Countermeasures to Protect Bridge Piers from Scour (2007)

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S U M M A R Y Overview This research accomplished its basic objectives of developing guidelines and specifications for design and construction, and guidelines for inspection, maintenance, and performance evaluation for a range of pier scour countermeasures including riprap, partially grouted riprap, articulating concrete blocks, gabion mattresses, grout mattresses, and geotextile sand containers. Local scour at bridge piers is a potential safety hazard to the traveling public and is a major concern to transportation agencies. Bridge pier scour is a dynamic phenomenon that varies with water depth, velocity, flow angle, pier shape and width, and other factors. If it is deter- mined that scour at a bridge pier can adversely affect the stability of a bridge, scour counter- measures to protect the pier should be considered. Because of their critical role in ensuring bridge integrity, and their potentially high cost, it is important that the most appropriate countermeasures be selected, designed, constructed, and maintained. In this study, existing design equations for sizing the armor component of the pier scour coun- termeasures of interest were used to develop a laboratory testing program. However, sizing the armor is only the first step in the comprehensive design, installation, inspection, and mainte- nance process required for a successful countermeasure. A countermeasure is an integrated system that includes the armor layer, filter, and termination details. Successful performance depends on the response of each component of the system to hydraulic and environmental stresses throughout its service life. In this context, filter requirements, material and testing spec- ifications, construction and installation guidelines, and inspection and quality control proce- dures are also necessary. To support the selection of an appropriate pier scour countermeasure for site-specific con- ditions, a countermeasure selection methodology was developed. It provides an assessment of the suitability of each of five specific countermeasure types based on a variety of factors involving river environment, construction considerations, maintenance, performance, and estimated life-cycle cost of each countermeasure. Research Approach This research was undertaken to extend the results and applicability of an earlier study (NCHRP Project 24-07), investigate additional countermeasure types, and develop detailed design guidelines. The research approach involved the following steps. 1. Transition from NCHRP 24-07 based on a review of the Users Guide and Final Report (both unpublished) from the University of Minnesota, St. Anthony Falls Laboratory Countermeasures to Protect Bridge Piers From Scour 1

22. Integration of European technology identified during a 1998 TRB/FHWA scanning review by including two research team members from Germany 3. Completion of a literature review and evaluation of current practice for pier scour coun- termeasures 4. Field site visits to countermeasure installations in the United States 5. Field site visits to research facilities and scour countermeasure installations at numerous project sites in Germany—sponsored by the German Federal Waterways Engineering and Research Institute (Bundesanstalt für Wasserbau or BAW) 6. Completion of extensive small-scale and prototype-scale laboratory investigations at the Colorado State University Hydraulics Laboratory 7. Integration of the survey of current practice with laboratory test results and other available guidance into a set of stand-alone Design Guidelines for five pier scour countermeasure systems 8. Development of a selection methodology for pier scour countermeasures considering site-specific conditions The following sections provide a brief overview of the five countermeasure systems and their applicability for bridge pier scour protection. Riprap When properly designed and used for pier scour protection, riprap has an advantage over rigid structures because it is flexible when under attack by river currents, it can remain functional even if some individual stones may be lost, and it can be repaired relatively easily. Properly con- structed riprap can provide long-term protection if it is inspected and maintained on a periodic basis as well as after flood events. For the DOTs, riprap has been the most common counter- measure installed at bridge piers. In this study standard (loose) riprap was used as a baseline and benchmark for evaluating the performance of other pier scour countermeasures. The study validated and extended existing guidelines for using riprap for pier scour protection. Partially Grouted Riprap and Geocontainers Partially grouted riprap consists of specifically sized rocks that are placed around a pier and grouted together with grout filling 50% or less of the total void space. In contrast to fully grouted riprap, partial grouting increases the overall stability of the riprap installation unit without sacrificing flexibility or permeability. It also allows for the use of smaller rock com- pared to standard riprap, resulting in decreased layer thickness. The system typically includes a filter layer—either a geotextile fabric or a filter of sand and/or gravel—specifically selected for compatibility with the subsoil. The filter allows infiltration and exfiltration to occur while providing particle retention. Tests conducted under this study confirm the applicability of partially grouted riprap as a scour countermeasure for bridge piers. Based on prototype-scale testing, the use of sand-filled geocontainers composed of non- woven needle-punched geotextile was confirmed to be an appropriate means of establishing a filter layer around a pier when placement of either standard riprap or partially grouted riprap must occur under water. Articulating Concrete Block Systems Articulating concrete block (ACB) systems provide a flexible armor for use as a pier scour countermeasure. These systems consist of preformed concrete units that either interlock, are held together by cables, or both. After installation is complete, the units form a contin-

uous blanket or mat. The term “articulating” implies the ability of individual blocks of the system to conform to changes in the subgrade while remaining interconnected. Block sys- tems are typically available in both open-cell and closed-cell varieties. There is little field experience with the use of articulating block systems as a scour coun- termeasure for bridge piers alone. More frequently, these systems have been used for bank revetment and channel armoring where the mat is placed across the entire channel width and keyed into the abutments or bank protection. Tests conducted under this study confirm the applicability of these systems as a scour countermeasure for bridge piers. Gabion Mattresses Gabion mattresses are containers constructed of wire mesh and filled with rocks. The length of a gabion mattress is greater than its width, and the width is greater than its thickness. Diaphragms are inserted widthwise into the mattress to create compartments. Wire is typically galvanized or coated with polyvinyl chloride to resist corrosion, and either welded or twisted into a lattice. Stones used to fill the containers can be either angular rock or rounded cobbles; however, angular rock is preferred because of the higher degree of natural interlocking of the stone fill. During installation, individual mattresses are connected together by lacing wire or other connectors to form a continu- ous armor layer. The wire mesh allows the gabions to deform and adapt to changes in the bed while main- taining stability. Additionally, when compared to riprap, less excavation of the bed is required and smaller, more economical stone can be used. The obvious benefit of gabion mattresses is that the size of the individual stones used to fill the mattress can be smaller than stone that would otherwise be required to withstand the hydraulic forces at a pier. There is limited field experience with the use of gabion mattress systems as a scour counter- measure for bridge piers alone. More frequently, these systems have been used for structures such as in-channel weirs or drop structures, or for channel slope stabilization. Tests conducted under this study confirm the applicability of these systems as a scour countermeasure for bridge piers. Grout-Filled Mattresses Grout-filled mattresses are composed of a double layer of strong synthetic fabric, typically woven nylon or polyester, sewn into a series of pillow-shaped compartments (blocks) that are connected internally by ducts. The compartments are filled with a concrete grout that flows from compartment to compartment via the ducts. Adjacent mattresses are typically sewn together prior to filling with grout. The benefits of grout-filled mattresses are that the fabric installation can be completed quickly, without the need for dewatering. Because of the flexibility of the fabric prior to fill- ing, laying out the fabric forms and pumping them with concrete grout can be performed in areas where room for construction equipment is limited. When set, the grout forms a single- layer veneer made up of a grid of interconnected blocks. The blocks are interconnected by ca- bles laced through the mattress before the grout is pumped into the fabric form. Flexibility and permeability are important functions for pier scour countermeasures. Therefore, systems that incorporate filter points or weep holes (allowing for pressure relief through the mattress) combined with relatively small-diameter ducts (to allow grout breakage and artic- ulation between blocks) are the preferred products. There is limited field experience with the use of grout-filled mattresses as a scour countermea- sure for bridge piers. More frequently, these systems have been used for shoreline protection, 3

4protective covers for underwater pipelines, and channel armoring where the mattresses are placed across the entire channel width and keyed into the abutments or banks. Tests confirm that grout-filled mattresses can be effective scour countermeasures for piers under clear-water conditions. However, when dune-type bed forms were present, the mattresses were subject to both undermining and uplift, even when they were toed down below the depth of the bed-form troughs. Therefore, study results do not support the use of these products as pier scour countermeasures under live-bed conditions when dunes may be present. Design Guidelines To guide the practitioner in developing appropriate designs and ensuring successful installation and performance of pier scour armoring systems, the findings of Chapter 2 and recommendations of Chapter 3 are combined to provide a detailed set of stand-alone appendixes: • Appendix C, Guidelines for Pier Scour Countermeasures Using Rock Riprap • Appendix D, Guidelines for Pier Scour Countermeasures Using Partially Grouted Riprap • Appendix E, Guidelines for Pier Scour Countermeasures Using Articulating Concrete Block (ACB) Systems • Appendix F, Guidelines for Pier Scour Countermeasures Using Gabion Mattresses • Appendix G, Guidelines for Pier Scour Countermeasures Using Grout-Filled Mattresses These application guidelines are presented in a format using the FHWA’s Hydraulic Engineering Circular No. 23 (HEC-23) as a guide. As appropriate, these guidelines can be considered by AASHTO, FHWA, and state DOTs for adoption and incorporation into manuals, specifications, or other design guidance documents. Conclusions and Recommendations A review of the conclusions and recommendations outlined for each countermeasure type in Chapter 4 reveals a range of commonalities and contrasts for these systems. In most cases a filter layer is essential for successful performance of all pier scour protection. How- ever for the countermeasures that incorporate rock particles, including gabions, the filter should extend only two-thirds of the distance from the pier to the perimeter of the armor. In contrast, ACB mats and grout-filled mattresses should have a filter underlying the full extent of the armor layer. In all cases, a granular filter should not be used when dune-type bed forms are expected in sand channels (i.e., under live-bed conditions). During testing, geotextile filters generally performed well for all countermeasure types when all compo- nents of the countermeasure system were properly designed and installed. For the ACB sys- tem, granular filters are not recommended under most conditions. Geotextile sand containers are strongly recommended as a proven technique for placing a filter under water for riprap or partially grouted riprap, and gabion and grout-filled mat- tresses. For the ACB systems, a conventional geotextile filter should be used because placement and grading tolerances would be difficult to meet if geotextile containers were used as a filter. For the pier scour countermeasures consisting of a thin veneer of armor (ACBs and the mattresses), termination details and, where necessary, anchor systems play a significant role in successful performance. It should be noted that testing of the grout-filled mattresses in both a “rigid” and “flexible” configuration yielded definitive results only for clear-water conditions. More research will be required before this countermeasure can be recom-

mended for pier scour protection under live-bed conditions. Similarly, the gabion mattress countermeasure, as tested, performed much better when the individual mattresses were physically connected to one another, compared to their performance as individual armor elements. However, laboratory testing could not provide guidance for the strength, com- position, or longevity of the “tie.” For all three of these manufactured systems, the product provider should supply appropriate test results along with installation and materials guid- ance. This information is essential for successful performance of these products. Suggested Research In developing the design guidelines, additional information, data, or field experience with various countermeasure systems would have supported more detailed guidance or specificity in several areas. Suggestions for future research that would permit extending the recommen- dations of this study in these areas are summarized in Chapter 4. 5