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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 abutmentâs approach embankment, and the abutmentâs construction configuration. These characteristics, together with the waterwayâs channel morphology, boundary sediments and soils, as well as flow-resistance features (e.g., vegetation state of the floodplain), influence the flow field around the abutment, and therefore, scour. A striking, and somewhat complicating, characteristic of bridge abutments is that few abutment situations are alike, as Figures 1-1 and 1-2 exemplify. Accordingly, the development of a method for estimating scour depth at abutments requires that the abutment forms, layouts, and construction configurations of common practical importance be identified. 2.1 ABUTMENT FORM Two general forms of abutment exist as illustrated in Figure 2-1: 1. Wing-wall abutments, including vertical-wall abutments; and, 2. Spill-through abutments Spill-through abutments have sloped sides, whereas wing-wall abutments have a vertical face and wing-walls that retain an earthfill approach embankment. The wing-walls can be oriented at various angles to the abutmentâs central panel, although a 45o angle is representative. A wing- wall abutment with wing-walls angled at 90o to its central panel is sometimes called a vertical- wall abutment, and it is fairly common for small abutments. Sheet-pile caissons extending into channels also may be viewed as a type of vertical-wall abutment. Various alternative names exist for these two general abutment forms. Figure 2-1. Plan views of the two common abutment forms: (a) Wing-wall; (b) Spill-through (Ettema et al. 2010). W
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. 2-1. Figure 2-2. Definitions of embankment length, floodplain width, and main channel width (Ettema et al. 2010). Bridge abutments can be characterized as conforming to the following layout arrangements, which can be represented in terms of the variables L, Bf, and B: 1. The abutment is located on the floodplain of a compound channel (L ⤠Bf). This layout is typical for spill-through abutments. It is usual for the abutment to be set back from the main-channel bank so that a vehicle (and wildlife) can pass between the abutment and the bank. A minimum setback distance of about 10 ft (3.05 m) is common practice, if site layout allows, but the setback distance on large rivers with wide floodplains may be considerably more ; 2. The abutment extends up to the bank of the main channel (L â Bf). This layout is typical for wing-wall abutments, especially for channels having a narrow, or no, floodplain. Wing-wall abutments are common for bridges over small streams; and, 3. The abutment is located in a rectangular channel, and no floodplain is present. This layout is not common, although it is essentially similar to a relatively short abutment on a wide floodplain and is representative of wide-braided channels. Also, it is similar to channel-control structures (e.g., spur-dikes, groins, barbs, hard-points), coffer-dams, and construction caissons. The nature of an abutment inevitably requires that the layout of an abutment be tailored to fit the local topography of a bridge site. Therefore, to varying extents each abutment inevitably differs in layout. Other variations in abutment layout can be found; e.g., many small bridges in Maine have wing-wall abutments that extend into the main channel (Lombard and Hodgkins 2008).
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), giving an overall top width of 40 ft (12.04 m). The side-slopes of earthfill approach embankments commonly are set at 2H:1V, though slopes range from about 2H:1V to 3H:1V. Figure 2-3 is an isometric view of the geometry used for spill-through abutments. The embankment geometry for wing-wall abutments is essentially similar to that shown in Figure 2-3, except that the vertical face of a wing-wall abutment retains the end of the embankment. Figure 2-3. Isometric view of spill-through abutment comprising a standard-stub column located within the end of an earthfill embankment (Ettema et al. 2010). Abutments usually comprise a concrete support wall founded on a pile cap supported by piles or on a spread footing, and adjoin an earthfill approach embankment. Pile supports are more common than are footing supports, unless the abutment is founded directly on rock. Spill- through abutments are formed around a âstandard-stub abutment,â which comprises a concrete stub supported by a pile cap on two rows of circular piles. The design and dimensions of a common standard-stub abutment column are shown in Figure 2-4. Wing-wall abutments usually have similar foundation layouts as the standard-stub abutments, except that they include wing- walls extending from the central stub. Figure 2-5 shows the design and dimensions of a common wing-wall abutment.
18 Figure 2-4. The geometry and dimensions of a standard-stub abutment commonly used for spill- through abutments (prototype scale indicated); design provided by the Iowa DOT (Ettema et al. 2010). Figure 2-5. The geometry and dimensions of a wing-wall abutment - compacted earthfill embankment extends back from the abutment structure (prototype scale indicated); design provided by the Iowa DOT (Ettema et al. 2010).
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. In this latter case, the piles directly support a cross beam, which in turn supports the beams of the bridge deck. Also, for some sites, wing-wall abutments may be supported by sheet piles driven in approximately the same plan layout as the abutment. The foregoing descriptions of common abutment forms and construction arrangements are not reflected in the leading design guides and bridge-monitoring guides addressing scour at bridge abutments. For example, FHWAâs (2009) guide for bridge inspectors does not fully portray the complexity of an abutment structure and its flow field, or possible failure mechanisms due to scour, as elaborated in this report. Chrisohoides et al. (2003) and Ettema et al. (2010), for example, provide useful visualizations of abutment flow, as currently understood. 2.4 PIER PROXIMITY Many bridges over rivers are constructed with a comparatively short first deck span, such that a pier is located very close to an abutment. There are construction-economy advantages in having the pier close to the abutment and riverbank, and the arrangement often facilitates a clear span over the river. This construction advantage, however, raises a question as to whether pier proximity could adversely influence abutment scour (and vice-versa). Figure 2-6 depicts an example of a bridge with a pier located close to an abutment. Figure 2-6. A spill-through abutment with a pier in close proximity; approximate layout proportions of L/Bf = 1.0; Bf/0.5B â 0.7, and L/W â 1.0, in which W = embankment top width (Ettema et al. 2010).
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. Figure 2-7. Variation of soil and sediment types at a bridge crossing (Ettema et al. 2010). Alluvial non-cohesive sediment (sands and gravels) most frequently forms the bed of the main channel, whereas the channelâs floodplain may be formed from considerably finer sediments (silts and clays), typically causing the floodplain soil to be more cohesive in character than the bed sediment of the main channel. The banks of the main channel usually are formed of the floodplain soils, and thus also may behave cohesively so as to stand at a fairly steep slope. Most abutments have an earthfill approach embankment formed of compacted soils. The soils may have been excavated from the floodplain or have been brought to the bridge site from elsewhere. The earthfill embankment is placed and compacted to a specific value of shear strength so as to support the traffic load. Direct, dynamic simulation of the strength behavior of an earthfill embankment or a floodplain soil poses a practical difficulty for laboratory experiments on scour at bridge abutments. The difficulty is to replicate, at a reduced scale, the shear strength of a representative earthfill embankment. To date, no study appears to have attempted experiments that closely replicated the strength behavior of an embankment with mixed soil types.
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. The flow width narrows and the flow accelerates through the contraction, generating macro-turbulence structures (eddies and various vortices spun from the contraction boundary) that shed and disperse within the flow. Flow contraction and turbulence at many bridge waterways, though, is complicated by the shape of the channel. It is common for waterways to traverse a compound channel formed of a deeper main channel flanked by floodplain channels, as shown in Figure 2-9. To varying extents, all flow boundaries are erodible. As this figure indicates, the major flow features of a short contraction prevail at a bridge waterway comprising a two-lane road. The contraction lengthens for dual-carriageway highways like freeways or expressways. Figure 2-8. Flow structure including macro-turbulence generated by flow around abutments in a narrow main channel. (Ettema et al. 2010). 3 The contraction is short in the streamwise direction
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. (Ettema et al. 2010). Though the short-contraction analogy is somewhat simplistic, an important point to be made is that the flow field around an abutment, like the flow field through an orifice, is not readily delineated as a contraction flow field separate from a local flow field established near the abutment. The two flow features (flow contraction and large-scale turbulence) are related and difficult to separate. Either of the flow features may dominate, depending on the extent of flow contraction and the characteristics of the abutment and its foundation. When an abutment barely constricts flow through the waterway, scour at the abutment may develop largely due to the local flow field generated by the abutment. This flow field is characterized by a local contraction of flow and by generation of large-scale turbulence. For a severely contracted bridge waterway, flow contraction dominates the flow field and a substantial backwater occurs upstream of the bridge. In this situation, the approach flow slows as it approaches the upstream side of the bridge, and then accelerates to a higher velocity as it passes through the bridge waterway. When the foundation of the end of an abutment comprises a solid contiguous form extending into the bed (flood plain or main channel), scour development may become similar to that at a wide pier where the flow becomes contracted and large-scale turbulence is produced. Such abutments include situations where a sheet-pile skirt is placed around the toe of the spill-slope of a spill- through abutment (to protect against spill-slope instability and failure), or when a wing-wall column is founded on sheet-piles. Embankment and abutment structures create potentially erodible short contractions. Higher flow velocities and large-scale turbulence around an abutment may erode the abutment boundary. Commonly, the bed of the main channel is more erodible than the floodplain, because the bed is formed of loose sediment, while the floodplain is formed of more cohesive soil often protected by a cover of vegetation. Accordingly, two prime scour regions typically develop, as borne out by field observations of scour, as indicated in Figure 2-10:
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) are sufficiently large; and, ⢠The other region is where the flow velocities (and unit discharges) and turbulence are greatest. This usually is near the abutment. For the simpler situation of an abutment well set back on a flood-plain, laboratory experiments indicate that deepest scour usually coincides with the region where flow contraction is greatest (Ettema et al. 2010, Melville et al. 2006). Figure 2-11 illustrates this for a spill-through abutment. For spill-through abutments comprising erodible embankments flow contraction dominates the abutment flow field. Once scour begins, the geometry of the bridge waterway (as a short contraction) changes. The deepened flow at the scour region draws more flow, because flow contraction is locally eased there. The extent and maximum depth of scour at abutments can be complicated by the mix of materials forming the compound channel and the abutmentâs embankment, and other considerations such as the proximity of a pier. Figure 2-10. Interaction of flow features causing scour and erodibility of boundary (Ettema et al. 2010).
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.
