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***Tuesday, April 10, 2012***

Topic: "Scour at Bridge Foundations on Rock: NCHRP Project 24-29 Overview"

Speaker: Jeffrey R Keaton, PhD, PE, PG, D.GE
Senior Principal Engineering Geologist, Vice President
AMEC Environment & Infrastructure, Inc., Los Angeles, CA

Location: Victorios Restaurant, North Hollywood

Date/Time: Tuesday, April 10, 2012

Rock scour is a rock-water interaction phenomenon. National Cooperative Highway Research Program Project No. 24-29 is geotechnical site characterization in scour-relevant terms for use by hydraulic engineers. Project goals are time-rate of scour and design scour depth at bridge foundations on rock for integration with Federal Highway Administration Hydraulic Engineering Circular HEC-18, “Evaluating Scour at Bridges”. Rock scour in natural channels is related to five processes: 1) physical and chemical weathering of exposed rock surfaces, 2) soluble-rock dissolution, 3) cavitation, 4), durable-rock quarrying and plucking, and 5) degradable-rock abrasion and grain-scale plucking. Defining ‘rock’ for scour purposes is just as problematic as it is for other engineering applications because physical properties range from strong soil to better than concrete. Bridge sites in Florida, Oregon, New York, Utah, and California visited in 2008 provided a range of conditions, data, and samples for the project. Check-list guidance aids in determining which scour processes can be dismissed and which deserve evaluation. Quarrying and plucking of jointed rock is a threshold process governed by turbulence intensity, flow velocity, and rock block size and geometry. Numerical modeling of threshold rock-block plucking and predicted scour depth relative to pier diameter was performed for this project by AquaVision Engineering, Lausanne, Switzerland. Hydraulic loading for degradable rock material is expressed as stream power (hydraulic shear stress × flow velocity) because it incorporates all flow parameters and can be accumulated over time. Probability weighted flood frequency captures the range of flow conditions and is converted to average annual scour. Long-term observations of scour divided by cumulative stream power for the same period represent the empirical scour number. Modified slake durability test results expressed as equivalent scour depth and stream power represent the geotechnical scour number. Design scour depth is the product of the probability weighted average annual scour and the remaining life of a bridge structure or the product of cumulative stream power for the life of a bridge and the appropriate scour number.
Speaker Biography:
Jeff Keaton ( is Geotechnical Practice Leader for AMEC. He has degrees in Geological Engineering, Geotechnical Engineering, and Geology. He has been employed by consulting firms for 42 years, and in AMEC's Los Angeles office of since July 2005 where he is a Senior Principal Engineering Geologist and Vice President. He was the president of AEG in 1992 and the Jahns Lecturer in 2004. He currently serves as Chair of the Technical Coordination Council of ASCE’s Geo-Institute.