Countermeasures to Protect Bridge Piers from Scour (2007)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

101 4.1 Applicability of Results to Highway Practice Approximately 83% of the 583,000 bridges in the NBI 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. 2001). The magnitude of these problems is demonstrated by the estimated average annual flood damage repair costs of ap- proximately $50 million for bridges on the federal aid system. Highway bridge failures caused by scour and stream insta- bility account for most of the bridge failures in this country. A 1973 study for the FHWA (Chang 1973) indicated that about $75 million were expended annually up to 1973 to re- pair roads and bridges that were damaged by floods. Extrap- olating the cost to the present makes this annual expenditure to roads and bridges on the order of $300 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 as- sociated 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 dur- ing the 1980s. Although it is difficult to be precise regarding the actual cost to repair damage to the nation’s highway system from problems related to pier scour, the number is obviously very large. In addition, the costs cited above do not include the extra costs that result from over-design of bridge foundations (i.e., deeper foundation depths, unnecessary or over- designed countermeasures) that result from our inability to select and design pier scour countermeasures with precision and confidence. This lack of knowledge often results in overly conservative design. For example, current FHWA policy considers riprap placed at bridge piers to be effective in reducing the risk from pier scour, but guidance dictates that riprap placed at bridge piers must be monitored by periodic inspection or with fixed instruments. This policy derives from experience with the difficulty of adequately sizing and properly installing riprap to withstand the turbulence and hydraulic stress generated in the vicinity of a bridge pier, particularly under flood-flow conditions. Similarly, a lack of specific design guidelines and specifica- tions for other potentially effective pier scour countermeasures has resulted in only limited application of countermeasures such as partially grouted riprap, articulating concrete block, gabion mattresses, grout-filled mattresses, and geotextile con- tainers. The guidelines, specifications, and recommendations from this research will provide a range of options to bridge owners for countering the effects of scour at piers and permit selecting the appropriate countermeasure for a specific prob- lem. The end result will be a more efficient use of highway resources and a reduction in costs associated with the impacts of pier scour on highway facilities. 4.2 Conclusions and Recommendations 4.2.1 Overview This research accomplished its basic objectives of develop- ing guidelines and recommended specifications for design and construction, and guidelines for inspection, mainte- nance, and performance evaluation for a range of pier scour countermeasures including riprap, partially grouted riprap, articulating concrete blocks, gabion mattresses, grout-filled 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 C H A P T E R 4 Conclusions and Suggested Research

varies with water depth, velocity, flow angle, pier shape and width, and other factors. If the determination is made that scour at a bridge pier can adversely affect the stability of a bridge, scour countermeasures to protect the pier should be considered. Because of their critical role in ensuring bridge integrity, and their potentially high cost, the most appropri- ate countermeasures must be selected, designed, constructed, and maintained. In this study, existing design equations for sizing the armor component of the pier scour countermeasures 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 maintenance process re- quired 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 con- text, filter requirements, material and testing specifications, construction and installation guidelines, and inspection and quality control procedures are also necessary. Each system typically includes a filter layer, either a geotextile fabric or a filter of sand and/or gravel, specifically selected for compati- bility with the subsoil. The filter allows infiltration and exfil- tration to occur while providing particle retention. To support the selection of an appropriate pier scour countermeasure for site-specific conditions, a countermea- sure selection methodology was developed. It provides an assessment of the suitability of each of five specific counter- measure types based on a variety of factors involving river environment, construction considerations, maintenance, performance, and estimated life-cycle cost of each counter- measure. Conclusions and recommendations for each of the pier scour countermeasures investigated are summarized in the following sections. In addition, some generalized observa- tions on pier scour protection systems are offered. For each pier scour countermeasure type, detailed design guidelines that incorporate these conclusions and recommendations and provide additional guidance are included as stand- alone appendixes. 4.2.2 Riprap When properly designed and used for pier scour protec- tion, 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 constructed riprap can provide long-term protection if it is inspected and main- tained on a periodic basis as well as after flood events. Tests conducted under NCHRP Project 24-07(2) validated and extended existing guidelines for using riprap as a scour countermeasure for bridge piers. Design Results of the tests confirmed that the HEC-23 (Lagasse et al. 2001) velocity-based procedure is appropriate for sizing riprap at piers, provided that the extent and thickness of the armor layer, the gradation, as well as the design of the filter, also follow recommended guidelines. Layout • Riprap areal coverage should extend a distance of at least two times the pier width in all directions around the pier. • Riprap should be placed in a pre-excavated hole around the pier so that the top of the riprap layer is level with the ambient channel bed elevation. • Placing the top of the riprap flush with the bed is ideal for inspection purposes and does not create additional ob- struction to the flow. • Riprap layer should have a minimum thickness of three times the d50 size of the rock. • The riprap thickness should be increased if the depth of the bed-form trough or contraction scour and long-term degradation is greater than the recommended thickness of three times the d50 size of the riprap. • Mounding riprap around a pier is not acceptable for design in most cases, because it obstructs flow, captures debris, and increases scour at the periphery of the installation. • For wall-type piers skewed to the flow, the riprap extent should be increased by a function of the skew angle, α; the pier width, a; and the pier length, L. Filter • The filter should not be extended fully beneath the riprap, but should be terminated two-thirds the distance from the pier to the edge of the riprap. • When a granular filter is used, the layer should have a min- imum thickness of four times the d50 of the filter stone or 6 in., whichever is greater. • Both riprap and granular filter thickness should be in- creased by 50% when placing under water. • Granular filters are not recommended when dune-type bed forms are present. 4.2.3 Partially Grouted Riprap and Geocontainers Partially grouted riprap consists of specifically sized rocks that are placed around a pier and grouted together with grout 102

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 compared to standard riprap, resulting in decreased layer thickness. Tests conducted under NCHRP Project 24-07(2) confirm the applicability of partially grouted riprap as a scour countermeasure for bridge piers. Design • Design guidance for partially grouted riprap comes, pri- marily, from the BAW in Germany. • The intent of partial grouting is to “glue” stones together to create a conglomerate of particles. Each conglomerate is therefore significantly greater than the d50 stone size and typically is larger than the d100 size of the individual stones in the riprap matrix. • For practical placement in the field, riprap having a d50 smaller than 9 in. (230 mm) exhibits voids that are too small for grout to effectively penetrate to the required depth within the rock matrix. • At the other extreme, riprap having a d50 greater than 15 in. (380 mm) has voids that are too large to retain the grout and does not have enough contact area between stones to effectively glue them together. • An appropriate riprap gradation is required to provide adequate void size. • With partially grouted riprap, there are no relationships per se for selecting the size of rock, other than the practical considerations of proper void size and adequate stone-to- stone contact area, cited above. Layout • Optimum performance of partially grouted riprap as a pier scour countermeasure was obtained when the riprap in- stallment extended a minimum distance of one and a half times the pier width in all directions around the pier. • Riprap should be placed in a pre-excavated hole around the pier so that the top of the riprap layer is level with the ambient channel bed elevation. • Placing the top of the riprap flush with the bed is ideal for inspection purposes and does not create any added ob- struction to the flow. • Mounding riprap around a pier is not acceptable for design in most cases, because it obstructs flow, captures debris, and increases scour at the periphery of the installation. • When used in a partially grouted application, the riprap layer should have a minimum thickness of two times the d50 size of the design riprap. When placement must occur under water, the thickness should be increased by 50%. • Where the grout must be placed under water, a recom- mended amount of Sicotan® admixture must be included in the mix to minimize segregation and improve the “stickiness.” Filter • As with standard (loose) riprap, a filter layer is typically re- quired. The filter should not be extended fully beneath the riprap; instead, it should be terminated two-thirds of the distance from the pier to the edge of the riprap. • With respect to filter materials, only geotextile filters were tested with the partially grouted riprap. 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. 4.2.4 Articulating Concrete Block Systems 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 continuous 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 systems are typically available in both open-cell and closed-cell varieties. There is little field experience with the use of ACB systems as a scour countermeasure for bridge piers alone. More fre- quently, 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 NCHRP Project 24-07(2) confirm the applicability of these systems as a scour coun- termeasure for bridge piers. Design Results of the tests confirmed that the factor of safety method recommended in HEC-23 (Lagasse et al. 2001) is ap- propriate for designing ACBs for hydraulic stability at piers, provided that the extent of the armor layer, as well as the design of the filter, also follows recommended guidelines. Testing also confirmed the importance of including block placement tolerance in the factor of safety calculations. Layout • The optimum performance of ACBs as a pier scour coun- termeasure was obtained when the blocks were extended a 103

distance of at least two times the pier width in all directions around the pier. • Because ACBs are essentially an erosion-resistant veneer that is one particle thick, the system edges must be toed down into a termination trench to prevent undermining and uplift around its periphery. • Blocks should not be placed on slopes greater than 2H:1V; when placed as pre-assembled mats, they should never be placed such that a portion of one mat lies on top of another mat. • When dune-type bed forms, contraction scour, and/or long- term degradation are present, the armor must be sloped away from the pier in all directions such that the depth of the ACB system at its periphery is greater than the depth of the bed-form troughs, or contraction scour and degradation. In some cases, this requirement may result in blocks being placed further than two pier widths away from the pier. Filter • The filter underlying the ACB system should be extended fully beneath the ACBs. • With respect to filter materials, only geotextile filters were tested with ACB systems. In most cases, granular filters are not recommended for use with ACBs because of the large open cells in typical ACB block systems. 4.2.5 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 thick- ness. 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 in- terlocking of the stone fill. During installation, individual mattresses are connected together by lacing wire or other connectors to form a continuous armor layer. The wire mesh allows the gabions to deform and adapt to changes in the bed while maintaining stability. Additionally, when compared to riprap, less excavation of the bed is re- quired and smaller, more economical stone can be used. The obvious benefit of gabion mattresses is that the size of the in- dividual stones used to fill the mattress can be smaller than stone that would otherwise be required to withstand the hy- draulic forces at a pier. There is limited field experience with the use of gabion mattress systems as a scour countermeasure 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 con- ducted under NCHRP Project 24-07(2) confirm the appli- cability of these systems as a scour countermeasure for bridge piers. Design • The guidance for pier scour applications provided in this document was developed primarily from the results of this study (NCHRP Project 24-07(2)). • The suitability of the basic design method, which is based on the concept of permissible shear stress, was confirmed for use at bridge piers by comparing the results of this test- ing program with the latest version of HEC-15 (Kilgore and Cotton 2005). • The durability of the wire mesh under long-term exposure to flow conditions specific to bridge piers has not been demonstrated; therefore, the use of gabion mattresses as a bridge pier scour countermeasure has an element of uncertainty. Layout • The optimum performance of gabion mattresses as a pier scour countermeasure was obtained when the mattresses were extended a distance of at least two times the pier width in all directions around the pier. • Because gabion mattresses are essentially an erosion-resist- ant veneer that behaves as a unit that is one layer thick, the system edges must be toed down into a termination trench to prevent undermining and uplift around its periphery. • Gabion mattresses should not be placed on slopes greater than 2H:1V, nor should they be placed in a manner that causes them to lie on top of adjacent mattresses. • When dune-type bed forms, contraction scour, and/or long-term degradation are present, the armor must be sloped away from the pier in all directions such that the depth of the gabion mattress system at its periphery is greater than the depth of the bed-form troughs, or con- traction scour and degradation. In some cases, this re- quirement may result in mattresses being placed further than two pier widths away from the pier. • To be effective, the gabion mattresses must be tied together using lacing wire or other types of mattress-to-mattress connectors. Field installations must use mattress-to- mattress connection materials that are at least as strong as the wire mesh composing the mattresses. Filter • As with riprap, the filter should only be extended two- thirds of the distance from the pier to the periphery of the gabion mattress installation. 104

• When a granular filter is used, the layer should have a min- imum thickness of four times the d50 of the filter stone or 6 in., whichever is greater. • The granular filter thickness should be increased by 50% when placing under water. • Granular filters are not recommended when dune-type bed forms are present. 4.2.6 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 in- stallation can be completed quickly, without the need for de- watering. Because of the flexibility of the fabric prior to filling, 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 cables laced through the mattress before the grout is pumped into the fabric form. Flexibility and per- meability are important functions for pier scour countermea- sures. Therefore, systems that incorporate filter points or weep holes (allowing for pressure relief through the mattress) com- bined with relatively small-diameter ducts (to allow grout breakage and articulation between blocks) are the preferred products. There is limited field experience with the use of grout-filled mattresses as a scour countermeasure for bridge piers. More frequently, these systems have been used for shoreline pro- tection, protective covers for underwater pipelines, and chan- nel armoring where the mattresses are placed across the en- tire 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 condi- tions. However, when dune-type bed forms were present, the mattresses were subject to both undermining and up- lift, even when they were toed down below the depth of the bed-form troughs. Therefore, this study cannot support the use of these products as pier scour countermeasures under live-bed conditions when dunes may be present. Design • The guidance for pier scour applications provided in this document has been developed primarily from HEC-23 (La- gasse et al. 2001) and the results of NCHRP Project 24-07(2). • The recommended design method is based on sliding sta- bility for both rigid and flexible grout-filled mattresses. Layout • The optimum performance of grout-filled mattresses as a pier scour countermeasure was obtained when the mat- tresses were extended at least two times the pier width in all directions around the pier. • Because these products are essentially an erosion-resistant veneer that behaves as a unit that is one layer thick, the sys- tem edges must be toed down into a termination trench to prevent undermining and uplift around its periphery. • Where long-term degradation and contraction scour is ex- pected at a bridge crossing, grout-filled mattresses must be sloped away from the pier in all directions such that the depth of the mattress system at its periphery is greater than the depth of anticipated contraction scour and degradation. • Grout-filled mattresses should not be laid on a slope steeper than 2H:1V. In some cases, this limitation may result in grout-filled mattresses being placed further than two pier widths away from the pier. • Mattresses should not be placed such that a portion of one mattress lies on top of an adjacent mattress. Filter • A filter layer is typically required for grout-filled mattresses at bridge piers. The filter should be extended fully beneath the system to its periphery. • When a granular filter is used, the layer should have a min- imum thickness of four times the d50 of the filter stone or 6 in., whichever is greater. • The granular filter thickness should be increased by 50% when placing under water. 4.2.7 Additional Observations on Pier Scour Protection Systems Countermeasures for scour and stream instability problems are measures incorporated into a highway-stream crossing system to monitor, control, inhibit, change, delay, or mini- mize stream instability and bridge scour problems. Although considerable research has been dedicated to development of countermeasures for scour and stream instability, many coun- termeasures have evolved through a trial-and-error process and lack definitive design guidance. In addition, some coun- termeasures have been applied successfully in one area but have failed when installations were attempted under different geomorphic or hydraulic conditions. This occurrence is par- ticularly true of pier scour countermeasures. In the mid- 1990s, FHWA guidance to the state DOTs cautioned that pier 105

scour countermeasures, such as riprap, may not provide ade- quate long-term protection, primarily because selection crite- ria, design guidelines, and specifications were not available. By the late 1990s, some progress had been made in devel- oping selection, design, and installation guidelines for pier scour countermeasures. For example, the publication of the first edition of HEC-23 in 1997 (Lagasse et al. 1997) was a first step toward identifying, consolidating, and disseminating in- formation on countermeasure guidance. In addition, the first phase of this study (Parker et al. 1998 and 1999) provided the initial results of laboratory and field research to evaluate the performance of pier scour countermeasures and develop design and implementation guidance. A wide variety of countermeasures has been used to con- trol channel instability and scour at bridge foundations. In HEC-23 (Lagasse et al. 1997) a countermeasure matrix is pro- vided to highlight the various groups of countermeasures and to identify their individual characteristics. In the matrix, countermeasures are organized into groups based on their functionality with respect to scour and stream stability. The three main groups of countermeasures are hydraulic coun- termeasures, structural countermeasures, and monitoring. Hydraulic countermeasures are those designed either to modify the flow (river training) or resist erosive forces caused by the flow (armoring). Structural countermeasures involve modification of the bridge structure (foundation) to prevent failure from scour. Monitoring describes activities used to fa- cilitate early identification of potential scour problems. When the second edition of HEC-23 was published (Lagasse et al. 2001), only structural countermeasures or monitoring options were considered to be well-suited countermeasure sys- tems to protect against pier scour. Hydraulic countermeasures (river training and armoring) were considered to have only a secondary benefit in preventing or controlling pier scour. For armoring countermeasures this consideration was, primarily, a result of the lack of definitive design guidance at the time. This project has focused on providing that design guidance for five pier scour armoring systems. Within the suite of pier scour armoring systems tested, riprap and partially grouted riprap provide protection by the bulk or mass of the armor layer, through the redundancy of multiple particles in a flexible, self-healing matrix underlain by an appropriate filter. From a hydraulic stability perspective, the other three armoring systems (articulating concrete blocks, and gabion and grout-filled mattresses) provide pro- tection through an armor layer essentially one particle (or one unit) thick. This thin veneer of armor often provides economy in both materials and installation but, if not provided with an appropriate filter and adequate transition or termination de- tails, could be subject to rapid, potentially catastrophic failure. A review of the conclusions and recommendations out- lined for each countermeasure type in the preceding sections reveals a range of commonalities and contrasts for these sys- tems. For example, in most cases a filter layer is essential for successful performance of all pier scour protection. However for the countermeasures that incorporate rock particles, in- cluding gabions, the filter should extend only two-thirds of the distance from the pier to the perimeter of the armor. In contrast, articulating concrete block 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 chan- nels (i.e., under live-bed conditions). During testing, geotex- tile filters generally performed well for all countermeasure types when all components of the countermeasure system were properly designed and installed. For the ACB system, 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 meeting placement and grading tol- erances would be difficult if geotextile containers are used as a filter. For the pier scour countermeasures consisting of a thin ve- neer 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 rigid and flexible configura- tions yielded definitive results only for clear-water conditions. More research will be required before this countermeasure can be recommended 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, labora- tory testing could not provide guidance for the strength, com- position, or longevity of the connecting material. For all three of the manufactured systems, the product provider should supply appropriate test results along with installation and ma- terials guidance. This information is essential for successful performance of these products. 4.2.8 Countermeasure Selection The countermeasure selection methodology developed as part of this study provides an assessment of the suitability of each of six specific countermeasure types based on a variety of factors. The output from the selection method provides a quantitative ranking of countermeasure types by computing a Selection Index. The Selection Index includes a fatal-flaw mechanism to identify situations where a particular counter- measure is unequivocally unsuitable due to one or more circumstances unique to the site being evaluated. The Selection 106

Index is intended to identify the countermeasure best suited for application at a particular site (see Appendix B). However, there is no substitute for experience and engineering judgment in countermeasure selection. The Selection Index should be considered only one indication of countermeasure suitability for site-specific conditions. The interactive Microsoft® Excel spreadsheet (available on the TRB website: http://www.trb. org/news/blurb_detail.asp?id=7998) will be helpful in apply- ing the selection methodology and adapting it to site-specific conditions. 4.2.9 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 rec- ommendations 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 HEC-23 as a guide. As appropriate, these guidelines are recommended for consideration by AASHTO, FHWA, and state DOTs for adoption and incorporation into manuals, specifications, or other design guidance documents. 4.3 Suggested Research The findings of Chapter 2 and the interpretation and appraisal of testing results in Chapter 3 are reflected in the recommended design methods, materials, and construction and inspection guidance presented in Appendixes C through G. In developing these guidelines, additional information, data, or field experience with various countermeasure sys- tems would have supported more detailed guidance or speci- ficity in several areas. The following suggestions for future research would permit extending the recommendations of this study in these areas: • Tests of simulated grout-filled mattresses at small scale indi- cated that, in the presence of dune-type bed forms, both flex- ible and rigid systems were vulnerable to undermining and uplift. Voids beneath these systems were noted even when the periphery was toed down below the depth of the bed-form troughs. It is suggested that these systems be investigated at near-prototype scale in the laboratory, and/or prototype scale at appropriate field sites to provide additional insight on their performance under live-bed conditions. • Limited testing of wall-type piers skewed to the flow direction confirmed that the lateral extent of armoring countermeasures must be increased for them to perform successfully. Recommendations provided in this study have been inferred based on consideration of the addi- tional scour potential caused by the skew. It is suggested that this subject be further investigated to either verify or modify the recommendations for skewed piers developed in this study. • Water quality measurements were taken during the place- ment of partially grouted riprap in flowing water under prototype-scale conditions in the laboratory. The data indicate that transient increases in pH, turbidity, and elec- troconductivity occur as grout is being placed, and for a short time afterwards. This impact was limited to a local area downstream of the installation and may be within ac- ceptable limits as established by permit agencies. Very little evidence of lateral dispersion was noted in the laboratory study. A limited number of candidate bridge sites could be identified for the installation of partially grouted riprap in the field under a wider variety of hydraulic and riverine conditions than could be investigated in the laboratory to verify the magnitude and extent of potential water quality impacts and to identify methods of mitigation and control, if such are needed. • Establishing a seal between the countermeasure and the pier has been noted as a necessary component of any installation to prevent the winnowing of bed material through gaps in the region of high turbulence and vortex action at the pier. The placement of a thin grout seal around the pier proved effective at preventing winnowing during the laboratory tests and can be performed under water if necessary in the field. However, many other materials may be suitable for creating an effective seal, for example, small sand-filled ge- otextile tubes, asphaltic mastic, or geotextile collars. Addi- tional techniques and materials should be investigated to expand the options for creating an effective seal between the pier and the scour countermeasure. • Physically attaching a scour countermeasure (such as a cabled articulating concrete block system, gabion mattresses, or grout-filled mattresses) to a bridge pier is often suggested as a means to increase anchorage and stability of the coun- termeasure. Recommendations made in this study discour- age physical attachment between countermeasure and pier; however, because it is a common practice, the potential for increased loading from pier scour countermeasures on the 107

bridge structure should be investigated, particularly in the case of countermeasure failure. • The permeability, flexibility, and long-term durability of pier scour countermeasures are identified as beneficial characteristics. In this study, these specific properties were neither quantitatively measured nor related to counter- measure performance. Instead, inferences regarding these aspects were drawn from the literature, as well as from qualitative observations made during the testing program. The following observations point to the need for future research: – Flexibility, abrasion resistance, and resistance to dam- age from impact or debris snagging on prototype-size gabion mattresses should be investigated more fully. Both welded wire and twisted wire products should be examined, along with various coatings that are currently commercially available for use with these systems. – The ability of commercially available grout-filled mat- tresses to fully articulate to accommodate edge scour, undermining, or differential settlement has not been adequately demonstrated. Also, the overall permeabil- ity of the system is not well characterized, and the lack of adequate permeability may have resulted in uplift- type failures observed in the laboratory-scale tests. Fur- ther research is suggested in these areas. – Open-cell articulated concrete blocks are generally as- sumed to have adequate permeability by virtue of the open cells, and to be durable, provided the concrete mix is well designed and quality controlled. The use of sys- tems composed of solid blocks that have very little open area may be questionable because of a marked decrease in permeability. In addition, the ability of some propri- etary products to fully articulate has been questioned, largely because of their “jigsaw puzzle” type of mechan- ical block-to-block interlock. Further research in these areas could resolve these uncertainties. • Improved predictive methods should be developed for quan- tifying dune bed-form geometry, as well as for providing practitioners a reliable method for recognizing the condi- tions under which the onset of dunes can be anticipated. Also, the potential interaction between bed forms and con- traction scour should be investigated to determine if these processes are independent or additive at the prototype scale. • ACB systems, gabion mattresses, and grout-filled mat- tresses were all observed to act as a scour-resistant veneer that behaves as a unit that is one layer thick. The prevention of voids beneath these systems is essential to their success- ful performance. Further research under a wider variety of conditions than could be accomplished under this study is warranted. 108