Review of Bridge Sites Visited for NCHRP Project 24- 29: Scour at Bridge Foundations on Rock

Jeffrey R. Keaton, MACTEC Engineering and Consulting, Inc., Su K. Mishra, Ayres Associates, Inc.

ABSTRACT:

Five bridge sites were visited in 2008 as part of National Cooperative Highway Research Program (NCHRP) Project 24-29: Scour at Bridge Foundations on Rock. I- 10 Chipola River Bridges, Jackson County, Florida, are founded on thick bedded Oligocene marine limestone that shows geologic evidence of dissolution. SR-22 Mill Creek Bridge, Polk County, Oregon, is founded on widely fractured Oligocene marine siltstone prone to slaking in air. I-90 Schoharie Creek Bridge, Montgomery County, New York, that failed in 1987 was founded on Quaternary ice-contact stratified drift armored by hard sandstone boulders and cobbles. The armor layer of boulders over the glacial till in New York provided a threshold control for scour and was used for evaluating excess stream power. Paleozoic marine sandstone is present across the channel at a US Geological Survey stream gage on Schoharie Creek. SR-262 Montezuma Creek Bridge, San Juan County, Utah, is founded on stratified Jurassic sandstone and claystone excavated in 1955 to create a channel which cut off a meander loop. Cavitation pits were observed on sculpted sandstone in Utah, but the primary control on scour was plunge pool excavation of fractured claystone interbedded with the sandstone. SR-273 Sacramento River Bridge, Shasta County, California, is founded on thinly bedded Cretaceous siltstone that slakes in water. Laboratory tests included slake durability, continuous abrasion, Rotating Erosions Test Apparatus (RETA), point load, and specific gravity. Reliable channel cross section data were available for bridges in Oregon, New York, and California for at least two dates several years apart.

 

INTRODUCTION

National Cooperative Highway Research Program (NCHRP) Project 24-29, Scour at Bridge Foundations on Rock, began in 2006 with objectives of developing a methodology for determining design scour depth and time-rate of scour in rock, and creating design and construction guidelines for application of the methodology. The status of this research in the spring of 2008 was described by Keaton and Mishra (2008), before field visits had been made to bridge sites. The objective of the current paper is to review some geologic and hydraulic conditions of the five sites visited during the summer and fall of 2008 and laboratory test results.

 

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