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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Appendix C

APPENDIX C

PIER SCOUR COUNTERMEASURE SELECTION METHODOLOGY

C.1 Overview

This selection methodology provides a quantitative assessment of the suitability of six armoring-type countermeasures for pier scour based on selection factors that consider river environment, construction considerations, maintenance, performance, and estimated life-cycle cost (Lagasse et al. 2007). 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 (see Section 3.2.5). 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 information. Detailed directions are included in the program file, and automated features are incorporated in the program to step the user through the process.

C.2 Selection Index

Five factors are used to compute a Selection Index (SI0 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 x S2 x S3 x S4)/LCC (C.1)

The countermeasure that has the highest value of SI is considered to be most appropriate for a given site, based not only on its suitability to the specific riverine and project site conditions, 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 factors 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.

In Section 3.2.5 the five factors that compose the methodology are described in detail. Flow charts illustrating selection factors S1 through S4 and reference to Excel spread sheet support follows.

C.3 Additional Considerations and Support

Federal or state regulations that preclude and use of a particular countermeasure because of environmental considerations and permitting issues are beyond the scope of NCHRP Project 24-07(2). The practitioner in any particular 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 methodology Excel spreadsheet capability to permit a user to introduce 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 available on the TRB website ( http://trb.org/news/blurb_ detail.asp?id=7998).

Schematics of two Flow Charts shown:   The first Flow Chart, Factor S1: Bed Material is used to determine the value for S1 - Factor for Bed Material size and transport.  If bed material D50 size greater than 2 mm, bed forms are minimal and there is a high potential for abrasion. In this case the recommended values of S1 for the five countermeasures are; Riprap 5, Partially Grouted Riprap 5, Articulating Concrete Blocks 4, Grout-Filled Bags 3, Grout Filled Mattresses 3, Gabions or Gabion Mattresses 0.   If bed material D50 size is less than 2 mm, there is a low potential for abrasion and if bed forms are likely then the recommended values of S1 for the five countermeasures are; Riprap 5, Partially Grouted Riprap 4, Articulating Concrete Blocks 4, Grout-Filled Bags 3, Grout Filled Mattresses 3, Gabions or Gabion Mattresses 4.  If bed material D50 size is less than 2 mm, there is a low potential for abrasion and if bed forms are not likely then the recommended value of S1 for the five countermeasures is 5.   The second Flow Chart, Factor S2: Ice Debris Load, is used to determine the value for S2 - Severity of debris or ice loading. If the expected loading from ice is High the recommended factor S2 values for the five countermeasures are; Riprap 3, Partially Grouted Riprap 4, Articulating Concrete Blocks 4, Grout-Filled Bags 3, Grout Filled Mattresses 4, Gabions or Gabion Mattresses 1. If loading from ice is expected to be low to moderate the recommended value of factor S2 is 5 for all countermeasures.

Schematic flow chart for Factor S3: Construction Considerations  The value of S3 is based on if the countermeasure placement is under water, if water velocities are greater or leas than 4 feet per second, if equipment access good or remote or restricted and if the footing is shallow or deep. Shallow footing or shallow pier is a spread footing for example.   For water velocities greater than 4 feet per second during construction and remote or restricted equipment access with Shallow footings the recommended value for S3 is zero - armoring is not recommended in these conditions.   For water velocities greater than 4 feet per second during construction and remote or restricted equipment access with Deep footings the recommended values for S3 are; Riprap 2, Partially Grouted Riprap 0, Articulating Concrete Blocks 0, Grout-Filled Bags 1, Grout Filled Mattresses 0, Gabions or Gabion Mattresses 0.   For water velocities greater than 4 feet per second during construction and good equipment access with Shallow footings the recommended values for S3 are; Riprap 1, Partially Grouted Riprap 0, Articulating Concrete Blocks 1, Grout-Filled Bags 1, Grout Filled Mattresses 0, Gabions or Gabion Mattresses 0.   For water velocities greater than 4 feet per second during construction and good equipment access with Deep footings the recommended values for S3 are; Riprap 5, Partially Grouted Riprap 0, Articulating Concrete Blocks 1, Grout-Filled Bags 2, Grout Filled Mattresses 0, Gabions or Gabion Mattresses 1.   For water velocities less than 4 feet per second during construction and remote or restricted equipment access with Shallow footings the recommended values for S3 are; Riprap 1, Partially Grouted Riprap 2, Articulating Concrete Blocks 2, Grout-Filled Bags 1, Grout Filled Mattresses 3, Gabions or Gabion Mattresses 1.   For water velocities less than 4 feet per second during construction and remote or restricted equipment access with Deep footings the recommended values for S3 are; Riprap 3, Partially Grouted Riprap 4, Articulating Concrete Blocks 2, Grout-Filled Bags 3, Grout Filled Mattresses 3, Gabions or Gabion Mattresses 1.   For water velocities less than 4 feet per second during construction and good equipment access with Shallow footings the recommended values for S3 are; Riprap 2, Partially Grouted Riprap 0, Articulating Concrete Blocks 0, Grout-Filled Bags 1, Grout Filled Mattresses 0, Gabions or Gabion Mattresses 0.   For water velocities less than 4 feet per second during construction and good equipment access with Deep footings the recommended values for S3 are; Riprap 5, Partially Grouted Riprap 5, Articulating Concrete Blocks 4, Grout-Filled Bags 5, Grout Filled Mattresses 4, Gabions or Gabion Mattresses 3.

Titled: Factor S3.1: Construction Considerations No Underwater Placement this is a continuation of flow chart for Factor S3: Construction Considerations. With no underwater placement of countermeasures, the value of S3 is affected by equipment access and if the footing is shallow or deep. Shallow footing or shallow pier is a spread footing for example.   For remote or restricted access and Shallow footings the recommended values for S3 are: Riprap 1, Partially Grouted Riprap 2, Articulating Concrete Blocks 2, Grout-Filled Bags 1, Grout Filled Mattresses 3, Gabions or Gabion Mattresses 1.   For remote or restricted access and Deep footings the recommended values for S3 are: Riprap 3, Partially Grouted Riprap 4, Articulating Concrete Blocks 3, Grout-Filled Bags 4, Grout Filled Mattresses 4, Gabions or Gabion Mattresses 3.   For good access and Shallow footings the recommended values for S3 are: Riprap 1, Partially Grouted Riprap 2, Articulating Concrete Blocks 5, Grout-Filled Bags 1, Grout Filled Mattresses 5, Gabions or Gabion Mattresses 2.   For good access and Deep footings the recommended value for S3 is 5 for all countermeasures.

Titled: Factor S4: Inspection and Maintenance.   Must inspection and maintenance be performed under water?  If yes the recommended values for S4 are: Riprap 5, Partially Grouted Riprap 4, Articulating Concrete Blocks 3, Grout-Filled Bags 2, Grout Filled Mattresses 2, Gabions or Gabion Mattresses 1.  If no the recommended value for S4 is 5 for all countermeasures.

Updated: 09/13/2011

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