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Preliminary Procedure to Predict Bridge Scour in Bedrock

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Preliminary Procedure to Predict Bridge Scour in Bedrock Report No. CDOT-R-SD-94-14 Preliminary Procedure to Predict Bridge Scour in Bedrock Steven P .. Smith Colorado Department of Transportation 4201 East Arkansas Avenue Denver, Colorado 80222 Interim Report December, 1994 Prepared in cooperation with the U.S. Department of Transportation Federal Highway Administratjon The contents of this report reflects the views of the author who is responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views of the Colorado Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. i TedmiC21 Report Documentation Page 1. Report No.. 2. Govenuaent Accession No .. 3. Recipient's Catalog No. CDOT-R-SD-94-14 4. Title and Subtitle S. Report Date Preliminary Procedure to Predict Scour in Bedrock December, 1994 6, Performing Organization Code 105.12 7. Autbor(s) 8. Performing Organization Rpt.No. Steven P. Smith, P .E. CDOT-RwSD-94-14 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) Colorado Department of Transportation Research Branch, 4201 E. Arkansas Ave. 11. Contract or Grant No. Denver, CO 80222 12. Sponsoring Agency Name and Address 13. Type of Rpt. and Period Covered Colorado Department of Transportation Interim~ 12/93 to 09/94 4201 E. Arkansas Ave. 14. Sponsoring Agency Code Denver, Colorado 80222 15. Supplementary Notes Prepared in Cooperation with the U.S. Department of Transportation Federal Highway Administration 16. Abstract The primary goal of this study is to develop a procedure to predict scour depths in bedrock that accounts for both the hydraulic conditions at the bridge site and the bedrock's abiHty to resist erosion. A methodology for determining material erodibility resulting from the erosive power of water has been presented by Annandale (1993, 1995). He introduced a relationship between stream power and a geomechanical material classification system known as the erodibility index. This report applies his findings to bridge scour analysis and presents an interim procedure for estimating bridge scour depths in bedrock and other materials defined by the erodibility index. Implementation: The recommendations in this report are currently being applied as an interim method for evaluating bridge scour at the Colorado Department of Transportation. Further laboratory study is planned to refine the relationship between energy dissipation at bridge piers and pier geometry. Findings from this study will be made available in a subsequent report. 17. Key Words 18. Distribution Statement Bridge Scour Energy Dissipation No Restrictions: This report is Erodibility Erodibility Index available to the public through Stream Power the National Technical Info. Service. Springfield, VA 22161 19.security Classif. (report) 20.security Classif. (page) 21. No. of Pages 22. Price Unclassified Unclassified 41 ii Acknowledgements The author would like to gratefully acknowledge Dr. George Annandale of HDR Engineering for his significant contribution. The author would also like to express his gratitude to Dr. William Hughes who participated as the author's academic advisor and Dr. Jonathan Wu and Dr. James Guo who participated on the academic committee for the author's Master's Report at the University of Colorado - Denver. The CDOT Research panel provided many excellent comments and recommendations for the study, particularly Mr. Roberto De Dios. Other members of the panel include Joe Siccardi (CDOT Bridge Branch)~ Peter Montoya (CDOT Bridge Branch}, Brandy Gilmore (CDOT Geotechnical Section), Bob Barrett (eDOr Region 3 Geologist), Tom Hunt (CDOT Research Branch) and Larry Arneson (FHWA). The author would also like to thank the members of the CDOT Hydraulics Unit and Geotechnical Section who provided review and comments for this report. iii ABSTRACT Scour at bridge crossings can lead to undermining of foundations and potentially to structure collapse. Bridge foundations must be designed to withstand the effects of scour from flooding events that can reasonably be expected to occur during a structure's life. Many equations are available to assist in the prediction of scour at bridge crossings. However, few account for the effects of gradation and none account for the effects of cohesion and consolidation. Currently, no quantitative procedure for determining bridge scour in bedrock or cohesive and consolidated material is in practice. The primary goal of this paper is to develop a procedure to predict scour depths in bedrock at bridges that accounts for both the hydraulic conditions at the bridge site and the bedrock's ability to resist erosion. A methodology for determining material erodibility resulting from the erosive power of water has been presented by Annandale (1993; 1995). He introduced a relationship between stream power and a geomechanical material classification system known as the Erodibility Index (Annandale, 1993; 1995; and Kirsten, 1982). This paper applies his findings to bridge scour analysis and presents an interim procedure for estimating bridge scour depths. This study involved a review of conventional scour prediction methods and available data. Preliminary methods for detennining stream power at bridge crossings are presented and an interim procedure for predicting bridge scour is outlined. This preliminary procedure will require refinement and calibration with additional laboratory data and field correlation. Although the main goal of this paper is to provide a method to predict scour at bridges in bedrock~ this procedure is equally applicable to scour prediction in all naturally occurring materials defmed by the Erodibility Index classification system. iv Notation A - net area of orifice (bridge opening) b - pier width Co - orifice coefficient Dso - median particle diameter ilE - energy loss per unit weight of water g - gravitational acceleration H - change in energy gradient through bridge contraction Ja - joint alteration number Je - number of joints per cubic meter In - joint set number Ir - joint roughness number Kl - correction factor for pier shape K2 ... correction factor for approach flow angle ~ - particlelblock size factor Kd - interparticle bond strength factor ~ - mass strength factor ~ ... Erodibility Index number ~ - relative shape and orientation factor I - unit channel length L - pier length P - stream power per unit channel width P a ... stream power in approach section Pp - stream power at base of bridge pier P pI _ stream power at pier base adjusted for pier shape and flow attack angle q - unit discharge of water RQD ... rock quality designation Sf - slope of energy grade line V ... mean channel velocity y - flow depth y - unit weight of water 't' . - shear stress <I> - equivalent residual friction angle v Table of Contents 1. INTRODUCTION 1 1.1 Scour 1 1.2 Existing Methods of Scour Prediction 2 1.3 Scope of Study 4 2. SCOUR AT BRIDGES 5 2.1 Material Erodibility 5 2.2 The Process of Erosion 6 2.3 The Erodibility Index 6 2.3.1 Mass Strength Factor 8 2.3.2 ParticleIBlock Size Factor 8 2.3.3 Interparticle Bond Strength Factor 9 2.. 3.4 Relative Shape and Orientation Factor 10 2.3.5 Summary of Erodibility Index 10 2.4 Erodibility Threshold 12 13 3. STREAM POWER 15 3.1 Scour at Bridge Contractions 17 3.. 1.1 Stream Power at Bridge Contractions 18 3.1.2 Pressure Flow Conditions 19 3.1.3 Contraction Scour Summary 20 3.2 Scour at Bridge Piers 20 3.2.1 The Horseshoe Vortex 21 3.2.2 Stream PowerJPier Scour Relationship 21 3.2.3 Application of Preliminary Stream PowerlPier Scour Relationship 22 3.3 Abutment Scour 26 4. PRELIMINARY PROCEDURE TO PREDICT SCOUR IN SEDIMENTARY BEDROCK 28 4.1 Procedure Outline 28 vi 4.2 Determination of the Erodibility Index 28 4.3 Determination of Contraction Scour 29 4.4 Determination of Pier Scour 30 4.5 Example Problem 32 4.5.1 Example Problem Discussion 36 5.. SUMMARY AND CONCLUSIONS 37 REFERENCES 39 Appendix A - Erodibility Index A.. l Appendix B - Analysis of Data B-1 Appendix C - Bridge Scour Prediction Procedure Worksheets C-l List of Figures Figure 2.1 The Process of Erosion 7 Figure 2.2 Erodibility Threshold 14 Figure 3.1 Preliminary Stream Power/Pier Scour Relationship 23 List of Tables Table 2.1 Determining the Erodibility Index 11 Table 3.1 Correction Factor for Pier Nose Shape 25 Table 3.2 Correction Factor for Angle of Attack of Flow 25 Table 4.1 Scour Prediction Procedure Outline 29 Table 4.2 Contraction Scour Procedure 30 Table 4.3 Pier Scour Procedure 31 Table 4.4 Example Problem - Summary of Geotechnical Properties 32 Table.4.5 Example Problem - Erodibility Index Calculations 33 Table 4.6 Example Problem - Contraction Sconr Calculations 34 Table 4.7 Example Problem ... Pier Scour Calculations 35 vii 1. INTRODUCTION Flood related scour is a major threat to bridges and the travelling pUblic. Analysis of scour requires an understanding of the interaction between the hydraulic forces and the variable properties of the channel bed and foundation materials that are found at bridge crossings. A large amount of literature is available regarding the analysis of bridge scour in non-cohesive materials but no procedure is currently in practice which relates the erosive power of water to the properties of bedrock or cohesive and consolidated material at bridges. A quantitative method of scour prediction is needed to determine scour depths in bedrock and other cohesive and consolidated materials. Such a method will provide an increased level of confidence in locating bridge foundations at depths which \ViII withstand SCOUT, but not be excessively conservative as to needlessly increase foundation costs. The purpose of this report is to provide a practical procedure which combines an assessment of both the unique hydraulic conditions at a bridge site and the erodibility of its channel bed and bedrock foundations to predict potential scour depths. This report discusses the basic concepts of scour at bridges, the processes of erosion, and the relationship between the erosive power of water and material erodibility as presented by Annandale (1993; 1995).
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