NextScour for improving bridge scour design in the United States

James Pagenkopf,Haoyin Shan,Kornel Kerenyi,Chao Huang

doi:10.1680/jfoen.20.00017

Abstract

The US Federal Highway Administration (FHWA) is developing the next-generation scour programme – that is, NextScour – to improve scour analysis for safe and economical bridge foundation design. NextScour recognises that scour phenomena consist of two major components: (a) water and hydraulic forces and (b) erosion resistance of soils and their associated geotechnical effects. Consequently, the programme consists of two focus areas: NextScour-Hydraulic and NextScour-Geotechnical. NextScour seeks to research and develop a design tool that computes hydraulic loads across the bathymetric domain. When linked to geotechnically derived subsurface erosion maps/stratigraphy and information of NextScour, the design tool produces instantaneous three-dimensional scour bathymetries around all bridge foundation elements. A bridge replacement project from the Virginia Department of Transportation provided an excellent case study of the potential cost saving by considering NextScour proof-of-concept results in the design process. The J. Sterling Jones Hydraulics Research Laboratory erosion tests determined the erosion resistance of the clay layer at the bridge foundation, and a hydraulic force depth decay function was developed along with the soil depth. The proof-of-concept analysis showed a potential reduced pier scour estimate by 15.7 feet (ft) (4.8 m) and contraction scour estimate by 16.4 ft (5.0 m), 44 and 65% reductions from the original scour analysis, respectively. NextScour empowers FHWA’s future scour analysis with the goal to improve significantly the accuracy of bridge scour estimates.

Highlights

The Federal Highway Administration (FHWA) Scour Program came into existence because of the 1987 scour-induced failure of the I-90 Schoharie Creek Bridge and the resulting findings by the National Transportation Safety Board (NTSB, 1987) that called for greater FHWA oversight
The General Thomas Highway (Route (Rte.) 671) Bridge scour study discussed later in this paper demonstrated the effectiveness of the NextScour approach for actual projects
The In-situ Scour Testing Device (ISTD) is a patented device developed by the FHWA (Shan et al, 2018)

Summary

Introduction

The Federal Highway Administration (FHWA) Scour Program (the programme) came into existence because of the 1987 scour-induced failure of the I-90 Schoharie Creek Bridge and the resulting findings by the National Transportation Safety Board (NTSB, 1987) that called for greater FHWA oversight. The initial scour-potential assessments were generally focused on the use of existing hydraulic methods These approaches were incrementally advanced over time but with little recognition of the potential geotechnical resistance or other engineering considerations that influence scour prediction. Current practice relies on empirical and simplified one-dimensional (1D) design equations in Hydraulic Engineering Circular Number 18 (HEC-18, Arneson et al, 2012) for predicting scour for individual foundation components by assuming a uniform layer of soil characterised using D50. This potentially results in a conservative scour design. NextScour institutes a new direction that refocuses and aligns those geotechnical and hydraulic components within a true multidisciplinary framework with outcomes that provide more certainty and reduced project costs

Next-generation scour design

Erosion analysis

NextScour case study

Conclusions