Scour at Bridge Foundations on Rock (2012)

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4Background and Objectives 1.1 Background The FHWA guidance for evaluating scour at bridges (FHWA, 1991b) recommends that every bridge over a waterway should be evaluated as to its vulnerability to scour and refers to Hydraulic Engineering Circular (HEC) 18: Evaluating Scour at Bridges (Richardson and Davis, 2001) for procedures. FHWA (1991b) notes that most waterways can be expected to experience scour over the service life of a bridge with the possible exception of “waterways in massive, competent rock formations where scour and erosion occur on a scale that is measured in centuries.” Some guid- ance is offered in HEC 18 (Chapter 2) regarding bridge footings on rock highly resistant to scour; reference is made to a memo by FHWA (1991a) in the discussion of bridge footings on erodible rock. The FHWA (1991a) memo states that several physical properties contribute to the scour resistance of rock; therefore, no single index property can be used for accurate characterization of rock masses. HEC 18 (Chapter 12) notes that additional research is needed for determining the scour resistance of rock. A number of bridges throughout the United States may be founded on erodible rock. Rock ero- sion processes include gradual dissolution by chemical weathering; disintegration and wearing away by impact and abrasion of bedload and suspended load particles; quarrying and pluck- ing of blocks of durable, jointed rock; and cavitation. Soft rock formations may scour rapidly during a single flood event, whereas hard rock formations may have no observable evidence of erosion after decades of floods. Geotechnical properties of most rock materials are not sufficiently well understood for the rock formations to be considered “scour-resistant,” let alone to define the time-rate of scour in susceptible formations. The results of this research demonstrate that rock scour is a rock-water interaction phenomenon; thus, weak rocks may be resistant to slow-moving water, whereas strong rocks may erode in response to swift, turbulent, high-power flows. State departments of transportation (DOTs) are required by FHWA to evaluate scour at bridge sites and protect bridge structures from failure. Thus, for conservatism, hydraulic engineers have chosen to consider all rock formations as if they were cohesionless sediments (i.e., sand) for the purpose of estimating scour depths. In many cases, this approach may be overly conserva- tive, with large predicted scour depths that result in excessive foundation costs for new bridges, expensive retrofitting of existing bridges, or a large number of bridges on the scour-critical list. At the beginning of the NCHRP rock scour project, two index methods, two specially developed laboratory testing devices, and one modified procedure for an existing laboratory device were available. The two index methods are similar and derived from mechanical excavation of rock (rippability) and unlined spillway erosion: the Headcut Erodibility Index (NRCS, 2001) and the Erodibility Index Method (Annandale, 2006). One of the laboratory devices generates hydraulic shear stresses on a rock core sample (the Rotating Erosion Test Apparatus; Henderson et al., 2000), whereas the other laboratory device generates abrasion from bedload saltation impacts on a rock C h a p t e r 1

Background and Objectives 5 disk (the bedrock abrasion mill; Sklar and Dietrich, 2001). The existing laboratory device for slake durability testing (ASTM D4644-08) was used to produce an abrasion number (Dickenson and Baillie, 1999). 1.2 Objectives Scour at bridge foundations traditionally is evaluated by hydraulic engineers with input from geologists and geotechnical engineers. NCHRP Project 24-29 focused on recognition of rock and rock-like materials that may be susceptible to scour processes and characterization of bridge foundation conditions in terms that accurately reflect the scour susceptibility and can be used by hydraulic engineers to calculate design scour depths. In essence, the research strives for geo- technical site characterization expressed in scour-relevant terms for use by hydraulic engineers. The objectives of NCHRP Project 24-29 were to develop • A methodology for estimating the time-rate of scour and the design scour depth of a bridge foundation on rock and • Design and construction guidelines for application of the methodology. The FHWA guidance for evaluating scour at bridges and stream stability is contained in three manuals, each of which is published as a hydraulic engineering circular, as follows: 1. HEC-18 Evaluating Scour at Bridges (Richardson and Davis, 2001) 2. HEC-20 Stream Stability at Highway Structures (Lagasse et al., 2001a) 3. HEC-23 Bridge Scour and Stream Instability Countermeasures (Lagasse et al., 2001b) Richardson and Davis (2001, Section 1.3, p 1.2) describe HEC-20 analysis procedures and then state In most cases, the analysis or evaluation will progress to the HEC-18 block of the flow chart. Here more detailed hydrologic and hydraulic data are developed, with the specific approach determined by the level of complexity of the problem and waterway characteristics (e.g., tidal or riverine). The “Scour Analysis” portion of the HEC-18 block encompasses a seven-step specific design approach which includes evaluation of the components of total scour (see Chapter 3 [in HEC-18]). Since bridge scour evaluation requires multidisciplinary inputs, it is often advisable for the hydraulic engineer to involve structural and geotechnical engineers at this stage of the analysis. Once the total scour prism is plotted, then all three disciplines must be involved in a determination of structural stability. Thus, it is clear that the hydraulic engineering community leads the way in bridge scour evaluations and also must lead the way in acceptance and implementation of the rock scour methodology described in this NCHRP report. The other key disciplines have important roles in supporting the hydraulic discipline leadership.