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.

• For aprons installed flush with the original streambed, guidance is as follows:
  
  
  • Because the apron is installed within the contraction scour zone, partial-width flush aprons will likely experience some edge failure even when W₂/y₀ is large. Therefore, partial-width flush aprons are not recommended.
    
    
  • Full-width flush installations were not tested. In addition to unknown hydraulic performance, their use may also generate environmental criticism that exposed riprap is not conducive to supporting aquatic habitat. Therefore, they are not recommended.

• For aprons buried to the estimated elevation of contraction scour, guidance is as follows:
  
  
  • Partial-width buried riprap aprons can be effective for W₂/y₀ greater than 6.2. Concern about edge failure is significantly reduced because the apron is buried to a depth below the contraction zone. This is the basis for option 2 described in appendix D.
    
    
  • Full-width buried riprap aprons are preferred for W₂/y₀ less than or equal to 6.2 and can be considered for all openings. This is the basis for option 3 described in appendix D.
    
    
  • Riprap size, apron extent upstream and downstream, apron thickness, and other parameters may potentially be less restrictive than specified in HEC-23 DG14, but insufficient data were available in this study to provide recommendations.⁽³⁾
    
    
• All aprons should conform to the channel bed and banks as closely as is reasonable while ensuring apron stability.
  
  
• Designers should consider the contraction ratio ₂ / ₁ so that significant contractions that may contribute to severe hydraulic or scour conditions might be avoided. Insufficient data were available in this study to provide quantitative recommendations.

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:

• Further developing the shear stress ratio conceptual model to a design tool that ultimately allows site-specific evaluation of stress ratios.
  
  
• Considering the aquatic organism passage and stream stability consequences of bridge abutment encroachment on the channel creating narrow (contracted) openings.
  
  
• Expanding the guidance to consider live-bed contraction scour conditions.
  
  
• Further testing pressure flow conditions.
  
  
• Considering differential guidance for rigid versus non-rigid shallow foundations if the two types of foundations might experience different failure mechanisms.