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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. Figures 2-1a and b illustrate the main features of wing-wall and spill-through abutment forms, respectively. Wing-wall abutments have vertical walls that retain the earthfill material comprising the embankment approach to the abutment. The walls can be angled from about 45 degrees to 90 degrees. The abutment face is vertical as well. The over- all form of the abutment is quite bluff, and causes large-scale turbulence structures to develop in the flow around the abut- ment. Wing-wall abutments are commonly used for small bridges with one to three spans, as the abutment form lends itself to placement on the banks of streams and creeks, or small rivers that do not have a pronounced floodplain. Spill-through abutments are formed as sloped earthfill placed fully around a pier-like abutment support (often called a standard stub abutment). This abutment form is commonly used for abutments set back on floodplains. The sloped earthfill material making up the earthfill embank- ment around the abutment needs protection from scour, as often does the floodplain immediately around the abutment. Accordingly, although some spill-through abutments have a simple grassed surface, it is usual for spill-through abut- ments to have their front slopes and flanks protected with riprap or sometimes with a concrete slab. 2.2 Abutment Setting Though there are essentially two basic abutment forms (spill-through and wing-wall), abutments may vary markedly in their setting at bridge waterways. Most abutment settings are unique in abutment placement, soil conditions, channel morphology, and surrounding vegetation. Therefore, the task of providing scour protection through the use of counter- measures inevitably entails tailoring countermeasure tech- niques to individual bridge sites. This consideration is a major theme in this report. A second, and related, theme concerns the size of river or stream channel to be bridged. Channel size has a bearing on abutment form and layout and thereby on the nature of the scour countermeasure options to be implemented. There is a correspondence between the size of the channel and the size of the countermeasure concept. For instance, spur dikes are likely not to be an appropriate countermeasure concept for flow and bank control along a small stream; rock hard-points or riprap are better suited scour countermeasures for small channels. For most state departments of transportation and counties in the United States, a small channel is one whose upstream watershed encompasses about 100 square miles or less. It is usual for such bridges to have one, two, or three spans. Usu- ally, small channels have a negligible floodplain, such that the bridge abutments are practically located at the top of the channel banks, more or less as indicated in Figure 2-2. To be sure, channel width and flow depth can increase as flow C H A P T E R 2 Abutment Forms and Scour (a) wing-wall (b) spill-through U U Spill-throughWing-wall Figure 2-1. Two common forms of abutment.
increases, yet the abutments cause minimal constriction of flow. As channel size increases, channels may become more com- pound in cross section, such that there is a main channel and a floodplain of variable extent, as shown in Figure 2-3. Abut- ments may still be located at the banks of a main channel, or they may be set back so as to constrict flow to some practical minimal extent and as to reduce the cost of the bridge. Spill- through abutments most commonly are used for bridges over comparatively large channels. Further, for bridges over com- pound channels, spill-through abutments usually are set back on the floodplain, as illustrated in Figure 2-3. To be sure, a great variety of channel sizes and abutment forms can be found; wing-wall abutments occasionally are used for bridges span- ning portions of floodplains. Also, water elevations and flow patterns can vary enormously with varying discharge in com- pound channels. The setting of a bridge across a waterway also may vary in accordance with highway or road orientation relative to the thalweg axis of a channel. These considerations determine abutment skewness to the flow and possibly the extent of set- back from the main channel of the waterway. The skewness angle of the abutment to the waterway can affect scour extent, depth, and location. When aligned upstream into the flow, the flow is retarded upstream of the abutment with scour occur- ring around the abutment tip. When an abutment is angled 8 Figure 2-2. Typical features of a bridge set over a small channel, with abutments near the edge of the channel. Figure 2-3. Typical features of a bridge over a relatively large and compound channel.
downstream, scour usually occurs around the tip and extends downstream of the abutment. Abutments perpendicular to the flow cause the deepest scour, occurring slightly down- stream of the tip. Figures 2-4 and 2-5 illustrate a spill-through abutment on a floodplain and a wing-wall abutment at a streambank, respectively. 2.3 Proximity of First Pier Many bridges over rivers are constructed with a compara- tively short first deck span such that a pier is located very close to an abutment. There are construction advantages in having the pier close to the abutment and riverbank, and the arrange- ment often facilitates a clear span over the river. That construc- tion advantage, however, can lead to a potentially severe scour situation whereby local scour at a pier adversely influences scour at a neighboring abutment, or vice versa. Figure 2-6 depicts a fairly common example of a bridge that has a pier located close to an abutment. For such cases, it is important that the protec- tion for the abutment and pier be jointly developed. A risk, otherwise, is that protection of the abutment may aggravate scour at the pier. 2.4 New Versus Existing Abutments Scour countermeasures may be applied to abutments being constructed for new bridges or be retrofitted to an existing bridge. It is increasingly usual for new abutments to be constructed with some form of scour countermeasure. For such bridges, it is somewhat easier to place the counter- measure, as the abutment site is usually accessible. However, present design practice entails that abutment (and pier) foundations be deep enough that the structural stability of an abutment is not reliant on the performance of a scour countermeasure. The role of the countermeasure is to reduce scour extent so as to minimize erosion of an approach embankment to an abutment or of a channel bank near an abutment. For existing abutments, scour countermeasures often are required to ensure that a potentially scour-threatened abut- ment does not fail or to aid in the repair of an abutment. Often a scour countermeasure can be a temporary action to prevent further erosion of an exposed embankment or erod- ing channel bank. However, a countermeasure may also be intended to extend the remaining design life of a bridge. One challenge associated with the application of a scour countermeasure for an existing bridge can be access to the abutment region requiring protection. This is a concern espe- cially if a countermeasure has to be placed in water flowing around the abutment. It is important that scour countermea- sures be practical to implement, in terms of both construction of the countermeasure components and placement of the countermeasure. 9 Figure 2-5. Wing-wall abutment at a streambank. Figure 2-6. Pier close to a spill-through abutment. Figure 2-4. Spill-through abutment on a floodplain.
2.5 Scour Processes and Abutment Failure Mechanisms There are many scour processes and abutment failure mechanisms of concern. The previously described flow pat- terns cause scour that, if unmitigated, can cause abutment failure. The scour processes are extensively described in the final report for NCHRP Project 24-20, âPrediction of Scour at Abutmentsâ; this study is a companion study to the present study. The ensuing descriptions of abutment scour are largely taken from the findings of NCHRP Project 24-20. 2.6 Channel and Bank Scour Processes Described here are the flow field and the scour processes leading to abutment scour. Given that several processes con- tribute to abutment failure, it is useful to first mention the several boundary materials forming the bridge waterway and then indicate the locations where abutment scour can be deepest. Figure 2-7 indicates the usual soil and sediment dis- positions in the vicinity of a bridge abutment, in this case for an abutment on a floodplain. The soils and sediments can have different erosion resistance and behavior. 2.6.1 Locations of Abutment Scour The abutment layout, the flow field, and the erodibility of sediment and soil at bridge sites may cause the deepest scour to occur at any, or all, of three locations near an abutment, as indicated in Figure 2-8: 1. In the main channel near the abutment (Scour Condition 1); 2. A short distance downstream of the abutment (Scour Condition 2); and 3. At the abutment itself (Scour Condition 3). Scour at these locations occurs at different rates and can differ in the maximum depth attained, in accordance with flow-field and soil conditions. If sufficiently deep, scour at each location can cause the slope-stability failure of the embankment adjoining the abutment. 2.6.2 Flow Field In its effect on flow in a channel, a bridge abutment may be likened to a short contraction, such as that indicated in Figure 2-9 for flow through a simple orifice. Two flow features are directly evident in the flow field through a contraction: 1. Flow contraction and 2. The generation and shedding of large-scale turbulence structures from the boundaries of the contraction. As shown schematically in Figure 2-10, the flow field at an abutment typically consists of an acceleration of flow from the upstream approach to the most contracted cross section somewhere at or just downstream of the head of the abutment, followed by a deceleration of flow. A flow- separation region forms immediately downstream of the abutment, and flow expands around the flow-separation region until it fully reestablishes itself across the compound channel. Just upstream of the abutment, a flow-separation point and a small eddy may develop. The size of the upstream eddy depends on the length and alignment of the abutment. The curvature of the flow along the interface between the stagnation region and the flow causes a secondary current that, together with the flow, leads to a spiral motion or vortex motion similar to flow through a channel bend. The vortex in flow around an abutment head is more localized and has a strong scouring action. The 10 Figure 2-7. Boundary soils and sediments forming the waterway at an abutment.
the wall. The downflow is much weaker for spill-through abutments because of their sloped face. The two flow features listed above (flow contraction and the generation and shedding of large-scale turbulence struc- tures from the boundaries of the contraction) are related and difficult to separate in the flow field. The region of flow con- traction is influenced by the area ratio of the approach flow and the contracted flow, as well as by the form and roughness of the contraction. The large-scale turbulence structures are also influenced by the form and roughness of the contraction. The orifice analogy is somewhat simplistic, but an important point to be made from it is that the flow field through a bridge waterway, like the flow field through an orifice, is not readily delineated as a contraction flow field and local flow field lim- ited to the near zone of the abutment. The effect of flow contraction on velocity through the con- traction can be explained in terms of a contraction coeffi- cient, C, as used in calibrating flow through an orifice. For fully turbulent flow, C is a function of orifice geometry. Like- wise, for abutments, the extent of flow contraction and the turbulence generated by the contracting flow depends on abutment shape. Either of the flow features listed above may become more pronounced, depending on the extent of flow contraction. When an abutment barely constricts flow through the waterway, scour at the abutment may develop largely as a consequence of the local flow field generated by the abut- ment. At the other extreme situation, flow contraction may dominate the flow field when the flow is severely constricted such that a substantial backwater rise in water level occurs. In this case, the approach flow slows as it approaches the upstream side of the bridge, then it accelerates to high speed as it passes through the bridge waterway. Except for bridges whose spans greatly exceed abutment length, the flow field at a typical bridge waterway will be influenced by the com- 11 Figure 2-8. Three main regions of abutment scour. Figure 2-9. Flow through a bridge opening is analo- gous to flow through an orifice contraction; the flow contracts and turbulence structures develop. Figure 2-10. Schematic of near-field flow around a spill-through abutment. vortex erodes a groove along its path and induces a complex system of secondary vortices. At abutments with wing-walls (Figure 2-11), the flow impinging on the wall may create a downflow (similar to at a bridge pier), which excavates a locally deepened scour hole at
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): ⢠Condition 1: Scour destabilization of the main-channel bank near the abutment, which is located close to the bank. The floodplain is relatively resistant to erosion compared with the bed of the main channel. Figure 2-12 illustrates the several-stage failure process, which involves scour leading to geotechnical failure of the main-channel bank and the embankment. Hydraulic scour of the main-channel bed causes the channel bank to become geotechnically unstable and collapse. The col- lapsing bank undercuts the abutment embankment, which in turn collapses locally. Soil, and possibly riprap, from the collapsed bank and embankment slide into the scour hole. ⢠For wing-wall abutments, located within the bank of the main channel, several erosion processes in addition to flow contraction can result in failure of the main-channel bank and the approach embankment: The local flow field generated at the corners of the abutment can cause local scour at those locations (see Figure 2-13) and Exposure of the piles beneath the abutment pile cap can cause riverbanks and embankment soil to erode out from beneath the pile cap (see Figure 2-14). ⢠Condition 2: Scour of the floodplain around an abutment well set back from the main channel. The floodplain scours near and slightly downstream of the abutment. The scour hole locally destabilizes the embankment side slope, causing embankment soil, and possibly riprap, to slide into the scour hole (see Figure 2-15). ⢠Condition 3: Scour at Locations 1 or 2 just mentioned may eventually cause the approach embankment to be washed out near the abutment, thereby fully exposing the abutment. In this condition, scour at the exposed stub or wing-wall abutment essentially occurs as if the abut- ment were a form of pier. Figure 2-16 illustrates this scour condition. ⢠Condition 4: Scour may occur at the embankment approach some distance from an abutment. This is shown in Figure 2-17. The embankment intercepts and deflects flow on the floodplain, but the unprotected floodplain near the embankment may experience eroding velocities that cause a local side slope failure of the embankment. This scour mechanism differs from those shown in Figures 2-15 and 2-16 because that scour does not occur at the bridge opening. In somewhat extreme cases, flow may erode through the embankment or wash out the embankment. ⢠Condition 5: Scour can occur when an approach embankment is overtopped by a high flow. Overtopping can occur because the embankment has a comparatively low crest elevation or because the bridge opening has become clogged with vegetation debris or perhaps (during the early spring season) with ice. In this condition, flow 12 Figure 2-11. Flow field past a wing-wall abutment.
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 possi- bly to the scour form that develops immediately down- stream of an unprotected outlet of a culvert. It is important to realize that a scour event (or series of events) at an abutment, however, may involve a sequence of all five scour conditions. When an abutment is close to the main channel, Condition 1 (Figure 2-12) may develop relatively quickly, with Condition 2 (Figure 2-15) occurring at a slower rate. Either separately or together, Scour Condi- tions 1 and 2 may eventually cause the approach embank- ment to undergo a slope-stability failure. If the em- bankment washes out enough to expose the abutment structure, scour may develop at the abutment structure as if the abutment were a form of pier (Condition 3, Figure 2-16). The combination of scour conditions is suggested earlier in Figure 2-8. The scour conditions described in this section may occur for pile-supported or spread-footing-supported abutments 13 (a) Hydraulic scour of the main-channel bed causes riverbank instability and failure. (b) The face of the abutment embankment fails. In this condition, the floodplain is much less erodible than is the bed of the main channel. Figure 2-12. The several-stage collapse process associated with one common condition of scour at a spill-through abutment in a compound channel.
and are of practical importance for the design and monitor- ing of bridge abutments. (a) Hydraulic scour of the main-channel bed causes riverbank instability and failure. (b) The face of the abutment embankment fails. In this condition, the floodplain is much less erodible than is the bed of the main channel. 2.6.4 Other Abutment Failure Processes Other possible scour conditions can be associated with abutments. These processes are attributable to several causes: ⢠General scour, ⢠Head-cut migration along a channel, ⢠Shifts in channel or channel-thalweg alignment, and ⢠Erosion associated with poorly maintained drainage chan- nels along the flanks of an abutment. General scour is scour that occurs irrespective of the exis- tence of the bridge. It includes long-term and short-term scour processes. Long-term general scour is scour that occurs over several years or longer and includes progressive degrada- tion and lateral bank erosion due to channel widening or meander migration. Progressive degradation is the almost per- manent lowering of the river bed at a bridge site owing to nat- ural changes in the watershed (meander-bend cutoff, head-cut progression, landslides, fire, climate change, etc.) or human activities (channel straightening, dredging, dam construction, agriculture, urbanization, etc.). 14 (a) Hydraulic scour of the main-channel bed causes riverbank instability and failure. (b) The channel bank and the face of the abutment embankment fail. Figure 2-13. The two-stage collapse process associated with one common condition of scour at a wing-wall abutment.
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 steepen- ing beginning at the downstream end (or base level) of the tributary channel. The steepening process forms a so-called knickpoint along the bed of the tributary channel; the knick- point is the location where there is a discontinuity in the channel bed of the tributary. As the downstream extent of the bed of the tributary channel erodes, the knickpoint is moved upstream. For a bed consisting of sandy alluvium, bed erosion and knickpoint movement occur relatively quickly. For a bed consisting of cohesive sediment (clay) or soft sedimentary rock, knickpoint movement can be relatively slow, and the upstream movement of the knickpoint occurs by means of a process called head-cutting. Head-cut migration along a channel occurs when flow plunges over a head-cut (that is, a vertical or near-vertical drop in the channel bed) and strikes the bed downstream, thereby eroding a scour hole. The scour hole deepens until the face of the head-cut becomes unstable geotechnically, then fails into the scour hole, and the head-cut progresses upstream. The upstream migration of a head-cut induces channel bed and bank instabilities, worsens erosion, and increases the sediment load delivered to downstream reaches. Figure 2-18 illustrates head-cutting occurring within the waterway of a small bridge. The head-cutting destabilized the bridge abutments and exposed the piling support of piers. Short-term general scour is scour that develops during a single or several closely spaced floods. It includes scour at a confluence, which includes shifts in channel thalweg or channel- thalweg alignment, shifts in bends, and scour arising from bed-form (dune or bar) migration. A common problem for bridge abutments is erosion attrib- utable to head-cutting at drainage channels along the flanks of an abutment. Figure 2-19 depicts a common situation found for small bridges. The erosion associated with poorly maintained drainage channel exposes the abutment to aggra- vated scour. 2.7 Need for Countermeasures To mitigate abutment scour and avoid abutment under- mining and failure, countermeasures are needed. These coun- termeasures can make it more difficult for the flow to cause scour or they can alter the flow pattern so as to lessen its scouring capacity, or a combination of both. Countermeasures can lessen scour by quite significant amounts and, therefore, provide a fairly inexpensive method of protecting the abutment from failure. Examples are provided in subsequent chapters of scour mitigation by countermeasures. 15 (a) Before scour. (b) Scour develops below the pile cap of a wing-wall abutment. (c) Embankment soil is sucked from beneath the pile cap and forms a cavity in the embankment. Figure 2-14. Collapse process.
16 (a) Hydraulic scour of the floodplain (b) Failure of the face of the abutment embankment. In this condition, the floodplain is as erodible (more or less) as is the bed of the main channel. The collapse of the embankment soil (and armor protection) into the scour hole modifies the scour area. Figure 2-15. The collapse process associated with a common condition of scour at a spill-through abutment in a compound channel.
17 Figure 2-16. Washout of the approach embankment can fully expose the abutment foundation, such that further scour pro- gresses as if the abutment were a form of pier. Figure 2-17. Floodplain flow impingement against a long approach embankment can result in erosion of the embankment.
18 Figure 2-18. Upstream progression of a head-cut through a bridge waterway with exposed pier supports and destabilized adjoining abutments. Figure 2-19. Erosion of side drainage upstream of right abutment, exposing the abutment to scour.