Pier and Contraction Scour in Cohesive Soils (2004)

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T R A N S P O R T A T I O N R E S E A R C H B O A R D WASHINGTON, D.C. 2004 www.TRB.org NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP REPORT 516 Research Sponsored by the American Association of State Highway and Transportation Officials in Cooperation with the Federal Highway Administration SUBJECT AREAS Highway and Facility Design • Bridges, Other Structures, and Hydraulics and Hydrology • Soils, Geology, and Foundations • Materials and Construction Pier and Contraction Scour in Cohesive Soils J.-L. BRIAUD H.-C. CHEN Y. LI P. NURTJAHYO AND J. WANG Texas A&M University College Station, TX

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. 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COOPERATIVE RESEARCH PROGRAMS STAFF FOR NCHRP REPORT 516 ROBERT J. REILLY, Director, Cooperative Research Programs CRAWFORD F. JENCKS, Manager, NCHRP TIMOTHY G. HESS, Senior Program Officer EILEEN P. DELANEY, Director of Publications HILARY FREER, Editor NCHRP PROJECT E24-15 PANEL Field of Soils and Geology—Area of Mechanics and Foundations STEVEN P. SMITH, PBS&J-Denver, Greenwood Village, CO (Chair) LARRY A. ARNESON, FHWA DARYL J. GREER, Kentucky Transportation Cabinet ROBERT W. HENTHORNE, Kansas DOT MELINDA LUNA, Lower Colorado River Authority WILLIAM L. MOORE, North Carolina DOT RICHARD A. PHILLIPS, South Dakota DOT MEHMET T. TUMAY, Louisiana State University J. STERLING JONES, FHWA Liaison Representative G. P. JAYAPRAKASH, TRB Liaison Representative AUTHOR ACKNOWLEDGMENTS Special thanks go to the NCHRP group associated with this proj- ect for their advice and comments throughout the study: the NCHRP Project Panel and NCHRP staff. Special thanks also go to our consultants: Peter Lagasse, Ayres Associates; Peter Smith, Parsons Brinckerhoff Quade & Douglas, Inc.; and Art Parola, Jr., University of Louisville. We also wish to acknowledge the help from people at Texas A&M University, including John Reed, Bob Randall, Andrew Faw- cett, Mike Linger, Richard Gehle, and Josh Reinbolt.

This report discusses the findings of a research project undertaken to investigate bridge scour in cohesive soils. The report presents a recommended method for pre- dicting the extent of complex pier and contraction scour in cohesive soils. This report will be of immediate interest to engineers with responsibility for predicting the extent of scour at bridge foundations and to engineers with responsibility for designing bridge foundations and bridge scour countermeasures. Scour at bridges founded in or on cohesive soil is a complex phenomenon that is not completely understood. Conventional approaches to scour prediction were devel- oped from laboratory experiments in cohesionless materials and are generally regarded as overly conservative when applied to cohesive soils. Accurate and accepted methods for predicting scour depths in cohesive soils that account for the soil’s greater scour resistance are not yet available to practicing engineers. The lack of an accurate predic- tive method often results in an overly conservative and sometimes unnecessarily costly bridge foundation. Research investigating the relationship between properties of cohe- sive material and the erosive power of flowing water is needed to improve the predic- tion of scour in cohesive soils. Under NCHRP Project 24-15, the Texas Transportation Institute developed a method for predicting pier and contraction scour in cohesive soil. The research team first reviewed the literature to identify existing knowledge in the subject area. The design of an erosion function apparatus (EFA) developed in earlier research was enhanced, and a new EFA was constructed and used in the development of erosion curves for specific soils. Laboratory flume tests were conducted, followed by numeri- cal simulations; and finally prediction equations were developed. The prediction method developed, termed SRICOS (Scour Rate In Cohesive Soils), was applied to several contraction and complex-pier configurations typically encountered by state highway agencies. An evaluation of the accuracy and precision of the SRICOS Method was conducted by comparing predicted and measured data. Exam- ple problems using the SRICOS Method were developed to assist practitioners in applying the method. A computer program, termed SRICOS-EFA was developed to automate the calculations used in the SRICOS Method and to assist in the imple- mentation of the research results. NCHRP Report 516 includes a discussion of existing knowledge and practice, a description of the erosion function apparatus, a discussion of laboratory tests and numer- ical simulations conducted, presentation of the SRICOS-EFA method, and one appen- dix, Appendix A: Photographs from the Flume Tests. Compilations of flume test data and case history data were provided by the Texas Transportation Institute but are not included in this publication; however, they are available on request from NCHRP. Sub- sequent to completion of NCHRP Project 24-15, the Texas Transportation Institute updated the SRICOS-EFA computer program and added other enhancements. The SRICOS-EFA computer program and User’s Manual are available from the Texas Transportation Institute via the internet at http://ceprofs.tamu.edu/briaud/sricos-efa.htm. FOREWORD By Timothy G. Hess Staff Officer Transportation Research Board

1 SUMMARY 8 CHAPTER 1 Introduction 1.1 Bridge Scour, 8 1.2 Classification of Soils, 8 1.3 The Problem Addressed, 8 1.4 Why Was This Problem Addressed?, 8 1.5 Approach Selected to Solve the Problem, 9 10 CHAPTER 2 Erodibility of Cohesive Soils 2.1 Erodibility: A Definition, 10 2.2 Erosion Process, 10 2.3 Existing Knowledge on Erodibility of Cohesive Soils, 10 2.4 Erodibility and Correlation to Soil and Rock Properties, 10 14 CHAPTER 3 Erosion Function Apparatus (EFA) 3.1 Concept, 14 3.2 EFA Test Procedure, 14 3.3 EFA Test Data Reduction, 14 3.4 EFA Precision and Typical Results, 15 17 CHAPTER 4 The SRICOS-EFA Method for Cylindrical Piers in Deep Water 4.1 SRICOS-EFA Method for Constant Velocity and Uniform Soil, 17 4.2 Small Flood Followed by Big Flood, 17 4.3 Big Flood Followed by Small Flood and General Case, 18 4.4 Hard Soil Layer Over Soft Soil Layer, 19 4.5 Soft Soil Layer Over Hard Soil Layer and General Case, 21 4.6 Equivalent Time, 21 4.7 Extended and Simple SRICOS-EFA Method, 22 4.8 Case Histories, 24 4.9 Predicted and Measured Local Scour for the Eight Bridges, 26 4.10 Conclusions, 29 30 CHAPTER 5 The SRICOS-EFA Method for Maximum Scour Depth at Complex Piers 5.1 Existing Knowledge, 30 5.2 General, 30 5.3 Flumes and Scour Models, 30 5.4 Measuring Equipment, 30 5.5 Soils and Soil Bed Preparation, 32 5.6 Flume Tests: Procedure and Measurement, 33 5.7 Shallow Water Effect: Flume Test Results, 34 5.8 Shallow Water Effect on Maximum Pier Scour Depth, 34 5.9 Shallow Water Effect on Initial Shear Stress, 36 5.10 Pier Spacing Effect: Flume Test Results, 37 5.11 Pier Spacing Effect on Maximum Scour Depth, 37 5.12 Pier Spacing Effect on Initial Scour Rate, 38 5.13 Pier Shape Effect: Flume Test Results, 39 5.14 Pier Shape Effect on Maximum Scour Depth, 39 5.15 Pier Shape Effect on Initial Scour Rate, 39 5.16 Pier Shape Effect on Pier Hole Shapes, 40 5.17 Attack Angle Effect: Flume Test Results, 40 5.18 Attack Angle Effect on Maximum Scour Depth, 40 5.19 Attack Angle Effect on Initial Scour Rate, 42 5.20 Attack Angle Effect on Scour Hole Shape, 42 5.21 Maximum Scour Depth Equation for Complex Pier Scour, 43 45 CHAPTER 6 The SRICOS-EFA Method for Initial Scour Rate at Complex Piers 6.1 General, 45 6.2 Existing Knowledge on Numerical Simulations for Scour, 45 6.3 Numerical Method Used in This Study, 46 6.4 Verification of the Numerical Method, 46 6.5 Shallow Water Effect: Numerical Simulation Results, 46 6.6 Shallow Water Effect on Maximum Shear Stress, 47 CONTENTS

6.7 Pier Spacing Effect: Numerical Simulation Results, 48 6.8 Pier Spacing Effect on Maximum Shear Stress, 49 6.9 Pier Shape Effect: Numerical Simulation Results, 49 6.10 Pier Shape Effect on Maximum Shear Stress, 51 6.11 Attack Angle Effect: Numerical Simulation Results, 52 6.12 Attack Angle Effect on Maximum Shear Stress, 53 6.13 Maximum Shear Stress Equation for Complex Pier Scour, 54 56 CHAPTER 7 The SRICOS-EFA Method for Maximum Contraction Scour Depth 7.1 Existing Knowledge, 56 7.2 General, 56 7.3 Flume Tests and Measurements, 56 7.4 Flume Tests: Flow Observations and Results, 57 7.5 Flume Tests: Scour Observations and Results, 59 7.6 Maximum and Uniform Contraction Depths for the Reference Cases, 60 7.7 Location of Maximum Contraction Depth for the Reference Cases, 63 7.8 Correction Factors for Transition Angle and Contraction Length, 64 7.9 SRICOS-EFA Method Using HEC-RAS Generated Velocity, 65 7.10 Constructing the Complete Contraction Scour Profile, 66 7.11 Scour Depth Equations for Contraction Scour, 66 68 CHAPTER 8 The SRICOS-EFA Method for Initial Scour Rate at Contracted Channels 8.1 Background, 68 8.2 Contraction Ratio Effect: Numerical Simulation Results, 68 8.3 Transition Angle Effect: Numerical Simulation Results, 68 8.4 Contracted Length Effect: Numerical Simulation Results, 69 8.5 Water Depth Effect: Numerical Simulation Results, 72 8.6 Maximum Shear Stress Equation for Contraction Scour, 72 76 CHAPTER 9 The SRICOS-EFA Method for Complex Pier Scour and Contraction Scour in Cohesive Soils 9.1 Background, 76 9.2 The Integrated SRICOS-EFA Method: General Principle, 76 9.3 The Integrated SRICOS-EFA Method: Step-by-Step Procedure, 76 9.4 Input for the SRICOS-EFA Program, 81 9.5 The SRICOS-EFA Program, 81 9.6 Output of the SRICOS-EFA Program, 84 85 CHAPTER 10 Verification of the SRICOS-EFA Method 10.1 Background, 85 10.2 Mueller (1996) Database: Pier Scour, 85 10.3 Froehlich (1988) Database: Pier Scour, 85 10.4 Gill (1981) Database: Contraction Scour, 85 10.5 Remarks, 88 89 CHAPTER 11 Future Hydrographs and Scour Risk Analysis 11.1 Background, 89 11.2 Preparation of the Future Hydrographs, 89 11.3 Risk Approach to Scour Predictions, 90 11.4 Observations on Current Risk Levels, 91 93 CHAPTER 12 Scour Example Problems 12.1 Example 1: Single Circular Pier with Approaching Constant Velocity, 93 12.2 Example 2: Single Rectangular Pier with Attack Angle and Approaching Hydrograph, 93 12.3 Example 3: Group Rectangular Piers with Attack Angle and Approaching Constant Velocity, 95 12.4 Example 4: Contracted Channel with 90-Degree Transition Angle and Approaching Constant Velocity, 99 12.5 Example 5: Contracted Channel with 60-Degree Transition Angle and Approaching Hydrograph, 103 12.6 Example 6: Bridge with Group Piers and Contracted Channel with Hydrograph in Contracted Section, 105

111 CHAPTER 13 Conclusions and Recommendations 13.1 Conclusions, 111 13.2 Recommendations, 113 114 REFERENCES 116 NOMENCLATURE 118 UNIT CONVERSIONS A-1 APPENDIX A Photographs from the Flume Tests