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Publication Number:  FHWA-HRT-17-054    Date:  October 2017
Publication Number: FHWA-HRT-17-054
Date: October 2017


Advanced Methodology to Assess Riprap Rock Stability At Bridge Piers and Abutments

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Riprap is one of the most common materials used to protect bridge abutment and pier foundations from scour. A key element of the design of riprap countermeasures is rock sizing, which is based on equations generally derived from simplified laboratory experiments. In this study, an advanced modeling approach is developed and applied to evaluate rock stability. The advantage of this approach is that it can incorporate site-specific conditions that complicate riprap design. This report describes this advanced, numerical modeling procedure for analyzing the stability of riprap at bridge abutments and piers. The report will be useful for designers and engineers responsible for protecting bridge foundations. The study described in this report was conducted at the Federal Highway Administration’s (FHWA) Turner-Fairbank Highway Research Center (TFHRC) J. Sterling Jones Hydraulics Laboratory.

Cheryl Allen Richter, P.E., Ph.D.
Director, Office of Infrastructure
Research and Development


This document is disseminated under the sponsorship of the U.S. Department of Transportation (USDOT) in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

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Technical Report Documentation Page

1. Report No.


2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Advanced Methodology to Assess Riprap Rock Stability at Bridge Piers and Abutments

5. Report Date

October 2017

6. Performing Organization Code
7. Author(s)

Oscar Suaznabar, Cezary Bojanowski, Steven A. Lottes, Jerry Shen, Kornel Kerenyi, and Roger Kilgore

8. Performing Organization Report No.


9. Performing Organization Name and Address

11848 Rock Landing Drive, Suite 303
Newport News, VA 23606

10. Work Unit No. (TRAIS)

11. Contract or Grant No.


12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

Pooled Fund Study Report
October 2011–September 2016

14. Sponsoring Agency Code


15. Supplementary Notes

The Contracting Officer's Representative (COR) was Kornel Kerenyi (HRDI-40).

16. Abstract

The objectives of this research study were to: (1) assess whether detailed fluid structure interaction (FSI) modeling can inform evaluation of rock riprap movement for both the analysis of existing riprap aprons and for the design of new riprap aprons; and (2) develop recommendations for the design, installation, and monitoring of riprap aprons at bridge piers and abutments, where feasible.


A new advanced computational methodology for assessing failure risk of geometrically complex riprap installations was developed for this study. The study demonstrated that detailed FSI modeling can inform evaluation of rock riprap movement for both the analysis of existing riprap aprons and for the design of new riprap aprons. The approach solved the FSI problem for the onset of rock riprap motion using weakly coupled computational fluid dynamics and computational structural mechanics software. The flow threshold for the onset of motion of riprap rocks was computed for a set of representative riprap rocks for both simplified laboratory and complex field conditions. Physical laboratory experiments were used to validate the numerical procedures. The FSI approach was also tested on a complex field case study of a riprap installation at a pier for a bridge over the Middle Fork of the Feather River. While the case study application was considered successful, the approach is limited by its high costs and limited availability. Therefore, good candidate applications for using FSI analysis to assess new or retrofit riprap installations would be those where the project cost is significant or the risks of failure are catastrophic.


The study also identified recommendations for improving the design, installation, and monitoring of riprap apron installations at bridge piers and abutments, where feasible. These included: (1) verifying as-built conditions for assuring that the intended level of protection has been achieved, (2) inspecting for changes in stream morphology that may significantly change conditions from those anticipated at design, (3) recording the date of rock riprap installations and monitoring the performance of the installations after major floods, (4) applying sonar technologies for riprap monitoring, and (5) avoiding rock riprap installations for new bridge piers as they are not recommended by FHWA policy.


The FSI numerical modeling approach has promise for supporting the design and evaluation of riprap installations for bridge abutments and piers. As computer capabilities increase and more detailed representations of rock riprap installations become more practical, the approach should continue to increase in its utility.

17. Key Words

Riprap, countermeasure, bridge pier, bridge abutment, fluid-structure interaction, computational fluid dynamics, computational structural mechanics, numerical modeling, flume modeling of riprap

18. Distribution Statement

No restrictions.

19. Security Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized



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