Countermeasures to Protect Bridge Abutments from Scour (2007) / Chapter Skim
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72.1 Common Forms of Abutments The two principal types of bridge abutment forms are wing-wall abutments and spill-through abutments. These abutment forms may be supported on piled or slab footings.
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increases, yet the abutments cause minimal constriction of flow. As channel size increases, channels may become more compound in cross section, such that there is a main channel and a floodplain of variable extent, as shown in Figure 2-3.
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downstream, scour usually occurs around the tip and extends downstream of the abutment. Abutments perpendicular to the flow cause the deepest scour, occurring slightly downstream of the tip.
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2.5 Scour Processes and Abutment Failure Mechanisms There are many scour processes and abutment failure mechanisms of concern. The previously described flow patterns cause scour that, if unmitigated, can cause abutment failure.
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the wall. The downflow is much weaker for spill-through abutments because of their sloped face.
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bined effects of flow contraction and flow features generated by the abutment. 2.6.3 Common Scour Conditions Causing Abutment Failure The foregoing considerations of scour location, based on flow field and boundary susceptibility to erosion, indicate that scour at the locations indicated in Figures 2-12 through 2-17 can lead to the following conditions of abutment failure (the scour conditions are elaborated in the final report for NCHRP Project 24-20)
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spilling over the abutment scours the floodplain along the downstream side of the embankment, and then the embankment side slope may undergo a side slope failure. This scour condition is akin to dam-breaching and possibly to the scour form that develops immediately downstream of an unprotected outlet of a culvert.
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and are of practical importance for the design and monitoring of bridge abutments.
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It is noted here that head-cutting of channel beds and channel migration are two types of channel degradation that are of major concern for bridges and account for numerous abutment failures. When a main-stem channel experiences bed degradation for some reason, the overall bed slope of a tributary channel then becomes steeper, with the erosion causing the steepening beginning at the downstream end (or base level)
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16 (a) Hydraulic scour of the floodplain (b)
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17 Figure 2-16. Washout of the approach embankment can fully expose the abutment foundation, such that further scour progresses as if the abutment were a form of pier.
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18 Figure 2-18. Upstream progression of a head-cut through a bridge waterway with exposed pier supports and destabilized adjoining abutments.
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