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19 CHAPTER 2 SCOUR AS A DESIGN CONCERN 2.1 Introduction When considering pier scour, it is necessary to consider pier structure and its influence on the scouring flow field, the foundation material supporting the pier, and the processes whereby flow erodes foundation material (sediment, soil, clay, rock) around the pier. 2.2 Pier Function and Structure For efficient structural performance, and minimal environmental impact on the channel spanned, bridges over rivers commonly have a comparatively short first span, with a pier placed near the toe of, or within, the spill-slope of a bridge abutment, as illustrated in Figures 1-2 and 1-3. The piers of a multi-span bridge typically are not positioned in the same local flow conditions or boundary material. Also, variations in pier location at a bridge commonly require variations in pier structure; i.e., differences in pile cap or footing elevation, and in pile length. Bridge piers support superstructure spans, doing so by transferring design loads to the channel boundary. Design loads include the deck weight and live-load, and the hydrodynamic load exerted by water flowing around the pier. Load transfer occurs typically via a simple slab footing, a set of end-bearing piles, or a set of friction-bearing piles. A consideration not well recognized is that pier structure affects the manner of pier scour failure, which in turn affects the form and dimensions of the scour hole. In other word, piers fail as scour develops (rather than a scour hole forming to equilibrium depth, then the pier collapses into it). This point is elaborated in Section 2.2. The simplest pier form is the single-column pier, especially cylinder of circular cross- section. However, simple circular piers are not usual. Circular cylinder elements do commonly exist as a pile extending to a pile cap, with the pile cap elevated above the water level, as shown in Figure 2-1. Most U.S. Departments of Transportation and other bridge-designing agencies use common designs involving piers of more complicated form. Figure 2-2 for example shows a common pier structure, comprising a pier column, pile cap, and cluster of piles (friction- or end-bearing). The layout and dimensions of common designs conform to the design loads anticipated for piers. A deficiency in the leading methods for scour-depth estimation is their basis largely on laboratory data, the great majority of which have involved simple cylindrical pier forms (circular or rectangular cylinders), usually not coinciding with common pier designs. Most field data are for piers of diverse forms. A research need to be mentioned early in this report is for scour estimation methods to link more expressly to common pier structures, such as shown in Figure 2-2. The effective form of pier structures may change during flow events, owing to the accumulation of woody debris or ice against the pier (Figure 2-3), or other factors such as flow direction and exposure of piles beneath a pile cap. Such changes influence the pier flow field, pier loading, and scour.
20 More complicated and unique pier forms often are required for less common bridge sites, such as for large bridges, and channel circumstances where piers must withstand additional loads. Bridges in large tidal flows, flows subject to dynamic ice conditions, and bridges possibly subject to vessel impact, for example, require more complicated piers. Figure 2-4 illustrates such an example. Figure 2-1 A simple pier form comprising two cylindrical cylinders Figure 2-2 A common pier structure used for two-lane bridges. The pier comprises a column supported by a pile group with a pile cap
21 Figure 2-3 A common pier form in a flow situation complicated by debris or ice accumulation Figure 2-4 Some bridges, such unusually large bridges, or bridges in unusual circumstances, require large piers of uncommon design 2.3 Design Depth for Pier Foundation The design scour depth must be taken into account when sizing and positioning the foundation base. For piers on spread footings, the top of the footing must be below the estimated design scour depth, so that the footing is not undermined. For piers on friction-
22 bearing piles, pile length must include design scour depth so as to ensure sufficient friction length of pile (Figure 2-5). Scour reduces the friction length of piles and increases the risk of pile buckling. To be kept in mind is that reduction of support will cause a pier to settle or tilt in various ways, as indicated in Figure 2-6. Pier settling and tilting may alter effective pier form, increase flow-field capacity to scour, and deepen scour. Field examples of pier settlement and tipping are shown in Figures 2-7 and 2-8, respectively. Figure 2-5 Scour reduces the effective length of friction-bearing piles
23 (a) (b) (c) (d) Figure 2-6 Scour reduces pier support, causing pier settlement (a) ï (b), bottom rotation of pier (a) ï (c), or top rotation of pier (a) ï (d)
24 Figure 2-7 Bridge pier settled vertically owing to scour reduction of pier support (a) (b) Figure 2-8 Pier tipped owing to scour: (a) forward tipping; and, (b), backward tipping. These photos raise interesting questions: How does scour develop when a pier rotates as it loses support or gets pushed back by flow pressure? Does pier tipping deepen scour? Also, (a) illustrates pier propensity to collect woody debris 2.4 Current U.S. Design Methods for Pier Scour For U.S. bridge design, the leading method for design estimation of pier scour is the method herein termed the Richardson and Davis (2001) method. It often is colloquially called the CSU method, because it stems from extensive research conducted at Colorado State University. This method currently is in FHWAâs design guide HEC-18, and is recommended by AASHTO.
25 Other methods may be used regionally, though in conjunction with the HEC-18 method; e.g., the method developed by Wilson (1995) who uses extensive field data from bridges in Mississippi. A method proposed by Sheppard and Miller (2006) is being used increasingly, notably in Florida (FDOT 2010). The method developed by Melville 1997, (also in Melville and Coleman 2000) is used in conjunction with the Richardson et al. method, though is not formally recommended by HEC-18 or AASHTO. It is used quite extensively in other countries besides the U.S. Melvilleâs method merged with that by Sheppard and Miller (2006) has evolved into the Sheppard-Melville method, consequent to NCHRP Project 24-32 (Sheppard et al. 2011). As mentioned in Section 1.2, a perceived significant deficiency of the existing methods is their predisposition to yield unacceptably conservative estimates of scour depth at wide piers. Additionally, concerns exist that the methods do not apply well to commonly used pier forms or piers of unusual form. An aspect inadequately reflected by the existing methods is the substantial differences pier flow field associated with variations in pier size, flow depth, and foundation material. 2.5 Need for a Structured Design Approach The potentially extensive set of parameters influencing pier depth, and the variable complexity of pier flow field, require a structured approach to design estimation of pier scour depth. The approach should not rest on a single, universal pier-scour relationship for estimating scour depth, but identify several levels of pier site complexity, and the current best methods for scour-depth estimation for each level. Such an approach is inadequately articulated in current design methods, such as HEC-18. Figure 2-9 outlines the main elements of the approach this report recommends. It relates pier site complexity (also pier size) essentially to the varying practicalities of two methods â formulation in a semi-empirical or rational equation, and simulation in model of the pier site. The design methodology should comprise the following levels of pier form and site complexity: 1. Simple or single-column pier forms; 2. Common pier forms comprising a more complex geometry; 3. Common pier forms in difficult situations; and, 4. Uncommon or Complex pier forms and situations. The methodology reflects the pier situations in the general views shown as Figures 1-2 and 1-3, which respectively illustrate the pier supports for a long multi-span bridge, and a short, three-span bridge. The central pier in Figure 1-2 could be considered a fairly simple pier in isolation, but the local flow fields at other piers are to varying extents affected by flow around the abutments and over the channelâs floodplain. The two piers in the shorter bridge (Figure 1-3) cannot be considered in isolation, and may be affected by flow around the abutment and over the floodplain. Further, the methodology entails using the methods having best present prospect for addressing pier-site complexities.
26 As the present report explains, the approach is not entirely new. Chapter 5 shows several methods that attempt to account for the influences of major parameters. Additionally, as explained in Chapter 5, it is reasonably common for difficult or complex pier circumstances to receive additional design attention; for instance, large piers of complex geometry, and piers in complicated channel bathymetries. The complexities listed in Section 1.4 make a single design relationship or method infeasible for estimating scour depth at all pier situations in bridge waterways. For situations involving a pier of relatively simple geometry in an uncomplicated channel, the methodology indicates that scour depth can be estimated using essentially a rational (parameter influence) equation comprising a linear combination of factors expressing parameter influences. Estimation accuracy diminishes as parameter number increases, in accordance with increasing pier or site complexity. For piers of increasing geometric complexity, pier geometries complicated by factors altering pier form (e.g., debris or ice), and site difficulties (e.g., in channel morphology), the design approach necessarily entails modeling the pier situation and the scour it generates. The methodology outlined in Figure 2-9 prompts several questions: 1. Which rational equation(s) should be used? 2. What are the limits for using a rational equation, and, consequently, when is it necessary to model a pier site? 3. How reliable are models (hydraulic and numerical) for simulating scour? 4. Can soil and rock properties be incorporated into equations or methods (including hydraulic and numerical models) for estimating scour depth? If substantial uncertainty attends the use of a method (equation and/or model), or site conditions, the methodology emphasizes the importance of site monitoring and management. Uncertainties frequently occur with approach-flow conditions, erosion characteristics of bed or floodplain material, and prospective changes in channel morphology. The ensuing three chapters review the scour processes and parameters to be considered for scour-depth estimation, indicate limits of present scour knowledge and formulation, and identify the leading rational equations for scour-depth estimation. Chapter 6 then elaborates the design methodology. 2.6 Synopsis of Post-1990s Research The numerous complexities associated with design estimation pier-scour depth cause scour to remain an active topic of civil engineering research. Though publication of research findings on scour extends back at least one hundred years (notably, Engels in 1894, at Dresden University, Germany (Freeman 1929)), papers dealing with investigation of scour processes and scour depth prediction are still regularly published in journals (e.g., ASCE Journal of Hydraulic Engineering [JHE], IAHR Journal of
27 Hydraulic Research [JHR]) and conference proceedings (e.g., Scour and Erosion , 2006, 2008). The literature on bridge scour is replete with papers presenting new flume data and observations on various aspects of scour, especially scour at vertical cylinders placed in sand beds. A survey of recent issues of Journal of Hydraulic Engineering and Journal of Hydraulic Research, the two leading hydraulic engineering journals, shows that, during 2008 and 2009, JHE and JHR published 17 and 19 journal papers on bridge scour. The papers, reflecting much of the research conducted since 1990, address the following aspects of scour: 1. Continued strong interest in basic scour processes; 2. Increased interest in pier flow field; 3. Influence of boundary complexities (non-uniform sediments, clay, rock); and, 4. Design for complex pier forms (large piers, overtopped bridges, debris and ice effects). Several comprehensive summaries of scour research and design relationships to estimate local scour at bridge foundations have been published since 1990. A selection of notable publications include Breusers and Raudkivi (1991), Lagasse et al. (1995), Richardson and Davis (1995), Parola et al. (1996), Hoffmans and Verheij 1997, Dey (1997), Raudkivi (1998), Whitehouse (1998), Hamill (1998), Richardson and Lagasse (1999), Melville and Coleman (2000), Sumer and Fredsoe (1997, 2002), FHWA (1996, 2001), Briaud et al. (2004a), Sheppard and Miller (2006), Lagasse et al (2009), and Sheppard et al. (NCHRP 24-32). Also, website information provides useful information about pier scour (e.g., the following website provided by the Federal Highway Administration, http://www.fhwa.dot.gov/engineering/hydraulics/bridgehyd/index.cfm). A useful development since 1990 is the availability of internet information regarding pier scour. An increasing amount of information can be accessed via the internet, thereby helping to improve understanding of scour processes and how to design for them.
28 Figure 2-9 Overview of structured design approach
