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25 CHAPTER 3. ABUTMENT SCOUR AS A DESIGN CONCERN The principal design concerns can be expressed in terms of set of questions: 1. What is the greatest scour depth that reasonably could occur near the abutment? 2. Will that scour depth pose a slope-stability problem for the embankment? 3. What scour depth should be used in estimating the required length of pile support? 4. What is the deepest scour that potentially could occur at the abutment column itself? 5. Does that scour occur when the embankment is breached so as to fully expose the abutment foundation? 3.1 DESIGN SCOUR DEPTHS When considering the possibility of embankment failure, two scour depths must be estimated, in accordance with the design concerns: 1. One scour depth is needed for stable embankment design; and, 2. The second scour depth is required for determining the length of piles underpinning the abutment column, or elevation of column footing (if a footing foundation is to be used). For design estimation of scour depths, it is necessary to consider the absolute elevations and locations attained by scour. The location of deepest scour relative to the concern of embankment stability differs from that associated with column stability. Additionally, the likely rates or sequences in which the scour develops are important, as explained in Ettema et al. (2010). 3.2 ESTIMATION OF SCOUR DEPTHS There are several approaches to estimate the two scour depths mentioned in Section 3.1. Scour depth associated with embankment stability subject to scour can be addressed in two ways, as described below. 1. Hydraulic then geotechnical calculations. Estimate the potential maximum depth of scour that may develop without immediately considering the geotechnical failure of the embankment on the floodplain near the abutment. Once this scour depth is estimated, its effect on the geotechnical stability of the main channel bank and embankment can be estimated. If the bank and embankment were found unstable, they would collapse. Failure of the headslope, or spill-slope, is an undesirable condition, which may have most serious consequences if road traffic is not immediately prevented from accessing the bridge approach. The integrity of the abutment column also may be affected by embankment failure, but this may not be the worst case for the column, as discussed below. Embankment failure acts to relieve flow contraction, diminish macro-turbulence generation, and consequently reduce the maximum scour depth attained. The geotechnical strengths of the embankment and floodplain soils, therefore, may
26 significantly influence abutment scour depth, as well as contribute uncertainty to scour- depth estimation; and, 2. Geotechnical calculation. For given (or measured) geotechnical strength properties of the embankment earthfill near the waterway, estimate the maximum limiting steepness for embankment stability. The maximum scour depth attainable then is determined in the context of the limiting maximum steepness of the embankment. No hydraulics calculation is needed, but the position of deepest scour must be estimated. An important point here is that the location of maximum scour depth has substantial bearing on embankment stability and thus the prospect of abutment failure. Once the embankment fails, flow contraction is relieved, flow area increases, maximum velocity near the abutment diminishes, and scour will not deepen. To be kept in mind, though, is the relative timing of scour development and embankment failure, and the undesirable consequences of full embankment failure. Scour depth associated with abutment-column stability should be considered in two ways. First, the abutment-column may be rendered unstable due to embankment failure as described above. Secondly, following embankment failure the abutment column may be exposed to the flow in the manner of a pier. This case must rely on a semi-empirical relationship such as used for estimating scour depth at a bridge pier, because an exposed abutment essentially is a pier. The complexity of flow field and sediment movement at a column is practically the same as at a bridge pier. These design concerns are drawn together in more detail in the NCHRP 24-20 report by Ettema et al. (2010) as a sequence of design steps that take into account abutment location, geotechnical properties of embankment and floodplain, and the erodibilities of main-channel bed and floodplain. Further elaboration of the research needs in this area can be found in Chapter 8. 3.3 AN ESSENTIAL DESIGN QUESTION An essential design question to be addressed by agencies designing bridge abutments â and not addressed during this evaluation study â concerns how abutment design should best take abutment scour into account. Many experiments and field observations of abutment failure indicate that failure typically occurs as the geotechnical collapse and washout of the abutmentâs earthfill embankment. Under severe situations, the abutment column also may fail in a manner similar to scour failure of a bridge pier. Embankment failure may limit the development of abutment scour to a potential maximum depth, because the exposed embankment soil erodes laterally, increasing the flow area and easing flow velocities in the area of deepest scour. The essential question leads to the following more specific questions: 1. What scour depth(s) should be considered for abutment design (the potential deepest scour, scour leading to embankment failure, or scour at an exposed abutment column)? 2. Is embankment failure (with bridge super-structure remaining intact) acceptable? 3. As the embankment near an abutment column often is a relatively weak or vulnerable location of bridge waterway, what design considerations should be contemplated in order
27 to strengthen embankments in the vicinity of an abutment column? Then, how would such strengthening affect abutment scour or scour at a nearby pier? It is noteworthy that all the illustrations of abutment scour in this report show failure of an abutmentâs earthfill embankment. The example shown in Figure 3-1 is representative of many abutment failures. Another example of embankment failure is shown in Figure 3-2 for a flood in the Atlanta metro area in 2009. Flow coming from the left floodplain as well as overtopping of the bridge severely eroded the left embankment, exposed the abutment and resulted in the approach span to the bridge deck falling into the stream. Figure 3-1. A common situation of abutment failure; scour has led to failure and partial washout of the earthfill spill-slope at this abutment. A basic question arises as to how abutment design should take scour into account. Figure 3-2. Failure of abutment fill in September 2009 Georgia flood accompanied by failure of approach roadway (Hong and Sturm 2010).
