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1 EXECUTIVE SUMMARY INTRODUCTION The objective of this study is to review the present state of knowledge regarding bridge- abutment scour and evaluate the leading methods currently used for estimating design scour depth. It focuses on research information obtained since 1990, and that must be considered in updating the scour-depth estimation methods recommended by AASHTO1 , and used generally by engineering practitioners. This summary defines the problem of abutment scour, describes the studyâs research approach, and presents its findings along with recommendations and suggestions for future research projects. The study builds on the three principal investigators extensive knowledge regarding abutment scour, and capitalizes on the insights of an expert panel consisting of leading academicians and engineering consultants who also have significant experience with abutment scour. An extensive and thorough series of 2-3 day workshops over the life of the project were conducted by the principal investigators with the results presented to the panel of experts and to the NCHRP review panel. In this manner, conflicting points of view and commonly held misinformation about abutment scour have been debated and clarified in completing this study. Collective physical insights have been integrated into an expert system of organizing, collating, and evaluating current knowledge to create a solid base from which future research needs are effectively identified and outlined in order to advance the methods needed for engineers to design safer bridges. PROBLEM STATEMENT The complexity of bridge abutment scour necessitates a thorough evaluation of the physical processes involved and their parameterization in scour depth estimation formulas. As river flow approaches a bridge, the streamlines converge due to the physical contraction in width and then diverge once through it. In this process, the flow passes around bluff bodies, generating, transporting, and eventually dissipating large-scale turbulence structures (large eddies shed in a recognizable pattern due to flow separation albeit intermittently with time). The flow is bounded by erodible boundaries of complex and changing form that have widely varying compositions and characteristics. Even the classification of abutment scour as an independent bridge scour component is problematic, because contraction scour and abutment scour are linked processes usually occurring together during flood events. Given the complexity of the various scour processes, and the difficulty of including all of those processes in a single empirical formula, it is not surprising that current abutment scour formulas commonly provide scour depth estimates that vary over a wide range of magnitudes. Furthermore, comparisons of abutment scour depth estimations from existing formulas with field data and with engineering experience produce mixed results, partly because of the misperception that abutment scour formulas based on simplified laboratory experiments apply to all types of abutment scour, even to the most complicated field situations, and partly due to the difficulty of estimating the flow and sediment parameters required in existing scour formulas. Even with the foregoing complexities, some progress has been made in understanding abutment and contraction scour in the past twenty years or so, but future advances require identifying the most useful concepts and then winnowing and unifying some of these concepts into an 1 AASHTO ~ Association of American State Highway and Transportation Officials
2 overarching design philosophy buttressed by fulfilling carefully focused and defined research needs. OBJECTIVES The studyâs specific objectives are: 1. Critically evaluate research completed since 1990 in abutment and contraction scour processes; 2. Compare current scour-prediction practice with the present understanding of scour processes through a clear delineation of the major variables governing abutment and contraction scour; 3. Provide recommendations for adoption of specific research results by AASHTO and use by the engineering community if possible; and, 4. Where knowledge gaps exist, develop a logical and comprehensive set of research needs and problem statements to fill them in the near future. RESEARCH APPROACH The research approach can best be described as one of expert systems analysis and evaluation. The three co-principal investigators, who have all done extensive research on abutment scour, pooled their knowledge and experience and augmented it with the insights of an expert panel consisting of leading academicians and engineering consultants who also have significant experience with abutment scour. The co-principal investigators conducted an extensive and thorough series of 2-3 day workshops over the life of the project and periodically presented the results to the panel of experts and to the NCHRP review panel in interactive oral presentations as ideas were refined and incorporated into the research product in a feedback loop. In this manner, conflicting points of view and commonly held misinformation about abutment scour were debated and clarified. Collected insights were integrated into an expert system of organizing, collating, and evaluating current knowledge to create a solid base from which future research needs could be effectively identified and outlined in order to advance the methodologies needed for engineers to design safer bridges. As part of the overall research approach, the following criteria were established to evaluate existing scour-depth prediction formulas in order to identify those that may provide promise and direction or even a framework for future research: 1. Adequacy of formulas in addressing parameters that reflect the important physical processes governing abutment scour; 2. Limitations of formulas in design applications with respect to ranges of controlling parameters on which they are based; 3. Categorization and acceptability of laboratory experiments and research methods that led to the formulas; 4. Attempts to verify and compare formulas with other lab data and field data, if any, with which a valid comparison can be made; 5. Applicability and ease of formula use for design (AASHTO manual)
3 RESEARCH FINDINGS At the outset, it was necessary to re-examine the definition of abutment scour because of its close association and interaction with contraction scour. Abutment scour is defined herein as scour at the bridge-opening end of an abutment, and directly attributable to the flow field developed by flow passing around an abutment. It includes the effects of flow acceleration due to channel flow constriction as well as local, large-scale turbulence effects due to flow separation which are present in varying relative proportions depending on the upstream approach flow distribution and flow distribution at the bridge section, abutment column type, foundation type and location, flow curvature, and near-field river morphology. Several of the most important research insights are summarized below. A New View of Abutment Scour Based on the foregoing definition of scour and documentation in this report of numerous failures of bridges due to abutment scour, one of the important initial findings is that many abutment failures occur due to scour and sliding of the earthfill embankment on the main stream side of the abutment into the scour hole, or outflanking due to erosion of the earthfill embankment on the floodplain side due to overtopping or inadequate drainage protection. Even more difficult to evaluate is the vulnerability to scour caused by lateral shifting of the channel thalweg such that it directs flow adversely towards abutments and embankments. Whereas much of the laboratory research of recent years has focused on solid abutments that extend into the soil foundation, such as with sheet piles or other fairly rigid foundations, more attention should be focused in the future on erodible embankments. Recognition of the difference between erodible and solid abutments provides a factor for classifying existing scour prediction formulas and introduces the importance of geotechnical failure caused by hydraulic scour. In addition, it suggests the need for estimating the strength of the embankment over the range of construction forms varying from unprotected, compacted soils of various types through rock riprap revetment to the solid abutment, and incorporating this estimate into a more comprehensive scour prediction formula. These considerations pose a fundamental design problem in that partial failure of the embankment that occurs as sliding of earthfill and/or riprap into the scour hole may ultimately reduce the total scour depth while complete failure of the embankment may be intolerable if it results in failure of the bridge approach slab or the first bridge span. Classification of Scour Formulas To apply the foregoing criteria for evaluation of the adequacy and limitations of existing scour prediction formulas, several classification schemes were developed as explained next. Classification by Abutment Scour Conditions For erodible embankments, three common conditions of abutment scour are identified 1. Scour Condition A. Scour of the main-channel bed, when the channel bed is far more erodible than the floodplain may cause the main-channel bank to become geotechnically unstable and collapse. The collapsing bank undercuts the abutment and embankment, which in turn collapses locally and results in soil and/or riprap sliding into the scour hole; 2. Scour Condition B. Scour of the floodplain around the abutment. This condition also is equivalent to scour at an abutment placed in a rectangular channel, if the abutment is set
4 far back from the main channel. Given the floodplain resistance to scour, this scour condition usually occurs as clear-water scour and can result in soil and riprap sliding into the scour hole as in Scour Condition A; 3. Scour Condition C. Scour Conditions A and B may eventually cause the approach embankment to breach near the abutment, thereby fully exposing the abutment column. For this condition, scour at the exposed stub column essentially progresses as if the abutment column is a pier, and it usually occurs as clear-water scour. Scour conditions A and B can also occur for solid abutments that are located near the main channel bank, or on the floodplain some distance from the bank. In this case, abutment failure would result if the scour hole were deep enough to undermine the solid foundation. Scour Condition C would tend to occur for a solid abutment in the case of outflanking of the abutment and erosion of the approach embankment. Classification by Types of Bridge Crossings The three scour conditions identified in the previous section may occur within the context of specific classes of bridge crossings: 1. Class I refers to narrower bridge crossings of incised channels, where the channel is reasonably well represented by a rectangular channel. This class also includes narrow crossings for conditions up to bank-full flows. 2. Class II refers to wider bridge crossings, where the channel is typically compound, comprising a main channel and wide flood channels. At such sites, significant flows may be diverted from the flood channels towards the main channel at the bridge section. 3. Class III refers to bridges spanning wide braided river channels, where the river channel can be approximated by a rectangular channel under extreme flood flow conditions. At such sites, the bridge foundations may be significantly skewed to the flow at lesser flood flow conditions. Class I and Class III are the simplest situations to model in the laboratory. Many of the existing laboratory data apply to these two classes, which have been modeled typically in rectangular flumes using rigid abutment models extending below the maximum measured scour depth. Equations derived from such data give the âmaximum possibleâ scour depth that can occur and should then be conservative for design. Such equations are not suitable for prediction of scour depths that develop where undermining of the pile cap or slab footing occur, because slope failure may then limit further scour. Scour Conditions A and C are the most likely for Class I and II bridge crossings. All three scour conditions (A, B, and C) are possible for Type II crossings of compound channels which consist of both a main channel and floodplain. Class II crossings are the most difficult in terms of scour prediction because of the interaction between the main channel and floodplain flows and the resultant redistribution of the flow in the contracted bridge section depending on how much of the floodplain flow is blocked by the embankment. A fourth scour condition might be added to the Type II crossing: Scour Condition AB for an abutment with a small setback distance in which both the floodplain and the bank of the main channel are erodible and the scour hole on the floodplain extends into the main channel.
5 Classification by Parameter Groups From the dimensional analysis of the abutment scour problem, it can be shown that specific groups of dimensionless parameters exist that define different aspects of the physics of the problem. These parameter groups are given as G1. Flow/sediment variable ratios such as the flow intensity defined as the ratio of the approach flow velocity to the critical velocity for initiation of sediment motion, V/Vc; G2. Relative abutment and sediment size scales given by the ratio of embankment length to sediment size, L/d; G3. Abutment and flow geometry variables such as the ratio of embankment length to flow depth, L/Y, and abutment shape and skewness factors, Ks and Kθ ; G4. Flow distribution ratios such as the ratio of the discharge per unit width in the approach flow section to that in the contracted bridge section, q2/q1; G5. An abutment stability parameter that quantifies the shear strength of the embankment relative to the intergranular grain stress due to the height of the embankment Parameter Group G1 is essential in establishing the potential for scour through the ratio of some flow variable, such as velocity or shear stress, to a variable indicating critical conditions for sediment movement. This parameter can take a variety of forms including a densimetric grain Froude number as well as the common V/Vc. Establishing the effect of the scale of the horseshoe vortex relative to sediment size is the intent of Parameter Group G2, but not enough is known at this stage to firmly establish what this parameter or parameters should be. The influence of flow contraction on abutment scour is incorporated into existing scour formulas by either Parameter Group G3 or G4 with the ratio of length scales utilized in the former and discharge ratios in the latter. Parameter Class G5 is somewhat unique in that it has not been utilized in existing abutment scour formulas although introduction of erodible embankments into the design problem suggests the need for a parameter of this type. Comparison, Evaluation and Selection of Scour Formulas From an extensive review and analysis of contraction scour, and consideration of the common parameter classes affecting abutment and contraction scour, it was concluded that the most promising treatment of the combined occurrence of bridge and abutment scour is to establish the total effect as an amplification factor times a reference contraction scour depth. The reference depth would be obtained from well-established contraction scour formulas that depend on the assumption of equilibrium sediment transport in the live-bed case and the occurrence of critical conditions in the equilibrium scour hole in the clear-water case. The amplification factor would be developed as a function of degree of flow contraction caused by the constricted bridge opening as well as the local turbulence generated by flow obstruction and separation as described previously. An extensive comparison of the performance of leading scour formulas against each other and against sound experimental data bases established a short list of scour prediction formulas that displayed similar trends in terms of the reference contraction scour depth formulation. This smaller list of formulas was further subjected to the classification schemes and the selection criteria developed for this purpose. Finally, a common parameter framework was established that
6 encompassed both solid abutments and erodible abutments. While no formula was found to satisfy all criteria, the framework developed as a result of this research approach suggests a path toward refining and unifying a small number of leading scour formulas. RECOMMENDATIONS 1. Contraction scour should be viewed as a reference scour depth calculation while abutment scour should be taken as some multiple of contraction scour rather than additive to it. 2. A small subset of abutment scour formulas, each member of which has certain desirable attributes, should be unified into a single formula in order to develop more realistic and robust procedures for abutment scour prediction. The following formulas are judged to be most promising in this regard, and with respect to the established criteria:: a. Ettema et al. (2010). It is the only formula that considers an erodible embankment; it has the desirable attributes of reflecting the physics of the abutment scour process both in terms of flow constriction and turbulent structures. b. Sturm (2004, 2006) It includes a method of accounting for flow re-distribution due to compound channel geometry, and it represents the upper limit of scour for a solid-wall foundation as opposed to an erodible embankment. c. Melville (1997) It is most applicable to short, solid-wall abutments and depends on abutment length rather than the flow distribution in the contracted section, but it can be viewed as comparable to the first two formulas. d. ABSCOUR (Chang and Davis 1998, 1999; MSHA 2010) It contains the desirable attribute of including the direct effect of flow re-distribution on the floodplain through the Laursen contraction scour formula and has a computer implementation. Although the Briaud (2009) formula does not satisfy the criterion for best parameter framework, it is one of the only databases for cohesive sediments, and the data could be useful in expanding the range of applicability of the final unified formula. 3. A flow chart should be developed to be used as a guide to evaluate abutment scour in an informed manner and to assist the judgment of design engineers including both a unified scour formula and geotechnical evaluation of scour. For more complex problems, hybrid numerical and physical models should become a readily accessible option. 4. In the near term, abutments should have a minimum setback distance from the bank of the main channel with riprap protection of the embankment including a riprap apron, and other effective scour countermeasures such as guidebanks should be considered (see Lagasse et al. 2009, HEC-23) 5. Further development of an educational curriculum for hydraulic engineers should be undertaken in order to emphasize the proper choice of parameters that go into any scour calculation and in the use of 2D and 3D numerical models to better evaluate the hydraulic parameters. At least in the short term, 2D numerical models should be used on all but the simplest bridge crossings as a matter of course. 6. A long-term field program of obtaining high-quality, real-time field data should be undertaken. While embarking upon such a program will be expensive and require
7 patience, the results will move the ultimate solution to the abutment scour problem forward more effectively than less-expensive post-flood surveys. CONCLUSIONS This study leads to the following main conclusions regarding its objectives: 1. Abutment-scour literature published since 1990 documents substantial advances in understanding abutment-scour processes: a. New insights exist regarding scour development at abutments with erodible, compacted earthfill embankments. Differences occur between scour at erodible abutments and scour at solid abutments on solid-wall foundations similar in nature to sheet piles or caisson structures; b. The flow field around an abutment has essentially the same characteristics as flow fields through short contractions. Notably, flow distribution is not uniform and generates large-scale turbulence. Deepest scour occurs approximately where flow contraction is greatest. As scour develops at abutments with solid-wall foundations, the large-scale turbulence may increase in strength and cause scour to deepen; c. At least three abutment scour conditions may develop at abutments with erodible embankments, depending on abutment location in a compound channel. Two conditions may result in embankment failure, while the third condition is pier-like scour at an exposed abutment column once an embankment has been breached; d. The roles of variables (e.g., embankment length) and dimensionless parameters (e.g., embankment length relative to flood-plain width and relative flow distribution in compound channels) defining scour processes have become better understood; e. The leading methods for estimating scour depth better reflect parameter influences; f. Improved insights exist regarding abutment scour in clay; g. Insight has been gained regarding the influence of some site complications (e.g., pier proximity); and, h. Numerical modeling is substantially growing in utility to reveal two- and three- dimensional features of flow distribution at abutments in ways that laboratory work heretofore has been unable to provide. 2. The following aspects of abutment scour processes remain inadequately understood: a. The role of embankment soil strength and flood-plain soil strength on scour development and equilibrium scour depth; b. Scour of boundary materials whose erosion characteristics are not adequately understood (some soils, rock); however, existing reliable data indicate that scour depths in cohesive soils and weak rock do not exceed those in cohesionless material; c. Quantification of factors further complicating the abutment flow field (such as debris or ice accumulation, submergence of bridge superstructure, channel morphology) and erodibility of flood-plain soils; and, d. Temporal development of abutment-scour depth, especially the relative timings for which scour develops at several locations around an abutment.
8 3. The evaluation (Chapter 5) outlines the well-understood relationships between scour depth and significant parameters, summarized in Table 5-1. Notable examples of recent information include similitude in hydraulic modelling of flow distribution through a contracted bridge waterway, and the importance of flood-plain and embankment soil strengths. Groups of primary parameters are identified in Table 5-2. They define the magnitude and approximate distribution of the abutment flow field, and therefore the potential maximum scour depth. 4. An important conclusion drawn from the evaluation (Chapter 6) is the need to define a set of methods for estimating abutment-scour depth associated with different abutment types, notably for abutments with erodible embankments and those with solid-wall foundations: a. For abutments with erodible embankments, the estimation methods proposed by Ettema et al. (2010) and ABSCOUR (MSHA 2010) should be further developed with a view to producing a set of methods for scour-depth estimation; b. For abutments with erodible embankments, further research is needed to develop and verify the geotechnical approach to scour depth estimation; and, c. For abutments with solid-wall foundations, the estimation methods proposed by Sturm (2006) and Melville (1997, also Melville and Coleman 2000) should be further developed with a view to producing a comprehensive method for scour- depth estimation. 5. The evaluation in this report draws attention to the importance of effective monitoring and maintenance of bridge abutments. Bridge waterway site complexity (flow field, foundation material, embankment material) can introduce significant uncertainty for scour-depth estimation. Moreover, risks attendant to channel changes and possible deterioration of the abutment structure introduce additional uncertainties as to abutment condition. Effective monitoring (inspection schedule and instrumentation) is needed to manage and mitigate the uncertainties. 6. It is important that the abutment designer recognize the limits of existing methods for scour-depth estimation and the capabilities of new field and numerical modelling tools through updated continuing education courses. 7. Detailed research needs related to Conclusions 4, 5, and 6 can be found in Tables 8-1, 8- 2, and 8-3 with expansion into research problem statements in Appendix C. The main research needs shown there are identified as Critical priority as decided by the NCHRP Project Panel. Work should commence on this road map for the future as soon as possible.
