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Appendix D. Hydraulic Requirements for Shallow Abutment Foundations

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

APPENDIX D. HYDRAULIC REQUIREMENTS FOR SHALLOW ABUTMENT FOUNDATIONS

Shallow foundations (e.g., spread footings) have been successfully used for bridge abutments at river and stream crossings. However, when a shallow abutment foundation is being considered for use in a river or stream environment, it is vitally important to fully understand the hydraulic design requirements. The guidance provided in this appendix identifies the major hydraulic requirements that, if followed, provide greater assurance that the shallow foundation will perform as intended. This guidance supersedes that currently provided in HEC-18, fifth edition, and HEC-23, third edition.^(2,3)

The design of bridges in a river environment is a very complex endeavor because of the complex interactions among the structural components, the soils in which they are founded, and the moving water that imparts hydraulic loading to both. For this reason, an interdisciplinary team of structural, geotechnical, and hydraulic engineers should always be fully engaged in the bridge scoping, design, and construction processes. From a strictly hydraulics perspective, it is of utmost importance that a qualified hydraulics engineer, experienced in river mechanics, sediment transport, and bridge hydraulics be part of the interdisciplinary team.

When bridges are constructed over a waterway, their foundations must be designed, detailed, and constructed in compliance with section 2.6 (Hydrology and Hydraulics) of the American Association of State Highway and Transportation Officials Load Resistance Factor Design Bridge Design Specifications or an FHWA division office-approved drainage or bridge manual.⁽²⁵⁾ In addition, the bridge impact on the numerous floodplain values that may be present must be evaluated in accordance with the U.S. Code of Federal Regulations (23 CFR Part 650 SubpartA).⁽²⁶⁾ These compliances are required for the project to be eligible for Federal assistance.

HYDRAULIC DESIGN CONSIDERATIONS

The minimum hydraulic design considerations that should be evaluated when deciding whether a shallow abutment foundation is appropriate for a waterway bridge follow:

Site selection: The optimum stream-crossing site has a fully stable channel, which is characterized by banks and bed that are not prone to change. Detailed information on what constitutes a stable channel is contained in FHWA publication, HEC-20, Stream Stability at Highway Structures.⁽²⁷⁾
Abutment location: Bridge abutments are typically set back from stream channel banks to minimize potential stability problems, scour, and impact loads. Impact loads can be expected on streams that transport ice, large cobbles, boulders, or large woody debris such as tree trunks. It is recommended that abutments be set back from the channel banks a minimum distance of twice the design flow depth in the channel or 25 ft (7.6 m), whichever is less. Flow depth should be established by water surface modeling.
Adverse flow conditions: Adverse flow conditions generate complex hydraulics and increase the potential scour and stream instability at a bridge site. Adverse flow conditions result from bridges that have the following characteristics: (1) are highly skewed to the flow, (2) severely constrict the flow, (3) encroach on flows in steep channels, and (4) produce overtopping of the bridge or an approach roadway. Crossings with one or more of these adverse conditions must be evaluated with advanced hydraulic modeling techniques (two-dimensional modeling) to identify accurate flow depths and velocities at the necessary locations.
Risk-based standards: Risk-based standards have been adopted so bridge owners can better balance the flood frequencies used for bridge design with the risks associated with the crossing (e.g., cost of the bridge, importance of bridge, traffic characteristics). Once the owner has identified the appropriate risk-based flood frequency standard for hydraulic design (i.e., bridge waterway capacity), FHWA has linked that flood frequency to recommended frequencies for the scour design and scour check floods. Reference chapter2 of HEC-18 (i.e., Evaluating Scour at Bridges) for a discussion of risk-based standards and the recommended relationship between the hydraulic design and the scour floods.⁽²⁾
Local drainage: The potential for unbalanced water pressure exists when an abutment wall can become partially submerged by a flood or when surface drainage is not controlled. All vertical-wall abutments should include provisions to accommodate surface and subsurface drainage. The following critical areas should be considered: behind the wall, at the base of the wall, and any location where a fill slope meets the wall face. For example, the design needs to include provisions for surface drainage along the fill slope adjacent to wing walls.

The listed descriptions of hydraulic design considerations reflect preferred conditions for shallow foundation abutments. The greater the departure from these preferred conditions, the more likely that alternative abutment types or drainage structure types (e.g., reinforced concrete box culverts or pipe culverts) should be considered. Under these circumstances, it is recommended that economic analyses be used to identify the preferred alternative.

HYDRAULIC DESIGN PROCESS

The bridge hydraulic design process applicable to shallow foundation abutments is illustrated in figure 123.

Figure 123. Flowchart. Bridge hydraulic design process.

The steps in this multidisciplinary process are described as follows:

Select design flow: A minimum of three flood frequencies must be evaluated for shallow foundation design: (1) hydraulic design flood, (2) scour design flood, and (3) scour check flood. The hydraulic design flood is used to identify the necessary size (i.e., length and elevation) and orientation of the bridge opening. The scour design and check floods, which are typically larger than the hydraulic design flood, are used to design scour countermeasures and determine the minimum depth of the spread footing foundation. The appropriate frequency for these floods is typically defined by the hydraulic standards established by the bridge owner. If such standards do not exist, the appropriated flood frequencies can be identified by conducting a risk assessment or analysis for the stream crossing. Reference chapter 2 of HEC-18 for detailed discussions of risk-based standards and the recommended relationship between the hydraulic design flood and the scour floods.⁽²⁾
Evaluate existing flow conditions and channel stability: Existing flow conditions and patterns should always be evaluated to establish a hydraulic baseline for the new or replacement bridge design. Also, stream channels should be stable, both horizontally and vertically, in the vicinity of the bridge to provide a suitable crossing environment for the life of the bridge. Details on the evaluation of channel stability are contained in HEC‑20.⁽²⁷⁾ Details on stream instability countermeasure design can be found in HEC-23 (Bridge Scour and Stream Instability Countermeasures).⁽³⁾
Select/modify bridge type and size: As indicated in figure 123, this step is the beginning of an iterative process that evaluates the hydraulics and potential scour resulting from the alignment and grade of the approach roadways, as well as the size and orientation of the bridge. Proposed layouts of these elements must be hydraulically modeled for a range of discharges that includes the hydraulic design flood and the scour floods to accurately estimate the hydraulic parameters (e.g., depths and velocities) affecting the bridge, the approach roadways, and the floodplain.
Perform hydraulic analysis: The hydraulic model used must be capable of developing water-surface profiles upstream, downstream, and through the bridge to identify reasonable estimates of the key hydraulic parameters. This requires that a one-dimensional water-surface profile model, such as HEC River Analysis System (i.e., HEC-RAS), be used at minimum. If adverse flow conditions exist, or are created at the crossing, a two-dimensional model, such as the Sedimentation and River Hydraulics—Two-Dimensional (i.e., SRH-2D), should be used. Also, if channel geometry can change over time, multiple hydraulic models may be needed to identify worst-case hydraulics and scour. The possible alignments, grades, and bridge geometries are evaluated with the hydraulic model(s) until an acceptable crossing configuration is found for the design floods. Refer to FHWA publication Hydraulic Design of Safe Bridges for detailed guidance on one- and two-dimensional hydraulic modeling.⁽²⁸⁾
Perform scour analysis: The hydraulics for floods up to and including the scour check flood standard are used to identify the worst-case scour depths for the applicable scour components and the total scour that can be generated at each bridge foundation. If a computed worst-case scour depth is not acceptable (i.e., too deep for the abutment to be economically built and/or protected), adjustments to the bridge type, size, or even location need to be made until an acceptable total scour value results. Such adjustments should be made only after in-depth consultations with the project geotechnical and structural engineers. Reference HEC-18 for detailed guidance on conducting scour evaluations and analyses.⁽²⁾
Set foundation elevation: There are two basic options for establishing the spread footing elevation: 1) the top of the footing may be set below the total scour depth for the scour check flood at the abutment, without the need for an abutment scour countermeasure, or 2) the top of the footing may be set at or below the contraction scour elevation for the scour design flood (includes any LTD), with a properly designed and constructed abutment scour countermeasure. As indicated in figure 123, the task of setting the final spread footing elevation can only be done effectively through in-depth consultation and coordination with the project geotechnical and structural engineers.
Design abutment scour countermeasures: When it is not practical to set the abutment foundation below the total scour depth, a designed abutment scour countermeasure is required to ensure stability during the scour check flood. See the sections entitled Scour Design and Scour Countermeasure Design later in this appendix for more information on the design of scour countermeasures for shallow abutment foundations.

APPLICABLE SCOUR COMPONENTS

For shallow abutment foundation design, the following primary scour components must be computed and evaluated: (1) LTD, (2) contraction scour, and (3) abutment scour. Both the contraction scour and abutment scour components are sensitive to what sediment transport regime exists upstream of the bridge (i.e., live-bed versus clear-water) and whether the scour floods are under free-surface flow or pressure flow conditions (e.g., bridge girders are in the flow) at the bridge. Because of the dramatic increase in potential scour depth, pressure scour should be avoided, if at all possible. In addition, the abutment scour component is sensitive to the location of the abutment relative to the main channel. When the abutment is located close to the channel, the scour is computed differently than when it is some distance away (i.e. more than twice the main channel flow depth for the scour check flood).

The manner in which these individual scour components are evaluated for floods up to and including the scour check flood standard is summarized as follows by flow condition.

Free-Surface Flow

LTD = Greater of the following two evaluations:
- Computed depth from equilibrium slope or armoring analyses, based on HEC-20 guidance.⁽²⁷⁾
- A specified depth for other degradation phenomenon, such as head cut depth, or historical observation.
Contraction scour (horizontal):
- For clear-water conditions = Clear-water contraction scour estimate.
- For live-bed conditions = Lesser of the following:
  ‒ Live-bed contraction scour estimate.
  ‒ Clear-water contraction scour estimate.
Abutment scour:
- Amplification factor, based on abutment location, multiplied by one of the following, as appropriate:
  ‒ Clear-water contraction scour estimate.
  ‒ Live-bed contraction scour estimate.

Pressure Flow

LTD (same as for free-surface flow).
Contraction scour (vertical) = Pressure scour.
Abutment scour (component presently undefined for pressure flow; research in progress).

As indicated, the conditions and interaction of the scour components can be complicated and must be identified and analyzed by a qualified hydraulics engineer. Refer to HEC-18 for detailed definitions of the individual scour components and conditions that apply to abutment analysis and design, and for the various methods available to compute the scour magnitude for each component.⁽²⁾

SCOUR DESIGN

The scour used to establish the spread footing foundation elevation is the worst-case combination of applicable scour components estimated for floods up to and including the appropriate scour flood standard (e.g., Q500 for scour check flood), and depends on the flow condition (i.e., whether free-surface or pressure flow exists at the bridge). The manner in which the individual scour components are combined for scour design is summarized below for each flow condition and illustrated in figure 124 through figure 127. The combinations listed are for abutments located close to the channel bank. Note that the underlining is a reminder to the user that measurements for scour check flood should be used and not those for scour design flood.

1 ft = 0.305 m.

Figure 124. Drawing. Free-surface flow with no scour countermeasure (option 1).

This illustration includes a plan and cross sectional view showing the free-surface flow with narrow-opening scour countermeasure (option 2). The plan view shows a rectangular shaped abutment protruding into the channel bank. A riprap apron is shown to extend from the front face of the abutment toward the embankment either 2 times y subscript 0(CF) or 25ft whichever is greater. 2 times y subscript 0(CF) is the flow depth in the bridge opening during the check flood. The riprap apron then extends by 2 times y subscript 0(CF) in the direction upstream and downstream from the wall. The plan view figure also illustrates the extent of the above ground riprap along the channel bank and the buried riprap within the main channel. The flow of the water goes from the top of the illustration to the bottom. The cross section view is sketched in alignment with the plan view. It shows a side view of an abutment supported on a shallow foundation with riprap layout on the wingwall and buried riprap in the stream channel. The top of the shallow foundation is built at the calculated depth of the contraction scour for the design flood plus the long-term degradation. The riprap is placed at 2 horizontal (H) to 1 vertical (V) slope down from the embankment until it reaches below the contraction scour for check flood + long term degradation depth and then becomes horizontal. The total horizontal span of the riprap is a minimum of 2 times y subscript 0(CF) perpendicular to the front face of the abutment into the main channel. The minimum length of the flat portion is 1 times the flow depth in the bridge opening during the check flood y subscript 0(CF). The top of the flat part of the riprap is built deeper than the top of the shallow foundation, at a depth of contraction scour for the check flood plus the long-term degradation. A filter is placed below the riprap with a riprap apron thickness of 2 times the median material size (D subscript 50).

1 ft = 0.305 m.

Figure 125. Drawing. Free-surface flow with narrow-opening scour countermeasure (option 2).

This illustration includes a plan and cross sectional view showing the free-surface flow with full width scour countermeasure (option 3). The plan view shows a rectangular footprint of a shallow foundation abutment protruding into the channel bank. A riprap apron is shown to extend from the front face of the abutment toward the embankment either 2 times y subscript 0(CF) or 25ft, whichever is greater. 2 times y subscript 0(CF) is the flow depth in the bridge opening during the check flood. The riprap apron then extends by 2 times y subscript 0(CF) in the direction upstream and downstream from the wall. The plan view figure also illustrates the extent of the above ground riprap along the channel bank and the buried riprap within the main channel. The flow of the water goes from the top of the illustration to the bottom. The cross sectional is sketched in alignment with the plan view. It shows a side view of an abutment supported on a shallow foundation with riprap layout on the wingwall and buried riprap in the stream channel. The top of the shallow foundation is built at the calculated depth of the contraction scour for the design flood plus the long-term degradation. The riprap is placed at 2 horizontal (H) to 1 vertical (V) slope down from the embankment until it reaches below the contraction scour for check flood + long term degradation depth and then becomes horizontal with the flat riprap zone extending across the channel. The top of the flat part of the riprap is built deeper than the top of the shallow foundation, at a depth of contraction scour for the check flood plus the long-term degradation. A filter is placed below the riprap with a riprap apron thickness of 2 times the median material size (D subscript 50).

1 ft = 0.305 m.

Figure 126. Drawing. Free-surface flow with full-width scour countermeasure (option 3).

This illustration includes a plan and cross sectional view showing the Pressure flow scour countermeasure design for a shallow foundation. The plan view shows a rectangular footprint of a shallow foundation abutment protruding into the channel bank. A riprap apron is shown to extend from the front face of the abutment toward the embankment either 2 times y<sub> </sub>subscript u(CF) or 25ft, whichever is greater. 2 times y subscript u(CF) is the flow depth directly before the bridge deck during pressure flow. The riprap apron then extends by y sub u(CF) in the direction upstream and downstream from the wall. The plan view figure also illustrates the extent of the above ground riprap along the channel bank and the buried riprap within the main channel. The flow of the water goes from the top of the illustration to the bottom. The cross sectional is sketched in alignment with the plan view. It shows a side view of an abutment supported on a shallow foundation with riprap layout on the wingwall and buried riprap in the stream channel. The water elevation in this view is shown to be above the deck of the bridge indicating a pressure flow condition. The top of the shallow foundation is built at the calculated depth of the vertical contraction scour for the design flood plus the long-term degradation. The riprap is placed at 2 horizontal (H) to 1 vertical (V) slope down from the embankment until it reaches below the contraction scour for check flood + long term degradation depth and then becomes horizontal with the flat riprap zone extending across the channel. The top of the flat part of the riprap is built deeper than the top of the shallow foundation, at a depth of vertical contraction scour for the check flood plus the long-term degradation. A filter is placed below the riprap with a riprap apron thickness of 2 times the median material size (D subscript 50).

1 ft = 0.305 m.

Figure 127. Drawing. Pressure flow scour countermeasure.

Free-Surface Flow

Option 1 (no countermeasure): Minimum depth to top of footing = Total scour at abutment = LTD + abutment scour for the scour check flood (see figure 124).
Option 2 (wide-opening countermeasure; for W₂/y₀ > 6.2 only): Minimum depth to top of footing = LTD + contraction scour depth for the scour design flood; minimum depth to top of abutment countermeasure apron = LTD + contraction scour for scour check flood (see figure 125).
Option 3 (narrow-opening countermeasure): Minimum depth to top of footing = LTD + contraction scour depth for the scour design flood; full-width countermeasure protection required from abutment to abutment; top of full-width countermeasure below LTD + contraction scour depth for scour check flood (figure 126).

Pressure Flow

Pressure flow countermeasure: Minimum depth to top of footing = greater of LTD or pressure scour depth for the scour design flood; full-width countermeasure protection required from abutment to abutment; top of full-width countermeasure below greater of LTD or pressure scour depth for scour check flood (figure 127).

It is important to note that scour depths must be tied to an appropriate reference elevation. The channel thalweg elevation should be used as the reference elevation for abutments located near the main channel.

SCOUR COUNTERMEASURE DESIGN

When a shallow abutment foundation requires installation of a scour countermeasure, the countermeasure must include a horizontal apron designed to be stable for the scour check flood. The apron should surround the entire abutment face and extend upstream and downstream of the abutment. For free-surface flow options 2 and 3, the extensions should be a distance equal to twice the main channel flow depth through the bridge, 2y₀. For pressure flow, the extensions should be a distance equal to twice the main channel flow depth upstream of the bridge, 2y_u. In addition, the same designed countermeasure should run up the channel bank and protect the abutment embankment. To do this effectively, the countermeasure should be configured to cover the embankment to an appropriate height (including freeboard) and for a distance of 2 times the main channel flow depth or 25 ft (7.6 m), whichever is greater, behind the abutment and parallel to the roadway.

Figure 124 through figure 127 illustrate the appropriate scour and countermeasure design configurations for the described flow conditions and options. Note that, although the countermeasure configurations are all similar, there are dimensional differences that make each case unique. Also note that the figures reflect the use of rock riprap as the countermeasure type. When riprap is used as the designed countermeasure, an appropriate filter must be placed under the riprap to prevent the underlying soils from being winnowed out through the interstitial openings between rocks.

There is one additional scour countermeasure design consideration for shallow foundations that support GRS and similar abutments. When the scour check flood overtops an approach roadway, a potential exists for the GRS abutment to be attacked from the back side. The likelihood depends on the depth of the overtopping flow, its duration, and the erosion resistance of the embankment. Research that defines the potential failure mechanism and most appropriate countermeasure configuration has yet to be conducted. However, the following additional countermeasure treatment is recommended when abutment embankment overtopping occurs for the worst-case scour condition:

Wrap the geotextile layers on back side of abutment full height to enable the abutment to stand unsupported should the embankment fail.
Extend the abutment embankment protection to the top of the embankment.

There is also one hydraulic/structural design consideration for GRS type abutments. When a scour flood inundates an abutment, either by free-surface overtopping flow or pressure flow conditions, the top of the GRS mass is subject to unknown hydrodynamic forces that may scour GRS material from around the beam seat. Should either of these flow conditions be present, it is recommended that the GRS abutment be constructed wide enough to accommodate placement of the scour countermeasure on top of the GRS mass.

Designing countermeasures for shallow foundation abutments in a river environment can also be a very complicated endeavor because of the complex interaction between the hydraulics, the multiple scouring mechanisms that are typically present, and the structural components. For these reasons, it is again of utmost importance that a qualified hydraulics engineer, experienced in river mechanics, sediment transport, and bridge hydraulics, perform the analyses required for countermeasure design.

At the time of this report, the complete details of scour countermeasure design specifically for shallow foundation abutments, which are too extensive to include herein, were being compiled and summarized for inclusion in the next edition of FHWA publication HEC-23.⁽³⁾

RIPRAP COUNTERMEASURE SPECIFICATIONS

As indicated in the preceding discussions, engineers must rely on countermeasures to ensure embankment, and at times, foundation stability during the scour check flood. Because of its flexibility, availability, and relative cost, the countermeasure of choice is often rock riprap. Accordingly, engineers must be aware that there are many sources of uncertainty associated with the design, manufacture, installation, performance, and maintenance of a riprap mass. Among the causes of premature riprap failure related to design and construction are the following:

Inadequate rock quality, size, and/or gradation.
Inadequate embedment and/or toe-down depths.
Inadequate thickness.
Segregation of rock sizes.
No or improperly installed filter.
Damaged filter material.

A designed granular or geotextile filter must be installed under all riprap installations to prevent finer-grained base soils from being winnowed out through the passages between individual rocks, causing premature failure.

Without comprehensive construction acceptance testing, there is little assurance that the riprap mass will perform as intended. Consequently, when a riprap countermeasure is used, rock quality, acceptance criteria, and testing frequency requirements must be developed and included in the construction contract specifications to properly control the manufacture, placement, and acceptance of the riprap. In addition, the size and gradation test methods to be used for accepting the riprap mass must be included in, or referenced by, the contract. Including such provisions in the contract will reduce the chances of premature riprap failure. The FHWA Office of Federal Lands Highways’ Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14, Sections 251 and 705, provide sampling, testing, and acceptance requirements; and material requirements, respectively, for rock riprap.⁽²⁹⁾

After construction, riprap countermeasure condition and channel instability should be assessed during each regular bridge inspection and after large flood events. Any countermeasure failure or significant change in channel stability should be noted and scheduled for repair or stabilization. Without proper inspection and maintenance, a scour countermeasure may fail or a channel may become unstable, which can lead to bridge abutment failure.

Page Owner: Office of Research, Development, and Technology, Office of Infrastructure, RDT

Topics: research, infrastructure, structures
Keywords: research, structures, Contraction scour, Abutment scour, Riprap apron, Vertical-wall abutment, Narrow bridge opening, Shallow bridge foundation, Buried riprap
TRT Terms: research, infrastructure, Facilities, Structures
Scheduled Update: Archive - No Update needed

This page last modified on 01/29/2018