Countermeasures to Protect Bridge Abutments from Scour (2007) / Chapter Skim
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38 5.1 Introduction The literature on scour at bridge abutments and similar structures, such as spur dikes, is extensive. Useful overviews of the literature are given by Melville and Coleman (2000)
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39 bank) eroding bank of meander bends, the placement of which reduces the erosive ability of the flow and may cause deposition near the bank.
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40 general lack of agreement on the important variables needed to predict maximum scour depth. This disagreement has been possibly settled by Melville (1992, 1997)
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Spur dikes situated at the bridge crossing oriented 90 degrees to the main channel are studied for the first time in this study, and the results are presented in Chapter 6. 5.2.4 Application to Abutment Scour Countermeasure Spur dikes are commonly used to maintain predetermined alignment of the upstream-channel approach to a bridge abutment.
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42 and Kennedy, 1983; Odgaard and Lee, 1984; Odgaard and Mosconi, 1987; Odgaard and Spoljaric, 1986; Odgaard et al., 1988; Odgaard and Wang, 1987, 1990a, 1990b, 1991) and for reducing sediment ingestion in intakes (Barkdoll et al., 1999)
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A factor associated with these concerns is the common propensity of scour at an abutment to attract a thalweg, especially during conditions of diminished or constricted flow (ice, debris, etc.)
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44 halt the upstream advance of a knickpoint have been required to change in recent years because of concerns that fish and other aquatic species can be able to move along a stream or river. Though there has been extensive work done on the use of grade-control structures to impede the upstream progression of bed degradation, there has not been much work done regarding the effect of bed degradation on the stability of bridge abutments.
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struction of a bridge waterway as a weir. The resulting bridge was called a Greenwood bridge, named for the county engineer who developed the concept for this form of bridge waterway.
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46 Figure 5-7. Vanes installed along the concave bank just upstream from the bridge crossing and their effectiveness in stabilizing eroded bank, Wapsipinicon River in Iowa.
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articulated concrete mattress on the lower portion because of the difficulty and uncertainty of placing riprap underwater in large depths and high velocities. Conversely, riprap can be used on the lower portion of the bank, with vegetation on the upper portion, a technique used on some smaller streams.
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48 such factors as stable stone size, lateral and vertical extent of protection, and alignment. • Riprap layout must be safe with regard to geotechnical stability, foundation settlement, and groundwater seepage.
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Riprap placed in an apron at the base of wing-wall abutments may be subjected to shear failure, edge failure, winnowing failure, and bed-form undermining (Parola, 1993; Chiew, 1995; Parker et al., 1998; Lauchlan, 1999)
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50 individual stones by impact and abrasion. This mode of failure is similar to shear failure, with an additional factor being the effect of the slope.
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51 Where y is flow depth. Neill (1973)
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52 Figure 5-17. Riprap size selection (Neill, 1973)
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riprap at bridge abutments. The recommendations cover some or all of the following riprap parameters: size, extent of protection, layer thickness, gradation, and filter design.
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54 Table 5-5. Equations for size of riprap.
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55 Figure 5-19. Plan view of the recommended extent of rock riprap apron (Lagasse et al., 1997)
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56 Where: y approach-flow depth, B upstream width of the flume, La abutment length, and rt radius of the spill-through abutment toe Wa, , and ai are defined in Figure 5-20. Layer Thickness To a certain extent, riprap layer thickness affects the stability and durability of riprap protection.
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Filter Design Filters include granular filters, which make use of the filtering effect of graded sediments, and synthetic filters, commonly called geotextiles. Filters are placed beneath riprap layers to meet one or both of the following objectives: • To prevent groundwater behind the riprap from transporting bank material through the riprap (i.e., piping)
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by Pagan-Ortiz (1991) , Macky (1986)
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placed on the floor of the flume. The gravel-covered observation area spanned the width of the flume and extended for 1.78 m of the length of the flume, with equal areas upstream and downstream from the abutment model.
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The tests were conducted to simulate typical construction practice rather than recommended construction practice. Thus, the protection measure was terminated at a level slightly below the existing bed levels.
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The abutment was 150 mm wide on an embankment 1.17 m wide, 0.25 m high, varying in length from 130 to 510 mm, with side slopes of 2:1 (H:V)
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Eve (1999) Eve studied criteria for selection of riprap protection at spill-through bridge abutments with launching apron protection under clear-water and live-bed conditions.
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abutment toe, which led to slumping of the sediment beneath the filter fabric. Three types of failure were observed in the live-bed tests: • Catastrophic rapid failure, which occurred without a geotextile where the embankment fill material was rapidly winnowed from between the riprap stones, leading to disintegration of the structure.
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the absence of a filter layer, the underlying sediment could be entrained by the winnowing process. Subsequent tests by Jones et al.
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Parker et al.
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5.8 Geobags Geobags can be used in lieu of riprap stone or other armor cover, such as cable-tied blocks, that in certain regions can be difficult and expensive to obtain. The potential advantages of the geobags are that they are readily transported (when empty)
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Where: b angle of the boundary on which the geobag is placed and C angle of repose of the sediment forming the boundary. For the experiments, and were 26.7 and 30 degrees, respectively.
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