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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 14 Rock Riprap at Bridge Abutments
Scour occurs at abutments when the abutment and embankment obstruct the flow. Several causes of abutment failures during post-flood field inspections of bridge sites have been documented (Parola et al. 1998):
Abutment damage is often caused by a combination of these factors. Where abutments are set back from the channel banks, especially on wide floodplains, large local scour holes have been observed with scour depths of as much as four times the approach flow depth on the floodplain. As a general rule, the abutments most vulnerable to damage are those located at or near the channel banks.
The flow obstructed by the abutment and highway approach embankment forms a horizontal vortex starting at the upstream end of the abutment and running along the toe of the abutment, and a vertical wake vortex at the downstream end of the abutment. The vortex at the toe of the abutment is very similar to the horseshoe vortex that forms at piers, and the vortex that forms at the downstream end is similar to the wake vortex that forms downstream of a pier. Research has been conducted to determine the depth and location of the scour hole that develops for the horizontal (so called horseshoe) vortex that occurs at the upstream end of the abutment, and numerous abutment scour equations have been developed to predict this scour depth.
Abutment failures and erosion of the fill also occur from the action of the downstream wake vortex. However, research and the development of methods to determine the erosion from the wake vortex has not been conducted. An example of abutment and approach embankment erosion of a bridge due to the action of the horizontal and wake vortex is shown in Figure 14.1. The types of failures described above are initiated as a result of the obstruction to the flow caused by the abutment and highway embankment and subsequent contraction and turbulence of the flow at the abutments.
The preferred design approach is to place the abutment foundation on scour resistant rock or on deep foundations. Available technology has not developed sufficiently to provide reliable abutment scour estimates for all hydraulic flow conditions that might be reasonably expected to occur at an abutment. Therefore, engineering judgment is required in designing foundations for abutments. In many cases, foundations can be designed with shallower depths than predicted by the equations when they are protected with rock riprap and/or with a guide bank placed upstream of the abutment designed in accordance with this design guide and Design Guideline 15. Cost will be the deciding factor (Richardson and Davis 2001).
The potential for lateral channel migration, long-term degradation and contraction scour should be considered in setting abutment foundation depths near the main channel. It is recommended that the abutment scour equations originally presented in HEC-18 (Richardson and Davis 2001) be used to develop insight as to the scour potential at an abutment.
Where spread footings are placed on erodible soil, the preferred approach is to place the footing below the elevation of total scour. If this is not practicable, a second approach is to place the top of footings below the depth of the sum of contraction scour and long-term degradation and to provide scour countermeasures. For spread footings on erodible soil, it becomes especially important to protect adjacent embankment slopes with riprap or other appropriate scour countermeasures. The toe or apron of the riprap serves as the base for the slope protection and must be carefully designed to resist scour while maintaining the support for the slope protection.
In summary, as a minimum, abutment foundations should be designed assuming no ground support (lateral or vertical) as a result of soil loss from long-term degradation, stream instability, and contraction scour. The abutment should be protected from local scour using riprap and/or guide banks. To protect the abutment and approach roadway from scour by the wake vortex several DOTs use a 50-foot (15-meter) guide bank extending from the downstream corner of the abutment (see Design Guideline 15). Otherwise, the downstream abutment and approach should be protected with riprap or other countermeasures.
The FHWA conducted two research studies in a hydraulic flume to determine equations for sizing rock riprap for protecting abutments from scour (Pagán-Ortiz 1991, Atayee 1993).The first study investigated vertical wall and spill-through abutments which encroached 28 and 56% on the floodplain, respectively. The second study investigated spill-through abutments which encroached on a floodplain with an adjacent main channel (Figure 14.2).
Encroachment varied from the largest encroachment used in the first study to a full encroachment to the edge of main channel bank. For spill-through abutments in both studies, the rock riprap consistently failed at the toe downstream of the abutment centerline (Figure 14.3). For vertical wall abutments, the first study consistently indicated failure of the rock riprap at the toe upstream of the centerline of the abutment.
Field observations and laboratory studies reported in HDS 6 (Richardson et al. 2001) indicate that with large overbank flow or large drawdown through a bridge opening that scour holes develop on the side slopes of spill-through abutments and the scour can be at the upstream corner of the abutment. In addition, flow separation can occur at the downstream side of a bridge (either with vertical wall or spill-through abutments). This flow separation causes vertical vortices which erode the approach embankment and the downstream corner of the abutment.
For Froude Numbers (V/(gy)1/2)#0.80, the recommended design equation for sizing rock riprap for spill-through and vertical wall abutments is in the form of the Isbash relationship:
For Froude Numbers >0.80, Equation 14.2 is recommended:
In both equations, the coefficient K, is a velocity multiplier to account for the apparent local acceleration of flow at the point of rock riprap failure. Both of these equations are envelope relationships that were forced to over predict 90% of the laboratory data.
The recommended procedure for selecting the characteristic average velocity is as follows:
Riprap is to be sized for an abutment located on the floodplain at an existing bridge. The bridge is 650 ft (198.12 m) long, has spill-through abutments on a 1V:2H side slope and 7 equally spaced spans. The left abutment is set back from the main channel 225 ft (68.58 m). Given the following tables of hydraulic characteristics for the left abutment, size the riprap.
Step 1. Determine the SBR (set-back distance divided by the average channel flow depth)
Step 2. Determine characteristic average velocity, V. SBR is greater than 5, therefore overbank discharge and areas are used to determine V.
V = Q/A = 7720/613.5 = 12.6 ft/s (3.84 m/s)
Step 3. Check SBR velocity against main channel velocity
Velocity in channel is greater than SBR velocity, therefore, use SBR velocity.
Step 4. Determine the Froude Number of the flow.
Fr = V/(gy)1/2 = 12.6/(32.2(2.7)) ½ = 1.35
Step 5. Determine the D50 of the riprap for the left abutment. The Froude Number is greater than 0.8, therefore, use Equation 14.2.
D50 = 0.4(2.7) = 1.1 ft (0.33 m)
Step 6. Determine riprap extent and layout.
Riprap design methods typically yield a required size of stone that will result in stable performance under the design loadings. Because stone is produced and delivered in a range of sizes and shapes, the required size of stone is often stated in terms of a minimum allowable representative size. For abutment scour protection, the designer specifies a minimum allowable d50 for the rock comprising the riprap, thus indicating the size for which 50% (by weight) of the particles are smaller. Stone sizes can also be specified in terms of weight (e.g., W50) using an accepted relationship between size and volume, and the known (or assumed) density of the particle.
For the shape, weight, density, and gradation of bridge abutment riprap, specifications developed for revetment riprap are applicable (Lagasse et al. 2006). These specifications are provided in Design Guideline 4 of this document (see Section 4.2.4).
Design Guideline 4 recommends gradations for ten standard classes of riprap based on the median particle diameter d50 as determined by the dimension of the intermediate ("B") axis. These gradations were developed under NCHRP Project 24-23, "Riprap Design Criteria, Recommended Specifications, and Quality Control." The proposed gradation criteria are based on a nominal or "target" d50 and a uniformity ratio d85/d15 that results in riprap that is well graded. The target uniformity ratio is 2.0 and the allowable range is from 1.5 to 2.5 (Lagasse et al. 2006).
Standard test methods relating to material type, characteristics, and testing of rock and aggregates recommended for revetment riprap are applicable to bridge abutment riprap (see Design Guideline 4). In general, the test methods recommended are intended to ensure that the stone is dense and durable, and will not degrade significantly over time.
Rocks used for riprap should only break with difficulty, have no earthy odor, no closely spaced discontinuities (joints or bedding planes), and should not absorb water easily. Rocks comprised of appreciable amounts of clay, such as shales, mudstones, and claystones, are never acceptable for use as riprap. The recommended tests and allowable values for rock and aggregate are summarized in Table 4.3 of Design Guideline 4.
Atayee, A. Tamin, 1993, "Study of Riprap as Scour Protection for Spill Through Abutment," presented at the 72nd Annual TRB meeting in Washington, D.C., January.
Atayee, A. Tamin, Pagán-Ortiz, Jorge E., Jones, J.S., and Kilgore, R.T., 1993, "A Study of Riprap as a Scour Protection for Spill Through Abutments," ASCE Hydraulic Conference, San Francisco, CA.
Barkdoll, B.D., Ettema, R., and Melville, B.W., 2007, "Countermeasures to Protect Bridge Abutments from Scour," NCHRP Report 587, Transportation Research Board, National Academies of Science, Washington, D.C.
Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Ruff, J.F., 2006, "Riprap Design Criteria, Recommended Specifications and Quality Control, NCHRP Report 568, Transportation Research Board, Academies of Science, Washington, D.C.
Melville, B.W., van Ballegooy, S., Coleman, S., and Barkdoll, B., 2007, "Riprap Size Selection at Wing-Wall Abutments," Technical Note, ASCE, Journal of Hydraulic Engineering, Vol. 133, No. 11, November.
Melville, B.W., van Ballegooy, S., Coleman, S., and Barkdoll, B., 2006, "Countermeasure Toe Protection at Spill Through Abutments," ASCE Journal of Hydraulic Engineering, Vol. 132, No. 3.
Pagán-Ortiz, Jorge E., 1991, "Stability of Rock Riprap for Protection at the Toe of Abutments Located at the Floodplain," FHWA Research Report No. FHWA-RD-91-057, U.S. Department of Transportation, Washington, D.C.
Parola, A.C., Hagerty, D.J., and Kamojjala, S., 1998, NCHRP Report 417, "Highway Infrastructure Damage Caused by the 1993 Upper Mississippi River Basin Flooding," Transportation Research Board.
Richardson, E.V. and Davis, S.R., 2001, "Evaluating Scour at Bridges," Hydraulic Engineering Circular 18, Fourth Edition, FHWA NHI 01-001, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.
Richardson, E.V., Simons, D.B., and Lagasse, P.F., 2001, "River Engineering for Highway Encroachments - Highways in the River Environment," Report FHWA NHI 01-004, Federal Highway Administration, Hydraulic Design Series No. 6, Washington, D.C.