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REPORT |
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Publication Number: FHWA-HRT-17-013 Date: February 2017 |
Publication Number: FHWA-HRT-17-013 Date: February 2017 |
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Vertical-wall bridge abutments with shallow foundations may present unique contraction scour issues in cases where the bridge opening is narrow and riprap aprons are used to protect the abutment from abutment scour. In addition, new approaches to abutment designs that feature non-rigid construction techniques require investigation to ensure they are constructed to protect against undermining. This report describes a study of methods for evaluating bridge abutments to assess their vulnerability and provides design guidance for their application. The study was conducted at the Federal Highway Administration’s Turner-Fairbank Highway Research Center J. Sterling Jones Hydraulics Laboratory. The report will be useful for engineers and other technical personnel involved with designing, constructing, and inspecting vertical-wall bridge abutments.
Cheryl Allen Richter, P.E., Ph.D.
Director, Office of Infrastructure
Research and Development
Notice
This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.
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Technical Report Documentation Page
1. Report No.
FHWA-HRT-17-013 |
2. Government Accession No. | 3 Recipient's Catalog No. | ||
4. Title and Subtitle
Hydraulic Performance of Shallow Foundations for the Support of Vertical-Wall Bridge Abutments |
5. Report Date February 2017 |
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6. Performing Organization Code | ||||
7. Author(s)
Oscar Suaznabar, Chao Huang, Zhaoding Xie, Jerry Shen, Kornel Kerenyi, Bart Bergendahl, and Roger Kilgore |
8. Performing Organization Report No.
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9. Performing Organization Name and Address Genex Systems, LLC |
10. Work Unit No. (TRAIS) |
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11. Contract or Grant No. DTFH61-11-D00010-T-5009 |
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12. Sponsoring Agency Name and Address
Federal Highway Administration |
13. Type of Report and Period Covered
Laboratory Report; October 2013–July 2016 |
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14. Sponsoring Agency Code
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15. Supplementary Notes The Contracting Officer’s Technical Representative was Kornel Kerenyi (HRDI-50). Nathan Tsou assisted with the literature search. |
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16. Abstract
This study combined abutment flume experiments with numerical modeling using computational fluid dynamics (CFD) to investigate flow fields and scour at vertical-wall abutments with shallow foundations. The focus was situations dominated by flow contraction in the bridge opening, turbulence around the bridge abutments, and variations in bed roughness and cross-section geometry resulting from riprap apron installation at the streambed elevation.
When riprap aprons are installed flush with the bed, the gap between the two abutment aprons is exposed to increased bed shear stress on the unprotected erodible channel bed in the gap, leading to greater contraction scour depths than would have occurred without the aprons. All riprap aprons installed flush with the streambed for abutment protection are vulnerable to movement of rocks at the edge of the apron (edge failure) because that edge lies within the contraction scour zone. Contraction scour may increase movement of rocks at the edge of the riprap apron. Contraction scour in the gap at the edge increases as the opening becomes narrower and the riprap coverage on the channel bed increases. Edge failure is of concern because riprap apron protection of shallow foundations is an integrated structural element of the bridge that must perform throughout the design life of the bridge.
This study had two phases. The first phase focused on flume abutment clear-water experiments using erodible uniform bed material to investigate different riprap installation geometries. The experiments facilitated evaluation of various field installations as well as the performance of installations based on the design guidelines from Bridge Scour and Stream Instability Countermeasures, Hydraulic Engineering Circular No. 23. The experiments also supported development of new riprap apron guidelines to address apron durability. In the second phase, a conceptual model was developed to define the flow-riprap interaction to inform development of design guidance. A CFD modeling approach was applied to validate the conceptual model and to support recommendations for riprap apron installation. This study supports the significant recommendation that locating the top of the riprap apron at the level of the estimated contraction scour depth should be preferred to surface installations. This buried apron would reduce vulnerability to edge failure. A buried apron might also be preferred from an environmental perspective because the riprap apron would be covered by natural streambed material. |
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17. Key Words
Contraction scour, Abutment scour, Riprap apron, Vertical-wall abutment, Narrow bridge opening, Shallow bridge foundation, Buried riprap |
18. Distribution Statement
No restrictions. This document is available through the National Technical Information Service, |
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19. Security Classification Unclassified |
20. Security Classification Unclassified |
21. No. of Pages 123 |
22. Price |
Form DOT F 1700.7 (8-72) | Reproduction of completed page authorized |
SI* (Modern Metric) Conversion Factors
ADV | acoustic Doppler velocimeter | ||
CFD | computational fluid dynamics | ||
CTB | cable-tied block | ||
DG | Design Guideline | ||
FHWA | Federal Highway Administration | ||
GRS | geosynthetic reinforced soil | ||
HEC | Hydraulic Engineering Circular | ||
IBS | Integrated Bridge System | ||
LTD | long-term degradation | ||
NCHRP | National Cooperative Highway Research Program | ||
PIV | particle image velocimetry | ||
RSF | reinforced soil foundation | ||
TFHRC | Turner-Fairbank Highway Research Center | ||
V:H | vertical-to-horizontal |
A0 | cross-sectional area of contracted section before contraction scour (ft2 (m2)) | |
A1 | cross-sectional area of upstream section (ft2 (m2)) | |
Â1 | cross-sectional area of upstream section for computational fluid dynamics experiments (ft2 (m2)) | |
A2 | cross-sectional area of contracted section without riprap at equilibrium contraction scour (ft2 (m2)) | |
A2R | cross-sectional area of contracted section with riprap after contraction scour to a depth of yc (ft2 (m2)) | |
A2b | cross-sectional area of contracted section before contraction scour (ft2 (m2)) | |
α1 | energy correction factor for upstream section (dimensionless) | |
α2B | energy correction factor for contracted section without riprap (dimensionless) | |
α2e | energy correction factor for equivalent contracted section with riprap (dimensionless) | |
α2R | energy correction factor for contracted section with riprap (dimensionless) | |
βB | channel shape factor for model without riprap (dimensionless) | |
βe | channel shape factor for equivalent model with riprap (dimensionless) | |
βR | channel shape factor for model with riprap (dimensionless) | |
CcB | contraction coefficient for model without riprap (dimensionless) | |
Cce | contraction coefficient for equivalent model with riprap (dimensionless) | |
CcR | contraction coefficient for model with riprap (dimensionless) | |
D50 | median noncohesive material size (bed material or riprap) (ft (m)) | |
Δ Z | vertical adjustment for equivalent depth (ft (m)) | |
ε | function related to ratio of roughness coefficient of riprap to that of erodible bed material (dimensionless) | |
Fr | Froude number (dimensionless) | |
Froude number for CFD experiments (dimensionless) | ||
g | acceleration from gravity (ft/s2 (m/s2)) | |
γ | unit weight of water (lbf/ft3 (N/m3)) | |
hB | head loss between upstream section and contracted section without riprap (ft (m)) | |
he | head loss between upstream section and equivalent contracted section with riprap (ft(m)) | |
hR | head loss between upstream section and contracted section with riprap (ft (m)) | |
Kr | coefficient equals to 0.89 for a spill-through abutment and 1.02 for vertical-wall abutment (dimensionless) | |
l1 | abutment length (ft (m)) | |
l2 | abutment width (ft (m)) | |
μ | viscosity (lbf∙s/ft2 (N∙s/m2)) | |
n | composite Manning’s roughness coefficient (dimensionless) | |
nB | roughness coefficient for bed material (dimensionless) | |
nR | roughness coefficient for riprap (dimensionless) | |
∅ | undefined function | |
Pd | downstream flow pressure (lbf/ft2 (N/m2)) | |
Pu | upstream flow pressure (lbf/ft2 (N/m2)) | |
Q | discharge (ft3/s (m3/s)) | |
Re | Reynolds number (dimensionless) | |
ρ | water density (lb/ft3 (kg/m3)) | |
Rh | hydraulic radius (ft (m)) | |
S | energy slope in contracted section (ft/ft (m/m)) | |
Sg | specific gravity of rock riprap (dimensionless) | |
τa | shear on front face of abutment (lbf/ft2 (N/m2 or Pa)) | |
τavg | average bed shear stress for model with riprap (lbf/ft2 (N/m2 or Pa)) | |
τB | average bed shear stress in middle portion of contracted section without riprap with scour at a depth of yc (lbf/ft2 (N/m2 or Pa)) | |
τc | critical shear stress on bed for model without riprap (lbf/ft2 (N/m2 or Pa)) | |
τo | average bed shear stress in contracted section without riprap before scour (lbf/ft2 (N/m2 or Pa)) | |
τR | average bed shear stress in contracted section between riprap aprons with scour at a depth of yc (lbf/ft2 (N/m2 or Pa)) | |
τrip | average bed shear stress on riprap (lbf/ft2 (N/m2 or Pa)) | |
V0 | average velocity in contracted section before contraction scour (ft/s (m/s)) | |
V1 | average velocity in upstream section (ft/s (m/s)) | |
1 | average velocity in upstream section for CFD experiments (ft/s (m/s)) | |
V2 | average velocity in contracted section without riprap at equilibrium contraction scour (ft/s (m/s)) | |
V2b | average velocity in contracted section before contraction scour (ft/s (m/s)) | |
V2R | average velocity in contracted section with riprap after contraction scour to a depth of the average equilibrium scour without riprap (yc) (ft/s (m/s)) | |
W1 | bottom width of upstream section (ft (m)) | |
W1T | top width of upstream section (ft (m)) | |
1 | average width in upstream section (ft (m)) | |
W2 | bottom width of contracted section (bridge opening width) (ft (m)) | |
WB | bottom width of bed material (ft (m)) | |
WR | riprap apron width (ft (m)) | |
y1 | average flow depth in upstream section (ft (m)) | |
y1M | maximum flow depth in upstream section (ft (m)) | |
ŷ1 | average flow depth in upstream section for CFD experiments (ft (m)) | |
y2 | average flow depth in contracted section with riprap after contraction scour (ft (m)) | |
y2M | maximum flow depth in contracted section with riprap after contraction scour (ft (m)) | |
y2R | average flow depth in contracted section with riprap at equilibrium contraction scour (ft (m)) | |
y2RM | maximum flow depth in contracted section with riprap at equilibrium contraction scour (ft (m)) | |
yc | average equilibrium contraction scour depth without riprap (ft (m)) | |
ycR | maximum contraction scour depth with partial-width riprap scour countermeasures (ft(m)) | |
ye | equivalent flow depth in contracted section (ft (m)) | |
y0 | average flow depth in contracted section before contraction scour (ft (m)) | |
y0M | maximum flow depth in contracted section before contraction scour (ft (m)) | |
ys | local scour depth (ft (m)) | |
z1 | reference elevation for upstream section (ft (m)) | |
z2 | reference elevation for downstream section (ft (m)) |