access for construction equipment, and overhead clearance. 6. Geotextile filters can also be installed as bags filled with sand or gravel. The geobags can be filled prior to placement, sewn shut, and dropped through the water column. Alternatively, empty geobags can be placed by divers and filled in place with a flexible tremie hose. Both approaches are commonly used in Europe, but are relatively unknown in the U.S. Wider use of the geobag or geocontainer approaches would constitute a significant advance in the state of practice for placing filters in flowing water in the U.S. 7. Practitioners in the U.S. indicate that underwater inspection of a filter installation is not usually required prior to placing the armor layer on top. This is an undesirable practice that must be rectified. Recommendations for filter inspection, as one component of a quality control program during construction, are provided in Chapter 4.
5.2 8. In summary, the current state of practice for filter installation provides a sufficient variety of filter material types and placement techniques to accommodate most underwater filter requirements including riverine as well as coastal and offshore applications. Indeed, there should be very few instances where a filter cannot be placed as an integral part of a properly designed and installed scour or erosion control countermeasure. 5.1.2 Applicability of Results to Highway Practice Approximately 82 percent of the 600,000 bridges in the National Bridge Inventory are built over waterways. Many, especially those on more active streams, will experience problems with scour, bank erosion, and channel instability during their useful life (Lagasse et al. 2012). The magnitude of these problems is demonstrated by the estimated average annual flood damage repair costs of approximately $50 million for bridges on the Federal aid system. Highway bridge failures caused by scour and stream instability account for most of the bridge failures in the United States. A 1973 study for the FHWA (Chang 1973) indicated that about $75 million were expended annually up to 1973 to repair roads and bridges that were damaged by floods. Extrapolating the cost to the present makes this annual expenditure for roads and bridges on the order of $400 to $500 million. This cost does not include the additional indirect costs to highway users for fuel and operating costs resulting from temporary closure and detours and to the public for costs associated with higher tariffs, freight rates, additional labor costs, and time. The indirect costs associated with a bridge failure have been estimated to exceed the direct cost of bridge repair by a factor of five (Rhodes and Trent 1993). Rhodes and Trent (1993) document that $1.2 billion was expended for the restoration of flood damaged highway facilities during the 1980s. Although it is difficult to be precise regarding the actual cost to repair damage to the nationâs highway system from problems related to the installation of scour and erosion control countermeasures with an inadequate (or non-existent) filter system, the number is obviously very large. The problems of design, specification, installation, and periodic inspection are compounded when the filter system must be installed underwater. The guidance and recommendations from this research will provide guidance to bridge owners for the proper selection, design, installation, and life cycle care of filter systems for a wide variety of countermeasure alternatives. This guidance will help ensure that effective scour and erosion protection can be constructed when the filter system must be installed underwater. The end result will be a more efficient use of highway resources and a reduction in costs associated with the impacts of countermeasure failures on highway facilities. NCHRP Report No. 887 "Guidance for Underwater Installation of Filter Systems" was published as a stand-alone guidance document intended to meet the specific needs of the practitioner with a focused presentation of the results of this research (Clopper and Lagasse 2018. 5.2 Suggested Research Field Demonstration Sites: An attempt to investigate actual field project sites as part of this research project was made, but issues involving construction schedules/delays, logistics, and liability concerns made this unworkable as a research effort.
5.3 We therefore suggest that state highway agencies/DOTs be solicited for participation in field- scale demonstration projects using various materials and methods described in this report. For example, potential volunteer project sites could be identified via outreach by the FHWA Resource Centers, FHWA Federal Land Highway Divisions, and the AASHTO Task Committee on Hydrology and Hydraulics. Implementation: Field demonstration sites based, in particular, on proof-of-concept laboratory studies undertaken as part of this research would supplement post-project efforts to implement the results of this project.