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Publication Number:  FHWA-HRT-17-013     Date:  February 2017
Publication Number: FHWA-HRT-17-013
Date: February 2017

 

Hydraulic Performance of Shallow Foundations for The Support of Vertical-Wall Bridge Abutments

CHAPTER 7. SUMMARY AND FUTURE RESEARCH

This report summarizes the results of physical and numerical modeling tests to evaluate the effects of riprap aprons on contraction scour for vertical-wall abutments with shallow foundations. The evaluations included aprons installed at the original bed surface and buried below the anticipated contraction scour depth. In both types of installations, full-width and partial-width installations were considered. Partial-width flush installations of riprap aprons introduced turbulence at the apron/bed interface and also prevented increased conveyance facilitated by contraction scour. These two effects resulted in scour observations in excess of predicted contraction scour. Potential mitigation strategies, including full channel width aprons installed flush with the streambed and buried aprons, were evaluated, and design guidance was proposed.

Recommended design guidance for riprap aprons recognizes that for these shallow foundations, the riprap apron is an integral component of the structure that must continue to perform throughout the life of the structure. As an integral component of the structure, the apron should not require maintenance as long as the design conditions are not exceeded. Detailed guidance is provided in appendix D. The following bullets summarize guidance highlights.

If riprap aprons or other scour countermeasures are not used to protect the abutments supported by shallow foundations, the foundations should be placed below the abutment (local) scour elevation as recommended in current guidance. Although this condition was not a focus of this study, the results from physical experiments 7 and 10 (with no riprap) confirmed that abutment scour might threaten shallow foundations. This condition is described as option 1 in appendix D.

This study evaluated free-surface conditions only. However, it is anticipated that pressure-flow conditions would exacerbate contraction scour compared with free-surface conditions. Because there are no experimental or field data to support the effectiveness of partial-width installations (flush or buried), it is recommended that only buried full-width installations be considered for pressure flow conditions at shallow foundations. This situation is described in appendix D.

This design guidance for free-surface conditions assumes that all approach flow passes under the bridge. If flow overtops or bypasses, the approach flow characteristics should be adjusted accordingly. The design guidance is also limited to clear-water scour conditions. The following further research is recommended to improve this guidance:

 

 

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