National Academies Press: OpenBook

Countermeasures to Protect Bridge Abutments from Scour (2007)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Page 6
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Page 6

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41.1 Introduction Scour of bridge abutments is a common cause of bridge failure, often resulting in substantial interruption of traffic, and sometimes loss of life, not to mention damage to vehicles. An abutment scour entails hydraulic erosion of the waterway boundary around an abutment and the geotechnical failure of both the soil upon which the abutment is placed and of the soil comprising the embankment at an abutment. Accord- ingly, it is necessary to protect the abutment structure against hydraulic erosion as well as geotechnical failure. This require- ment potentially makes the design of scour countermeasures for abutments a more difficult proposition than countermea- sure design for piers. The present project reviews existing countermeasure concepts, explores prospective new concepts, and further develops existing concepts. It is pertinent to indicate at the outset here that, after exten- sive investigation, the main advances stemming from this project concern the further development of existing counter- measure concepts. There are, though, several additional rela- tively new concepts introduced. These concepts are not highly innovative or elegant, but are sensible and practical. This final report documents the extensive amount of investigation and testing completed for the project. The investigation included heuristic, laboratory exploration of countermeasure concepts not documented in the literature on scour, detailed testing aimed at further developing concepts of proven merit, and identification of concepts that have been implemented in a somewhat ad hoc manner at existing abutments. No panacea was found in the sense that many counter- measure configurations did mitigate scour at the abutment location but shifted the scour somewhere else. Care must be taken to ensure that a pier or other structure is not located in the scour location in order to avoid potential collapse. Work for this project was performed at various locations. The riprap and cable-tied block experiments were carried out at the University of Auckland, New Zealand. Experiments on geobags and a large-scale experiment on riprap on spill- through abutments were carried out at the University of Iowa. Experiments on parallel walls and spur dikes were performed at the U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) National Sedimentation Laboratory. The researchers involved visited the other laboratories involved, and the results are synthesized in this final report. The researchers involved and their positions, institutions, and roles in the study are listed in Table 1-1. 1.2 Problem Statement Most of the techniques and guidelines that are available for protecting bridge abutments against scour have been developed from small-scale, hydraulic modeling conducted in laboratories, and a limited amount of empirical data along with anecdotal observations has been acquired from field sites. Though quite useful advances have been made with scour protection (hereinafter termed scour “countermea- sures”), there is a widely recognized need for a further exten- sive study, one that links modeling efforts in the laboratory to both priorities of countermeasure needs and observed field performance of countermeasures. In addition, there is a perceived need to explore innovative concepts for scour countermeasures. There are two broad approaches for protecting bridge abutments from scour: 1. Mechanically stabilizing the abutment slopes with armor units, such as riprap, gabions, cable-tied blocks, or grout- filled bags, and 2. Aligning the upstream flow by using guidebanks,dikes, spurs, or in-channel devices such as vanes and bendway weirs. Neither of these broad approaches has been totally suc- cessful, as bridge abutments and their approach embank- ments are the most commonly damaged bridge components C H A P T E R 1 Introduction

during floods. It has been recognized, therefore, that along with new countermeasure concepts, better design and con- struction guidelines need to be developed to protect bridge abutments and approach embankments from scour damage and to reduce the depths to which expensive deep founda- tions may have to be placed. In addition, there are substantial needs for guidelines and selection criteria that address limitations imposed by envi- ronmental regulation, relative cost, availability, serviceability, constructability, and design constraints. Such guidelines will assist practitioners in preventing, reducing, or mitigating the damage incurred with abutment failure owing to scour. 1.3 Objective and Scope The objective of this project is to develop and validate selec- tion criteria and guidelines for the design and construction of countermeasures to protect bridge abutments and approach embankments from scour. The following countermeasure concepts were considered: • Armoring countermeasures such as riprap, cable-tied blocks, and geobags at abutments, including the type and extent of filters that could be used under the protective armoring countermeasure layer, the size of the armoring countermeasure, and the extent to which the armoring countermeasure should be placed up an abutment embankment slope, into the channel, and on the approach embankment; • Methods to increase or maintain the geotechnical stability of the earthfill embankment at an abutment; • Flow-altering devices such as spur dikes, including the size, type, placement, and feasibility of using these devices; and • Nontraditional concepts for abutment scour countermea- sures, such as collars and parallel walls, and temporary countermeasures, such as geocontainers and extra-large armor units, including the size, type, placement, and feasi- bility of using these concepts. 1.4 Relationship to Prior NCHRP Studies Several prior studies have investigated scour countermea- sures for bridge abutments or provide suggestions for counter- measures. Notably, the FHWA Hydraulic Engineering Circular No. 23 (HEC 23) addresses various countermeasures for the prevention of river-bend migration and has some relation to the current topic only in that some of the same countermeasures may be applicable.Their design,however, for use at bridge abut- ments is made clear in the present study. Only single-channel flow conditions are mentioned; therefore, HEC 23 does not address the complicated flow situation of overbank flow inter- acting with the bridge abutments, as the current study does. NCHRP Project 24-7 also discusses countermeasures, but in relation to bridge piers. There are similar concepts in the mitigation of scour at piers and abutments, but the issue of compound channel flow is less important for piers due to their farther distance from the bank. 5 Table 1-1. Personnel involved in project. Personnel Position Institution Role in Study Brian Barkdoll PI Michigan Tech University Project management; experiments on parallel wall, spur dikes, and collars Carlos Alonso Co-PI National Sedimentation Lab Parallel wall, spur dikes, and collars Robert Ettema Co-PI University of Iowa Geobags and large-scale riprap Roger Kuhnle Co-PI National Sedimentation Lab Parallel wall, spur dikes, and collars Bruce Melville Co-PI University of Auckland Riprap and cable-tied blocks Art Parola Co-PI Riverine Inc. 2D modeling of flow through bridge opening Recep Korkut Graduate Student University of Iowa Geobag and riprap apron at wing- wall abutments Hua Li Graduate Student Michigan Tech University Parallel wall, spur dikes, and collars Emelio Martinez Graduate Student University of Iowa Countermeasure concepts for wing- wall abutments Reinaldo Morales Graduate Student University of Iowa Large-scale experiment on apron performance at a spill-through abutment Sjoerd van Ballegooy Graduate Student University of Auckland Riprap and cable-tied blocks

NCHRP Project 24-7(2) further reported on pier selection criteria, guidelines for design and construction, inspection, maintenance, and performance evaluation. NCHRP Project 24-23 is concurrent with the present study and investigates scour depth prediction only and does not address countermeasures, which the present study does. Other studies besides NCHRP reports are in the literature and are described further in a later chapter, but none of these studies address abutment scour countermeasures for over- bank flow as the current study does. Overbank flow is espe- cially important for abutment scour, since the return flow from the floodplain is one of the more significant causes of scour, as is elaborated later in this report. 1.5 Abutment Forms Given the wide variety of abutment forms existing and the various conditions of scour, it is understandable that previous studies focused on simplified abutment shapes and geome- tries. The scour-inducing flow patterns around abutments on compound channels can be quite complicated and counter- intuitive. This simplification, however, has led to conclusions that are perhaps not realistic. The current study has chosen abutment shapes that replicate those found on actual water- ways, namely wing-wall (vertical face with angled walls into the bank) and spill-through (angled face) shapes. Conditions of scour for spill-through and wing-wall abutments include abutments threatened by main channel flow (in the cases of single and compound channels), failure of bank, failure of floodplain, and failure of embankment. 1.6 Countermeasure Concepts The primary concepts for abutment scour mitigation are bank hardening, embankment stabilizing, and flow alter- ing. Bank-hardening countermeasures seek to armor or strengthen the surface of a bank so as to withstand the hydrodymanic forces imposed by flow round an abutment. Primary examples of bank-hardening countermeasures are riprap, cable-tied blocks, geobags, partially grouted riprap, and interlocking devices (e.g., Toskanes). Embankment-stabilizing countermeasures entail increas- ing the slope stability of the earthfill embankment at an abutment by ensuring that an embankment does not fail geotechnically as scour hole forms at the toe of the embankment. Flow-altering countermeasures seek to reduce the flow- induced stresses on the bed and banks to a level that will not erode the bed or bank material. Common flow-altering countermeasures are spur dikes, stone walls, collars, sub- merged vanes, and guidebanks. All of the above-listed bank-hardening, embankment- stabilizing, and flow-altering countermeasures (except per- haps collars) can also be used for channel and bank sta- bilization. A stable channel is paramount in protecting a bridge because if the channel migrates around and bypasses the bridge crossing, then a new bridge must be built. 1.7 Research Approach The research was broken into seven distinct tasks. • Task 1 Literature Review. All available research was read and summarized to aid in the selection of the research approach and the selection of countermeasures for study. • Task 2 Screening Approach. After consideration of all countermeasures, a select few were chosen for further study. • Task 3 Research Alternatives. Various research options were considered in order to make an informed choice of research approach and the associated work plans. • Task 4 Interim Report. An interim report was prepared for consideration by the NCHRP review panel for the project, and consent was received to proceed. • Task 5 Execution of Work Plans. The approved work plans were carried out between the various parties involved in the form of laboratory work and two-dimensional modeling. • Task 6 Selection Criteria and Guidelines. Upon consider- ation of the laboratory results and two-dimensional modeling, design guidelines were prepared for each of the countermeasures studied. • Task 7 Final Report. This final report was written to docu- ment the findings and recommendations of the project. 1.8 Overview of Report Following from the introduction given above, Chapter 2 presents abutment forms and failure processes. Chapter 3 out- lines countermeasure concepts and criteria. Chapter 4 presents the results of a survey of state departments of transportation. Chapter 5 contains a comprehensive literature review. Chapters 6–9 contain laboratory results of preliminary ex- periments, wing-wall abutment experiments, spill-through abutment experiments, and flow modification experiments, respectively. Chapter 10 presents the design guidelines stem- ming from the project, and Chapter 11 presents the project’s conclusions and recommendations. 6

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TRB's National Cooperative Highway Research Program (NCHRP) Report 587: Countermeasures to Protect Bridge Abutments from Scour examines selection criteria and guidelines for the design and construction of countermeasures to protect bridge abutments and approach embankments from scour damage. The report explores two common forms of bridge abutments--wing-wall (vertical face with angled walls into the bank) and spill-through (angled face).

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