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75 CHAPTER 9. CONCLUSIONS 9.1 INTRODUCTION This report comprises an extensive, broad review of scour at bridge abutments, conducted with the intent of providing recommendations for updating AASHTO manuals Policy for Design of Highway Drainage Facilities and Recommended Procedures for Design of Highway Drainage Facilities, so that these manuals present the best available guidelines for abutment scour estimation and countermeasure design, and indicate clear direction as to further research. The present chapter presents the main conclusions of the review. A key observation providing context for the conclusions is that, though substantial progress has been made in understanding abutment scour, the state of knowledge regarding abutment-scour estimation lags that for pier scour. Indeed, the review cannot arrive at a definite recommendation regarding the adoption of one method for design estimation of depth of abutment scour. For several reasons, as discussed in this report, the existing methods for abutment-scour estimation are inadequately formulated or require further verification: 1. Scour at abutments is strongly influenced by abutment construction. Most abutments consist of an abutment column set amidst an erodible earthfill embankment. However, most abutment formulations are based on laboratory data obtained using abutment models with a solid-wall foundation (similar to half a wide pier); 2. Scant few laboratory studies have replicated abutments with erodible earthfill embankments; 3. Abutment scour at abutments with erodible embankments can result in at least three failure modes, and scour at solid-wall abutments produces yet a different scour form; 4. For abutments with earthfill embankments, which form the majority of abutments in the U.S., two failures may occur (the earthfill embankment and the abutment column); and, 5. Additional processes cause abutments to erode. A particularly common process causing abutment erosion is the lateral migration of the approach channel. The mixing of processes associated with channel migration, the flow field through a bridge waterway, and the flow field at an abutment at times complicates interpretation of field data on abutment scour. The reportâs main contribution is its focus on the various scour processes occurring at abutments as actually constructed, and its clear indication as to directions for future research needed to improve the reliability of methods for design estimation of abutment scour. Additionally, an important observation expressed in the report is the necessity for abutment design practice to address the essential question â How should abutment design account for abutment scour? This essential question quickly leads to a set of specific questions (Section 3.3) concerning the acceptability (or otherwise) of embankment failure. Acceptable reliability of abutment design in relation to scour behooves bridge designers to address this question.
76 9.2 CONCLUSIONS The review and evaluation leads to the following main conclusions regarding the six objectives stated in Section 1.3: 1. Abutment-scour literature published since 1990 documents substantial advances in understanding abutment-scour processes. In particular, with regard to Objective 1, knowledge has advanced regarding the following aspects of abutment scour: i. 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 caisson structures; ii. 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; iii. 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 failed and been breached. These scour-induced failures differ substantially from those in previous studies of abutments modeled as solid bodies with a solid-wall foundation for the case of sheet-piles or other solid, high strength foundations resistant to erosion; iv. 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; v. The leading methods for estimating scour depth better reflect parameter influences; vi. Improved insights exist regarding abutment scour in clay; vii. Insight has been gained regarding the influence of some site complications (e.g., pier proximity); and, viii. Numerical modeling is substantially growing in utility to reveal two- and three- dimensional features of flow distribution of flow at abutments in ways that laboratory work heretofore has been unable to provide. These advances address a significant portion of the general statement in NCHRP Project 24-8, Scour at Bridge Foundations: Research Needs (Parola et al. 1996) regarding abutment scour research needs. They also address aspects of abutment scour not envisioned for NCHRP 24-8, especially the roles of embankment erosion during abutment scour. However, further significant research has yet to be done in these areas. 2. The following aspects of abutment scour processes remain inadequately understood: i. The role of embankment soil strength, and flood-plain soil strength on scour development and equilibrium scour depth;
77 ii. 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; iii. 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, iv. Temporal development of abutment-scour depth, especially the relative timings for which scour develops at several locations around an abutment. 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 modeling 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 5-1. 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: i. 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; ii. For abutments with erodible embankments, further research is needed to develop and verify the geotechnical approach to scour depth estimation; and, iii. 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 review 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. Finally, 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 modeling tools through updated continuing education courses.
