Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011) / Chapter Skim
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15 CHAPTER 2. ABUTMENT FORM AND CONSTRUCTION The main design characteristics of an abutment can be described in terms of abutment form, the overall layout of an abutment's approach embankment, and the abutment's construction configuration.
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16 2.2 ABUTMENT LAYOUT In a somewhat simplified manner, it is useful to discuss abutment layout in terms of the length, L, of approach embankment, floodplain width, Bf, main channel width, Bm, overall width of the main channel and floodplain at a bridge crossing of a waterway, B, and embankment top width, W These variables are indicated in Figure 2-2 except for W which is shown in Fig.
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17 2.3 ABUTMENT CONSTRUCTION It is usual for the top width of the earthfill embankment to accommodate minimally a road width of 24 ft (7.22 m) plus two shoulders of width 8 ft (2.41 m)
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18 Figure 2-4. The geometry and dimensions of a standard-stub abutment commonly used for spillthrough abutments (prototype scale indicated)
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19 The elevation of the pile cap and the detailed arrangement of piles may vary from bridge site to bridge site. At some sites, the pile cap is located at, or near, the top elevation of the floodplain, whereas at other sites the piles extend upward through the embankment earthfill.
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20 2.5 SEDIMENT AND SOIL BOUNDARY MATERIAL The boundary material of the main-channel, floodplain, and embankment components of a bridge-waterway boundary usually comprise different zones of alluvial sediments and soil, as indicated in Figure 2-7. Abutment scour usually occurs within several zones of sediment and soil, leading to different erosion processes and varying rates of erosion.
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21 2.6 FLOW FIELD Flow through a bridge waterway narrowed by a bridge abutment and its embankment is essentially flow around a short streamwise contraction3 . Figure 2-8 schematically illustrates the characteristic flow features and the connection between the contraction and the formation of a complex flow field around the abutments.
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22 Figure 2-9. Flow structure including macro-turbulence generated by floodplain/main channel flow interaction, flow separation around abutment, and wake region on the floodplain of a compound channel.
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23 • One region is where the boundary is least resistant to hydraulic erosion. This could be the main bed if flow velocities (and unit discharges)
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24 Figure 2-11. For a spill-through abutment well set back on a flood-plain, deepest scour usually occurs where flow is most contracted through the bridge waterway.
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