Bridge Scour Analysis for Rocks: Classification, Method Selection, and Practical Case Studies

Despite the availability of several scour calculation methods, bridge scour analysis for scour susceptible rocks is much less understood than its counterpart for sands, because scour calculation equations were traditionally obtained using flume tests with sands. As a result, there are three major knowledge gaps in the bridge scour analysis for susceptible rocks: (1) a rock classification system is needed for scour considerations to serve as a pre-screening process before conventional scour analysis involving sampling, erodibility testing, and scour calculation, (2) there has been a lack of information to guide the scour analysis for rocks including detailed procedures, needed and available data, and interpretation of results, especially for real bridge sites, and (3) detailed comparisons between the available rock scour calculation methods and the widely used sand-based scour depth calculation equations are rare. This paper presents a study to bridge the above knowledge gaps. First, a rock classification system was proposed for the bedrock of Michigan to exemplify the use of a pre-screening tool for rock preliminary scour susceptibility determination. Then, two methods were selected for two modes of scour that are common to bridges: the Erodibility Index Method for the quarrying and plucking mode of rock scour and the NCHRP-717 method for the abrasion mode. Case studies were conducted at two real bridge sites using these methods as well as sand-based equations. These case studies are intended to demonstrate practical implementation using available project data, compare the outcomes of different scour-analysis methods, and identify common data limitations and implementation challenges.