Countermeasures to Protect Bridge Piers from Scour (2007)

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B-1 The selection methodology provides a quantitative assess- ment of the suitability of six armoring-type countermeasures based on selection factors that consider river environment, construction considerations, maintenance, performance, and estimated life-cycle cost. With the exception of life-cycle costs, the methodology analyzes the design factors by stepping the user through a series of decision branches, ultimately resulting in a site-specific numerical rating for each selection factor. The following countermeasures are evaluated by this methodology: • Standard (loose) riprap • Partially grouted riprap • Articulating concrete blocks • Gabion mattresses • Grout-filled mattresses • Grout-filled bags To facilitate the decision-making process, the procedure was automated using a Microsoft® Excel spreadsheet format. In the spreadsheet, the decision-making process can easily be modified to consider new situations or include additional in- formation. Detailed directions are included in the program file, and automated features are incorporated in the program to step the user through the process. Five factors are used to compute a Selection Index (SI) for each countermeasure: S1: Bed material size and transport S2: Severity of debris or ice loading S3: Constructability constraints S4: Inspection and maintenance requirements LCC: Life-cycle costs The Selection Index is calculated as SI = (S1 × S2 × S3 × S4)/LCC The countermeasure that has the highest value of SI is con- sidered to be most appropriate for a given site, based not only on its suitability to the specific riverine and project site condi- tions, but also in consideration of its economy. The approach is sensitive to assumptions regarding initial construction cost, remaining service life, assumed frequency of maintenance events, and extent of maintenance required. Each of these fac- tors requires experience and engineering judgment, as well as site- or region-specific information on the cost of materials and delivery, construction practices, and prevailing labor rates. It should be noted that the methodology can be used simply to rank the countermeasures in terms of suitability alone by assuming that the life-cycle costs are the same for all countermeasures. The following sections describe the five factors that com- pose the methodology. Flowcharts illustrating selection fac- tors S1 through S4 are enclosed. Bed Material Bed material is included as a selection factor for two rea- sons. Abrasion caused by the transport of coarse bed sedi- ments will cause the wire mesh on a gabion mattress to weaken and break, whereas other countermeasure types are relatively resistant to degradation by abrasion. For this reason, when bed material is greater than 2 mm, gabion mattresses are elim- inated from the selection process. Bed material size also assists in distinguishing whether dune-type bed forms are antici- pated. Grout-filled mattresses are susceptible to failure in the presence of bed forms because the mattresses do not articulate as well as other countermeasures. When the bed material is less than 2 mm and bed forms are not anticipated, all coun- termeasures included in the selection process are deemed equally viable. Ice and/or Debris Loading Debris in this context is considered floating material such as logs, other woody materials, man-made materials that are typically transported during floods, or ice. The intent of this selection factor is to recognize that high debris loads can be A P P E N D I X B Countermeasure Selection Methodology

detrimental to gabion mattresses, as indicated in the coun- termeasure selection matrix of HEC-23 (Lagasse et al. 2001). When a user indicates that anticipated debris loading is high, gabion mattresses receive a low rating of “1” but are not elim- inated from the selection process. When debris loading is not anticipated, all countermeasures included in the selection process are deemed equally viable. Construction Constraints Construction constraints take into account the different needs and challenges required for placing a countermeasure in the dry versus installation under water or, in the extreme case, in flowing water. All ratings that consider construction con- straints are divided into two categories: piers that have shallow footings versus piers that are more deeply embedded. This cat- egorization is necessary because riprap-based countermeasures are typically thicker than alternative countermeasures and they require pre-excavation that may undermine the footer. In addition, the requirements for specialized equipment are addressed. For example, the equipment requirements, place- ment techniques, and construction QA/QC requirements for partially grouted riprap are straightforward for working in the dry; however, placement under water requires construction equipment and placement technologies that are much more sophisticated. Subgrade preparation requirements and place- ment tolerances also vary among countermeasure types. For example, a relatively thin veneer of articulating concrete blocks requires finer grading techniques than an equivalent, and much thicker, riprap layer. Working beneath a bridge deck that affords little head- room will dictate the type of equipment that can be used for countermeasure installation. Last, alternative placement techniques, particularly for rock riprap, typically dictate the strength requirements for geotextiles to meet criteria for ge- otextile survivability during installation. The decision box for flow velocity is intended to reflect the relative difficulty in placing a mattress system, such as ACBs, gabion mattresses, or grout mattresses under fast-flowing water (V > 4 ft/s). When the countermeasure does not need to be placed under water and access for construction equip- ment of all types is good, all countermeasures included in the selection process are deemed equally viable. Inspection and Maintenance Inspection and maintenance guidelines vary greatly among countermeasure types. Underwater or buried installations re- quire different considerations to ensure that the counter- measure can be adequately inspected, compared to surficial treatments in ephemeral or intermittent stream environ- ments. The numerical values assigned to this selection factor reflect the relative difficulty of repairing and/or replacing “manufactured” countermeasures, such as ACBs, gabion mattresses, and grout mattresses, versus the relative ease of adding more riprap stone. The maintenance required for gabion mattresses as pre- sented may be somewhat higher than for other forms of revetment because the wire mesh used to construct the gabion is susceptible to vandalism. When the countermea- sure can be inspected and maintenance performed in the dry, all countermeasures included in the selection process are deemed equally viable. Life-Cycle Costs The Selection Index calculation is similar to the “Risk Pri- ority Number” method suggested by Johnson and Niezgoda (2004). Johnson and Niezgoda use the concept of “risk cate- gories” in contrast to this selection methodology concept of “suitability categories” to relate various factors. Both meth- ods represent relatively simple techniques for selecting pier scour countermeasures. However, because of the complexity of determining costs associated with countermeasure design and implementation, Johnson and Niezgoda discussed life- cycle costs but did not include those costs in the scope of their procedure. Without consideration of life-cycle cost, the suitability of a countermeasure is dictated solely by the environment of the river and its interaction with the bridge structure, combined with the strengths and vulnerabilities of the countermeasure. This selection methodology attempts to simplify the life-cycle cost estimation process through a series of spreadsheets that assist the user in evaluating regional availability of materials, installation expenses, and an estimation of maintenance based on experience and engineering judgment. Life-cycle cost information can be difficult to quantify. Ini- tial construction costs are relatively easy to develop; however, even for a specific countermeasure, these costs can vary widely depending on regional availability of materials, site condi- tions, and access or constructability constraints. Therefore, a particular countermeasure might be very cost effective in one locale and prohibitively expensive in another. Extending these issues to life-cycle maintenance requires an even broader set of assumptions. This portion of the assessment attempts to ease this process for the practitioner by providing templates for cost estimation. Estimating life-cycle costs for pier scour countermeasures requires consideration of three major components: • Initial construction materials and delivery costs • Initial construction installation costs associated with labor and equipment • Periodic maintenance during the life of the installation B-2

Each of the above components comprises multiple elements, which differ among the various countermeasure types. For example, quantities and unit costs of alternative materials will vary depending on the specific project conditions, as well as local and regional factors. Experience with these factors, as well as project-specific knowledge of the bridge site, are required in order to be as accurate as practicable when using this selection methodology. The following Issues should be considered when develop- ing life-cycle cost estimates: • Availability of materials of the required size and weight • Haul distance • Site access • Equipment requirements for the various countermeasures being considered • Construction under water vs. placement in the dry • Environmental and water quality issues and permitting requirements • Habitat and/or migration issues for threatened and endan- gered species • Traffic control during construction and/or maintenance activities • Local labor rates • Construction using in-house resources versus outside contract • Design life of the installation • Anticipated frequency and extent of periodic maintenance and repair activities Quantifying each of these factors requires experience and engineering judgment. For this reason, these variables are user inputs in the life-cycle cost worksheets. The default val- ues that are provided in the Excel spreadsheet program can and should be changed by the user to reflect both site-specific and state or regional conditions. Additional Considerations Federal or state regulations that preclude the use of a par- ticular countermeasure because of environmental consider- ations and permitting issues are beyond the scope of NCHRP Project 24-07(2). The practitioner in any particu- lar state must be aware of circumstances that may warrant the exclusion of a countermeasure for consideration at a specific site. A feature allowing the user to easily include an additional design consideration, such as state-specific environmental concerns, to the computation of the Selection Index was added to the Excel-based selection methodology program. Inclusion of an additional selection criterion will require the user to assign values in the context of the selection factors for all countermeasures considered. In addition, a feature was added to the selection method- ology Excel spreadsheet capability to permit a user to intro- duce another countermeasure and generate selection factor values for that countermeasure. Inclusion of an additional countermeasure will require the user to assign values in the context of the design considerations and selection factors. The supplementary countermeasure feature and design consideration feature can be used independently or together, as described in the countermeasure selection Excel file avail- able on the TRB website (http://trb.org/news/blurb_ detail.asp?id=7998). References Johnson, P.A., and Niezgoda, S.L. (2004). “Risk-Based Method for Se- lecting Scour Countermeasures,” ASCE Journal of Hydraulic Engi- neering, Vol. 130, No. 2, pp. 121–128. Lagasse, P.F., Zevenbergen, L.W., Schall, J.D., and Clopper, P.E. (2001). “Bridge Scour and Stream Instability Countermeasures,” Second Edition, Hydraulic Engineering Circular No. 23, FHWA-NHI-01- 003, Washington, D.C. B-3

B-4 Factor S1: Bed Material No Yes Are bed forms likely? No Low potential for abrasion Yes Bed forms minimal High potential for abrasion Is bed material primarily coarse sand or gravel with a d50 greater than 2 mm? 5 5 5 5 5 5 5 4 4 3 3 4 5 5 4 3 3 0 Recommended values for S1 Riprap Partially Grouted Riprap Articulating Concrete Blocks Grout-Filled Bags Grout-Filled Mattresses Gabions, Gabion Mattresses Factor S2: Ice/Debris Load Expected loading from ice or debris High 5 5 5 5 5 5 3 4 4 3 4 1 Recommended values for S2 Riprap Partially Grouted Riprap Articulating Concrete Blocks Grout-Filled Bags Grout-Filled Mattresses Gabions, Gabion Mattresses Low to Moderate

B-5 Factor S3: Construction Considerations No Continue to S3.1 on next page CM placement under water? Recommended values for S3 Riprap Partially Grouted Riprap Articulating Concrete Blocks Grout-Filled Bags Grout-Filled Mattresses Gabions, Gabion Mattresses *Note: Armoring countermeasures not recommended for these conditions. SF = Shallow Pier, e.g. Spread Footing DF = Deep Footing SF* 0 0 0 0 0 0 Remote or restricted Good V > 4 ft/s during installation? Yes No Equipment access Remote or restricted Good Equipment access SF 1 2 2 1 3 1 SF 2 0 0 1 0 0 SF 1 0 1 1 0 0 DF 5 5 4 5 4 3 DF 3 4 2 3 3 1 DF 5 0 1 2 0 1 DF 2 0 0 1 0 0 Yes

B-6 Factor S3.1: Construction Considerations No Underwater Placement DF 5 5 5 5 5 5 Recommended values for S3 Riprap Partially Grouted Riprap Articulating Concrete Blocks Grout-Filled Bags Grout-Filled Mattresses Gabions, Gabion Mattresses SF= Shallow Pier, e.g. Spread Footing DF= Deep Footing SF 1 2 2 1 3 1 SF 1 2 5 1 5 2 DF 3 4 3 4 4 3 No CM placement under water? Remote or restricted Equipment access Good

B-7 Factor S4: Inspection and Maintenance Recommended values for S4 Riprap Partially Grouted Riprap Articulating Concrete Blocks Grout-Filled Bags Grout-Filled Mattresses Gabions, Gabion Mattresses No Yes Must inspection and/or maintenance be performed under water? 5 5 5 5 5 5 5 4 3 2 2 1