Contenido Evaluating Scour At Bridges

This document is the fifth edition of HEC-18. It presents the state of knowledge and practice for the design, evaluation and inspection of bridges for scour. There are two companion documents, HEC-20 entitled "Stream Stability at Highway Structures," and HEC-23 entitled "Bridge Scour and Stream Instability Countermeasures." These three documents contain updated material from previous editions and continued research by NCHRP, FHWA, State DOTs, and universities. This fifth edition of HEC-18 also contains revisions obtained from further scour-related developments and the use of the 2001 edition by the highway community. The major changes in the fifth edition of HEC-18 are: expanded discussion on the policy and regulatory basis for the FHWA Scour Program, including risk-based approaches for evaluations, developing Plans of Action (POAs) for scour critical bridges, and expanded discussion on countermeasure design philosophy (new vs. existing bridges). This fifth edition includes: a new section on contraction scour in cohesive materials, an updated abutment scour section, alternative abutment design approaches, alternative procedures for estimating pier scour, and new guidance on pier scour with debris loading. There is a new chapter on soils, rock and geotechnical considerations related to scour. Additional changes include: a new approach for pier scour in coarse material, new sections on pier scour in cohesive materials and pier scour in erodible rock, revised guidance for vertical contraction scour (pressure flow) conditions, guidance for predicting scour at bottomless culverts, deletion of the "General Scour" term, and revised discussion on scour at tidal bridges to reflect material now covered in HEC-25 (1st and 2nd Editions).


TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

LIST OF SYMBOLS..

ACKNOWLEDGMENTS
GLOSSARY .

CHAPTER 1 INTRODUCTION

1.1 PURPOSE.
1.2 BACKGROUND..
1.3 COMPREHENSIVE ANALYSIS.
1.4 PROCEDURAL GUIDANCE
   1.4.1 Objectives of a Bridge Scour Evaluation Program
   1.4.2 Bridge Scour and the NBIS..
1.5 ADVANCES IN THE STATE-OF-PRACTICE FOR ESTIMATING SCOUR  AT BRIDGES
1.6 MANUAL ORGANIZATION
1.7 DUAL SYSTEM OF UNITS.

CHAPTER 2 - DESIGNING AND EVALUATING BRIDGES TO RESIST SCOUR

2.1 SCOUR DESIGN PHILOSOPHY AND CONCEPTS FOR NEW BRIDGES
2.2 GENERAL DESIGN PROCEDURE
2.3 DESIGN CONSIDERATIONS.
   2.3.1 General
   2.3.2 Piers
   2.3.3 Abutments
   2.3.4 Superstructures
2.4 DETAILED PROCEDURES AND SPECIFIC DESIGN APPROACH
   2.4.1 Step 1: Determine Scour Analysis Variables.
   2.4.2 Step 2: Determine the Magnitude of Long-Term Degradation or Aggradation
   2.4.3 Step 3: Compute the Magnitude of Contraction Scour
   2.4.4 Step 4: Compute the Magnitude of Local Scour at Piers
   2.4.5 Step 5: Determine the Foundation Elevation for Abutments.
   2.4.6 Step 6: Plot the Total Scour Depths and Evaluate the Design
2.5 SCOUR EVALUATION PHILOSOPHY AND CONCEPTS FOR EXISTING BRIDGES
   2.5.1 Overview
   2.5.2 Countermeasures for Scour Critical Bridges
   2.5.3 Bridges with Unknown Foundations

CHAPTER 3 - BASIC CONCEPTS AND DEFINITIONS OF SCOUR..

3.1 GENERAL
3.2 TOTAL SCOUR
   3.2.1 Aggradation and Degradation
   3.2.2 Contraction Scour..
   3.2.3 Local Scour
   3.2.4 Other Types of Scour..
   3.2.5 Lateral Stream Migration .
3.3 LONG-TERM STREAMBED ELEVATION CHANGES (AGGRADATION AND DEGRADATION)
3.4 CLEAR-WATER AND LIVE-BED SCOUR
3.5 CONTRACTION SCOUR .
   3.5.1 Basic Conditions for Contraction Scour
3.6 LOCAL SCOUR..
   3.6.1 Scour at Bridge Piers and Abutments
   3.6.2 Bridge Pier Flow Field..
   3.6.3 Bridge Abutment Flow Field..
3.7 LATERAL SHIFTING OF A STREAM.

CHAPTER 4 - SOILS, ROCK, AND GEOTECHNICAL CONSIDERATIONS

4.1 GENERAL
4.2 SCOUR PROCESSES
   4.2.1 Cohesionless Soils
   4.2.2 Cohesive Soils
   4.2.3 Rock
4.3 ERODIBILITY.
   4.3.1 Velocity.
   4.3.2 Shear Stress.
   4.3.3 Stream Power
   4.3.4 Erosion Rates
   4.3.5 Devices to Measure Erodibility.
4.4 SOIL PROPERTIES
   4.4.1 Particle Size
   4.4.2 Plasticity and the Atterberg Limits
   4.4.3 Density and Compaction
   4.4.4 Shear Strength
   4.4.5 Hydraulic Conductivity
4.5 CLASSIFICATION OF SOILS.
   4.5.1 Unified Soil Classification System
   4.5.2 AASHTO Classification System.
4.6 ROCK PROPERTIES..
   4.6.1 Igneous, Sedimentary, and Metamorphic Rocks.
   4.6.2 Rock Mass Descriptions and Characteristics
4.7 CLASSIFICATION OF ROCK.
   4.7.1 Rock Mass Rating System
   4.7.2 Erodibility Index Method ..
4.8 SUMMARY ..

CHAPTER 5 - LONG-TERM AGGRADATION AND DEGRADATION .

5.1 INTRODUCTION....
5.2 LONG-TERM BED ELEVATION CHANGES .
5.3 ESTIMATING LONG-TERM AGGRADATION AND DEGRADATION
5.3.1 Bridge Inspection Records.
5.3.2 Gaging Station Records
5.3.3 Geology and Stream Geomorphology
5.3.4 Computer Models
5.3.5 Aggradation, Degradation, and Total Scour
5.3.6 Inspection, Maintenance, and Countermeasures..

CHAPTER 6 - CONTRACTION SCOUR.

6.1 INTRODUCTION.
6.2 CONTRACTION SCOUR
   6.2.1 Contraction Scour Conditions .
   6.2.2 Contraction Scour Cases
6.3 LIVE-BED CONTRACTION SCOUR
6.4 CLEAR-WATER CONTRACTION SCOUR.
6.5 CONTRACTION SCOUR WITH BACKWATER
6.6 CONTRACTION SCOUR EXAMPLE PROBLEMS .
   6.6.1 Example Problem 1 - Live-Bed Contraction Scour
   6.6.2 Example Problem 2 - Alternate Method
   6.6.3 Example Problem 3 - Relief Bridge Contraction Scour.
   6.6.4 Comprehensive Example.
6.7 CONTRACTION SCOUR IN COHESIVE MATERIALS
   6.7.1 Ultimate Scour
   6.7.2 Time Rate of Scour
6.8 CONTRACTION SCOUR IN ERODIBLE ROCK
6.9 SCOUR AT OPEN-BOTTOM CULVERTS
   6.9.1 Laboratory Investigations of Scour at Open-Bottom Culverts
   6.9.2 Clear-Water Scour Equation for Open-Bottom Culverts
6.9.3 Example Problem
6.10 PRESSURE FLOW SCOUR (VERTICAL CONTRACTION SCOUR) .
6.10.1 Estimating Pressure Flow Scour
6.10.2 Pressure Flow Scour Example Problems.

CHAPTER 7 - PIER SCOUR .

7.1 GENERAL .
7.2 HEC-18 PIER SCOUR EQUATION
7.3 FLORIDA DOT PIER SCOUR METHODOLOGY..
7.4 PIER SCOUR AT WIDE PIERS
7.5 SCOUR FOR COMPLEX PIER FOUNDATIONS
   7.5.1 Introduction.
   7.5.2 Superposition of Scour Components Method of Analysis.
   7.5.3 Determination of the Pier Stem Scour Depth Component .
   7.5.4 Determination of the Pile Cap (Footing) Scour Depth Component .
   7.5.5 Determination of the Pile Group Scour Depth Component..
   7.5.6 Determination of Total Scour Depth for the Complex Pier..
7.6 MULTIPLE COLUMNS SKEWED TO THE FLOW.
7.7 SCOUR FROM DEBRIS ON PIERS .
   7.7.1 Debris Accumulation on Piers
   7.7.2 Debris Size and Shape...
   7.7.3 Effective Pier Width with Debris
7.8 TOPWIDTH OF SCOUR HOLES
7.9 PHYSICAL MODEL STUDIES ..
7.10 PIER SCOUR EXAMPLE PROBLEMS..
   7.10.1 Example Problem 1 - Scour at a Simple Solid Pier .
   7.10.2 Example Problem 2 - Angle of Attack
   7.10.3 Example Problem 3 - Scour at Complex Piers (Solid Pier on an Exposed Footin
   7.10.4 Example Problem 4 - Scour at a Complex Pier with Pile Cap in the Flow
   7.10.5 Example Problem 5 - Scour at Multiple Columns..
   7.10.6 Example Problem 6 - Florida DOT Pier Scour Methodology .
   7.10.7 Example Problem 7 - Pier Scour with Debris
   7.10.8 Comprehensive Example
7.11 PIER SCOUR IN COARSE BED MATERIALS
7.12 PIER SCOUR IN COHESIVE MATERIALS .
7.13 PIER SCOUR IN ERODIBLE ROCK .
   7.13.1 Quarrying and Plucking
   7.13.2 Example Problem - Erodibility Index Method for Rock
   7.13.3 Abrasion
   7.13.4 Example Problem - Long-Term Abrasion of Rock

CHAPTER 8 - EVALUATING LOCAL SCOUR AT ABUTMENTS

8.1 GENERAL

8.2 ABUTMENT SCOUR EQUATIONS
   8.2.1 Overview .
   8.2.2 Abutment Scour Parameter Determination
8.3 ABUTMENT SITE CONDITIONS.
8.4 ABUTMENT SKEW
8.5 ABUTMENT SHAPE.
8.6 ESTIMATING SCOUR AT ABUTMENTS
   8.6.1 Froehlich's Abutment Scour Equation
   8.6.2 HIRE Abutment Scour Equation
   8.6.3 NCHRP 24-20 Abutment Scour Approach ..
8.7 ABUTMENT SCOUR EXAMPLE PROBLEMS.
   8.7.1 Example Problem 1 - Froehlich Equa
   8.7.3 Example Problem 3 - NCHRP Live-Bed Scour.
   8.7.4 Example Problem 4 - NCHRP Clear-Water Scour (Particle Size)
   8.7.5 Example Problem 5 - NCHRP Clear-Water Scour (Shear Stress)
   8.7.6 Comprehensive Example..

CHAPTER 9 - SCOUR ANALYSIS FOR TIDAL WATERWAYS

9.1 INTRODUCTION
9.2 OVERVIEW OF TIDAL PROCESS.
   9.2.1 Glossary .
   9.2.2 Definition of Tidal and Coastal Processes
   9.2.3 Aggradation, Degradation, and Scour in Tidal Waterways.
9.3 LEVEL 1 ANALYSIS.
   9.3.1 Tidally Affected River Crossings.
   9.3.2 Tidal Inlets, Bays, and Estuaries..
9.4 LEVEL 2 ANALYSIS
   9.4.1 Introduction.
   9.4.2 Evaluation of Hydraulic Characteristics
   9.4.3 Design Storm and Storm Tide
   9.4.4 Scour Evaluation Concepts
9.5 TIME DEPENDENT CHARACTERISTICS OF TIDAL SCOUR.
9.6 LEVEL 3 ANALYSIS
9.7 TIDAL HYDROLOGY, HYDRAULICS, AND SCOUR AT BRIDGES
9.8 HIGHWAYS IN THE COASTAL ENVIRONMENT..

CHAPTER 10 - SCOUR EVALUATION, INSPECTION, AND PLAN OF ACTION

10.1 INTRODUCTION.
10.2 OFFICE REVIEW..
10.3 BRIDGE INSPECTION
    10.3.1 Safety Considerations
    10.3.2 FHWA Recording and Coding Guide
    10.3.4 Assessing the Substructure Condition
    10.3.5 Assessing the Condition of Countermeasures
    10.3.6 Assessing Scour Potential at Bridges
    10.3.7 Underwater Inspections.
    10.3.8 Notification Procedures
10.4 MONITORING BRIDGES FOR SCOUR.
    10.4.1 General ..
    10.4.2 Portable Monitoring Devices
    10.4.3 Fixed Monitoring Devices
    10.4.4 Selection and Maintenance of Monitoring Devices
10.5 CASE HISTORIES OF BRIDGE INSPECTION PROBLEMS.
   10.5.1 Introduction...
   10.5.2 Schoharie Creek Bridge Failure.
   10.5.3 Hatchie River Bridge Failure.
   10.5.4 Arroyo Pasajero Bridge Failure.
   10.5.5 Conclusions..
10.6 PLAN OF ACTION .
   10.6.1 Background .
   10.6.2 Developing a Plan of Action
   10.6.3 Maintaining a Plan of Action .

CHAPTER 11 - LITERATURE CITED...

APPENDIX A - METRIC SYSTEM, CONVERSION FACTORS, AND WATER
PROPERTIES
APPENDIX B - EXTREME EVENTS .
APPENDIX C - CONTRACTION SCOUR AND CRITICAL VELOCITY EQUATIONS
APPENDIX D - COMPREHENSIVE SCOUR PROBLEM
APPENDIX E - UNKNOWN FOUNDATIONS