FHWA NextScour Case Studies: Bridging Hydraulic Loads with Soil Erosion Resistance

Abstract

The Federal Highway Administration (FHWA) is developing NextScour, a scour research initiative to improve scour analysis and provide more accurate scour depth estimates for bridge foundation design. NextScour considers the two major components of scour: hydraulic loads and the erosional resistance of soils. Through an ongoing transportation pooled fund study, FHWA partnered with several state departments of transportation to provide soil and erosion testing services for bridge scour evaluations, which has resulted in several excellent case studies for the NextScour initiative. This paper summarizes the evaluations of two bridge replacement projects in Michigan (MI) and North Carolina (NC). The proposed bridge site in MI had a calculated scour depth that exceeded 11 m; however, a layer of medium-to-hard clay at depths of 4.6–6.1 m below the channel was identified in the geotechnical site investigation. The NC bridge site has a shallow medium to very stiff clay layer 1.2 m underneath the fine sand of the riverbed. As part of the subsurface exploration, samples of both clayey soils were collected and tested by FHWA to obtain the erosion resistance and other geotechnical properties. The resulting critical shear stress measurements suggested that both clay layers would be resistant to scour from the design floods, potentially saving costs on the foundation designs for both bridge replacements. This paper presents an overview of the NextScour initiative, discusses current and proposed methods to evaluate hydraulic loads and soil erosion resistance, details the two case studies, and provides a summary of the hydraulic and geotechnical testing conducted within the context of NextScour.