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70 CHAPTER 8. RESEARCH AND EDUCATION NEEDS 8.1 INTRODUCTION The preceding chapters show that several groups of factors cause abutment scour to persist as a major cause of bridge waterway failure. One group concerns the need for improved understanding of the processes causing abutment scour and failure. Another group concerns the development of more reliable design methods, including the integration of abutment-scour counter-measures into abutment design. Yet a further group concerns inadequate monitoring and maintenance of bridge waterways so as to avert abutment failure for reasons associated with maintenance of approach-channel conditions. This chapter outlines the primary research and education needs associated with the groups and, thereby, with overall improving the reliability of design estimates of abutment-scour depth. Brief problem statements associated with each research need are given in Appendix C. Education is included here because some aspects of the substantial advances in abutment-scour knowledge that have occurred since 1990 have yet to be integrated adequately into abutment design and monitoring practice, particularly in regions with limited access to design expertise. Bridge abutments continue to fail for the same reasons they did prior to 1990. 8.2. SCOUR PROCESSES The primary research needs regarding scour processes concern better definition of abutment scour for the categories of abutment scour outlined in Chapter 6, how scour varies with the parameters determining the potential maximum scour depth, and what considerations limit scour depth at abutments. These needs entail understanding the abutment flow field and its variation with the parameters determining the potential maximum scour depth. They also concern better knowledge about how foundation material erodes. Table 8-1 lists research topics and priorities regarding scour processes. Research needs of highest priority are designated as Critical. The main research needs outlined in Table 8-1 can be summarized as follows: i. Laboratory experimental studies aimed at elucidating scour processes (flow field, erosion, geotechnical instability), filling existing data gaps, and for diagnostic comparison with selected field investigations; ii. Forensic field investigations of abutment failures during significant flood events at bridges featuring particular abutment types; and, iii. The use of numerical models to illuminate flow at abutments, and possibly scour development and embankment failure at abutments.
71 Table 8-1. Prioritized list of research and education needs addressing improved understanding of abutment-scour processes. Aspect Research Need Priority Laboratory studies L1. Additional laboratory hydraulic experiments on realistic abutment foundation structures and abutment shapes with and without countermeasures, methods of modeling embankment material; geotechnical stability aspects; modeling of intermediate length and short erodible embankments and wide abutments Critical L2. Overtopping of erodible embankments and abutment scour under pressure scour conditions High Field studies FS1. Field studies with continuous hydraulic and scour monitoring that assess uncertainties in measurement and that can be compared with laboratory hydraulic models Critical FS2. An overall survey to determine the statistical distribution of embankment failure (including types of failures) relative to other modes of bridge waterway failure. Critical Numerical studies N1. Investigation of sound use of 2D (depth-averaged models) for determining flow distribution through bridge waterways for the short term combined with 3D CFD models and laboratory turbulence measurements to shed further light on hydraulic model scaling issue for the long term Critical N2. Education of engineers concerning limitations of 1D abutment scour prediction formulas and the potential and applicability of 2D and 3D numerical modeling in combination with laboratory hydraulic modeling High 8.3 DESIGN ESTIMATION OF SCOUR Before outlining research tasks to improve design methodology as an integral part of the research needs outlined in the previous section, it is useful to refer back to the essential question raised in Section 3.3 â How should abutment design best take abutment scour into account? This basic question leads to a set of specific questions outlined in Section 3.3, and provides important context for research aimed at improved design estimation of scour depth. Most research to date has focused on identifying scour depths at solid-body abutments, without indication of how such scour depths relate to abutment design; though the designer would ensure that the abutment column foundation extends adequately below the scour depth. There is a need to include assessment and improvement of design practice, and to determine guidelines, on how best to address scour at abutments as a part of the research needs in Table 8-1. The present review (Chapter 3) describes several basic abutment designs (erodible embankment at abutment column, caisson-type (solid wall foundation) abutments). Each basic design likely requires its own tailored relationship for estimating abutment scour depth, but these relationships may possibly be placed in the same form with different values of specific coefficients. Chapter 7 summarizes a set of scour estimation methods for the basic types of abutments, and indeed indicates that currently no single estimation method in its present form suffices for all abutments. Table 8-2 lists the design-related research tasks needed to improve and validate the design methodologies recommended in Chapter 7. Such improvement and validation should occur concurrently and in concert with satisfaction of the research needs described in Table 8-1. The
72 listed priorities in Table 8-2 coincide with those of the overall research needs given in Table 8-1. A theme evident in the research needs described here is that of merging existing methods. The leading methods summarized in Chapter 7 contain important insights and reflect the effects of primary parameters, and should be brought into closer relationship with each other and refined. The present evaluation does not advocate the development of yet more methods. The main design-related tasks that should be coupled to research needs in the previous section are: i. For abutments with erodible embankments with abutment columns, merge and validate and/or refine the scour estimations methods proposed by Ettema et al. (2010) and ABSCOUR (MSH 2010) as guided by the scour process research in Table 8-1. These methods both treat abutment scour as an amplification of contraction scour, but the adjustment coefficients reflecting effects of turbulence need to be unified, and less physically based adjustment coefficients that lack specific experimental validation need to be re-evaluated. In addition, geotechnical scaling of laboratory results for erodible embankments to the field require estimation of laboratory embankment strength and erodibility ii. For abutments with solid-wall (or caisson-like) foundations, validate and/or refine the sour estimation methods proposed by Sturm (2006) and Melville (1992) using field and laboratory data, and explore unifying these formulas with those of Ettema et al. (2010) to provide a range of scour depth estimates that depend on the strength and type of foundation of the embankment; iii. Include scour counter-measures in bridge abutment design. Abutments fitted with an armored apron may be intermediate in form between the abutment types mentioned in items i and ii, above; iv. Use two-dimensional flow (depth-averaged) numerical models for estimating flow distribution and scour depth at abutments.
73 Table 8-2. List of design-related research tasks addressing improved design estimation of abutment scour depth coupled to research needs in Table 8-1. Aspect Research Need Design-related Research Task Priority Erodible embankment abutments L1, FS1 1. Determine if and how the ABSCOUR method (MSHA 2010) and that proposed by Ettema et al. (2010) can be merged and further developed. From diagnostic field studies determine method veracity. Critical L1, FS1 2. Further develop and check the validity of the geotechnical approach to estimating scour depth. From diagnostic field studies determine method veracity. Critical L1, FS1 3. Refine the methods in Task 1 for the limiting case of a short abutment as the channel becomes very wide. From diagnostic field studies determine method veracity. Critical L2, FS1 4. Ascertain how the methods in Task 1 apply, or should be adjusted, for embankments under pressure scour conditions and possibly over-topping. From diagnostic field studies determine method veracity. High Solid body abutments L1, FS1 5. Determine the extent to which the methods proposed by Sturm (2006) and Melville (1992, 1996) can be merged and further developed for solid-wall abutments and then combined with Task 1. in a comprehensive design procedure. From diagnostic field studies determine method veracity. Critical L2, FS1 6. Ascertain how the methods in Task 5 apply, or should be adjusted, for embankments under pressure scour conditions and possibly over-topping. From diagnostic field studies determine method veracity. High Abutments fitted with scour counter-measures L1, FS1 7. Determine how the methods in Tasks 1 and 5 should be adjusted, for embankments fitted with scour counter-measures, notably an armored apron around the abutment toe or sheet-pile skirt. From diagnostic field studies determine method veracity. Critical 2-D flow numerical methods N1 8. Utilize a 2-D flow model to determine peak values of flow velocity, unit discharge or shear stress in the vicinity of an abutment, especially if the abutment is located in a channel of irregular geometry, in order to estimate amplification of contraction scour at an abutment. Critical 8.4 MONITORING AND MAINTENANCE OF BRIDGE ABUTMENTS Numerous scour-induced failures of bridge abutments often result as a consequence of inadequate monitoring and maintenance of approach channel features (especially the lateral shifting of a channel), and concomitantly at times the deterioration of the abutment embankment (as can be caused by inadequate handling of drains along embankment flanks). Therefore, an important group of research needs relates to improving ways to monitor and maintain bridge waterway conditions in order to avert abutment failure. Developments in monitoring techniques and maintenance methods can help to reduce abutment failure. The ensuing research (and education) needs aim to improve implementation of monitoring and maintenance practice.
74 i. Development of instrumentation and techniques to better facilitate routine observation and recording bridge-waterway conditions, and identify waterway and embankment deteriorations that may increase abutment susceptibility to failure scour; ii. Development of instrumentation and techniques for determining abutment state during extreme flood-flow events; iii. Education of appropriate technical staff about abutment scour processes including those linked to changes in channel alignment and abutment condition (exposure to flow, geotechnical weakening). Also, education regarding monitoring instrumentation and effective options for abutment maintenance; and, iv. Determination of additional effective maintenance methods for mitigating abutment failure owing to scour. Table 8-3. Prioritized list of research and education needs addressing improved methods for monitoring and maintenance (needs I1, I2, and I3 can be combined). Aspect Research Need Priority Instrumentation for routine monitoring I1. (a) New instrumentation and techniques for remote-sensing of abutment and bridge waterway state, and for accessible data and image storage; (b) instrumentation for monitoring embankment soil conditions (leverage off COE levee studies) and (c) low- cost instrumentation and techniques for small bridges or bridges in regions with limited resources to monitor bridges. High Instrumentation for monitoring during flood flows I2. (a) Instrumentation for obtaining waterway bathymetry data during flood flows; (b) instrumentation for monitoring embankment soil parameters during flood hydrograph passage. High Education I3. Training of appropriate staff to conduct monitoring activities, and complete effective abutment maintenance. High Maintenance M1. Innovative and efficient methods for repairing, stabilizing, or replacing weakened components of abutments (e.g., strengthening weakened spill-slope soil at abutment column) Medium
